Ballistic Coefficient Study for Earth Entry

It has been suggested that inflatable or extendible heat shields can be used to lower the entry ballistic coefficient,  and thereby lower entry heating,  perhaps to the point of not needing heat protection on a stage or other item returning from low Earth orbit. 

To that end,  I used my spreadsheet version of the old H. Julian Allen and A. J. Eggers 1950’s-vintage entry model,  at fixed entry speed and angle below horizontal,  with a constant entry interface altitude.  I kept the object mass and hypersonic drag coefficient constant,  and used a fixed nose radius to heat shield diameter ratio. 

All I varied was the diameter (and nose radius right with it).  This produced a set of ballistic coefficients β = M/(C_D*A) that decreased dramatically from a near-Apollo value of 300 kg/m²,  down to very low values at very large diameters.  See Figure 1 below for the scope investigated and inputs used (all figures are located at end of this article).

The trajectory model uses a simple scale-height type exponential model of density with altitude.  It presumes a constant angle below horizontal in a 2-D Cartesian modeling set up.  It presumes the drag coefficient (and thus the ballistic coefficient) is constant with speed.  It corresponds to a certain velocity-altitude trend that is doubly exponential.  This is only approximate,  but it really is in the ballpark!  End-of-hypersonics for a blunt object is usually local Mach 3,  which for Earth,  is just about 1 km/s,  but I arbitrarily took this down to 0.7 km/s (about Mach 2.1),  which is well into the range where ribbon chutes can be deployed.

The results I obtained for each of the four ballistic coefficient cases are given in Figures 2 through 5 below.  I expected to see the end of hypersonics altitudes increase,  and the peak stagnation heating rates decrease,  as the ballistic coefficients reduced,  and they did.  I also expected to see peak deceleration gees increase as ballistic coefficients decreased,  but that is not what I got:  peak gees stayed just about the same for all 4 cases.

I then ran stagnation surface temperatures at those peak heating rates,  for a low emissivity and a high-emissivity case.  I did the analysis in US Customary after converting the heating rates,  then converted the temperatures back to metric.  These show a strong decrease as ballistic coefficients get very low,  but are still problematic for anything but high-temperature steels and exotic alloys!   They are reported in Figure 6 below.

I also ran the average pressure exerted upon the heat shield at that observed constant 6.3 gee peak deceleration.  This is nothing but mass times gees times the acceleration of gravity,  then divided by the heat shield blockage area.  These are not as problematic as the stagnation point temperatures,  by far.  They are also reported in Figure 6.

Whether the inflatable or extendible heat shield concepts are survivable,  I leave to others. 

Figure 1 – Inputs Used for Entry Ballistic Coefficient Study

Figure 2 – Entry Trajectory Results for the Highest Ballistic Coefficient

Figure 3 – Entry Trajectory Results for a Lower Ballistic Coefficient

Figure 4 – Entry Trajectory Results for the Next-to-Lowest Ballistic Coefficient

Figure 5 – Entry Trajectory Results for the Lowest Ballistic Coefficient

Update 4-12-2025:  Oops,  found an error concerting to degrees C in my data.  Revised Figure 6 replaces the original.  

Figure 6 – Temperature and Pressure Results for the Ballistic Coefficient Study

Update 4-12-2025:

I went ahead and estimated the attached-flow heating rates as stagnation divided by 3,  and the wake zone heating rates as stagnation divided by 10.  This is only an educated guess,  but it is rough ballpark correct. 

From these I computed surface temperatures that equilibriate the convective heating with thermal re-radiation to surroundings at 300 K Earth temperatures.  There is no ablation,  no transpiration cooling,  and no conduction into an interior heat sink.  These temperatures are shown in Figure 7.  Bear in mind that they are very approximate! 

Figure 7 – Temperature Trends Around the Entering Structure

The pictures I’ve seen of inflatable and extendible heat shield concepts seem to fall in the range of 2 to 3 for shield/capsule diameter ratio.  2.5 diameter ratio is about an area ratio near 6.  Factor 6 below typical capsule ballistic coefficients (near 300 kg/m²) would be about 50 kg/m².  Bigger diameter ratio may be too fragile to serve,  since I have not seen any concept images with ratios any bigger than about 3.

At a rather low ballistic coefficient of about 50 kg/m²,  assuming a dark and emissive surface,  we are looking at surface temperatures near 1100 C at stagnation,  near 790 C for attached-flow regions near the rim of the shield,  and near 500 C for all the surfaces immersed on the wake zone behind the heat shield.  If the surfaces are not highly-emissive,  those temperatures will be significantly higher yet!  That is what the plot indicates.

The table just below gives some typical “max service temperatures” for a variety of possible materials of construction.  It would seem that there are no flexible materials one could use to construct inflatable or extendible heat shields for Earth entry from low orbit,  which would not be damaged or destroyed by only one use. 

Carbon cloth might work,  but would suffer both serious oxidation damage,  and heat-induced embrittlement,  preventing any re-use.  It might actually suffer burn-through holes,  if too thin or too-lightweight a weave.

TV Commercial Content Is Out of Control!

As of Wednesday 9 April 2025,  I just timed the commercial content of a local 5:00 PM news broadcast as just about 10.5 minutes out of a 30 minute slot.  And I just timed the commercial content of a national broadcast news program as 10 minutes out of a 30 minute slot starting at 5:30 PM.  That corresponds to 36.7% commercial local,  and 33.3% commercial nationally. 

In both cases,  the majority of the commercial content was in the second half of the 30 minute time slot,  which really means during that second half of the news program,  there was actually more commercial time than there was news time!

These stations are supposedly licensed by the FCC to broadcast “in the public interest”.  Commercials in a news program are not in the public interest,  they clearly are in the profit interest of the advertisers.  It is only the news content that is actually in the public interest!  That is obvious even to the casual observer,  no matter how the lawyers might spin it!

In the 1950’s and 1960’s,  a typical hour-long prime-time program had 51 or 52 minutes of content,  and 8 or 9 minutes of commercials.  That corresponds to 13-15% commercials.  (Most programs were only a half-hour long back then,  but at about that same percentage commercials.)  You can time the DVD’s of those old programs for yourself! 

In the 1980’s,  this had changed to around 42 to 44 minutes of program,  and 16-18 minutes of commercials in an hour time slot in prime time.  That is 27-30% commercials,  about double the commercial percentage of 2+ decades earlier! 

Now,  4 decades since the 80’s,  it is even higher at 33-37%,  right there in prime time news broadcasts!  Outside prime time,  it appears to be even higher commercial content!  When flipping channels,  it often seems like there’s more commercials than content.

As a case in point,  on Thursday 4-10-2025,  I scrolled through all 36 channels that I can pick up with a rabbit-ears antenna,  between 2:10 and 2:15 PM.  Two of those are shopping channels (all commercials),  and one is a weather update (no commercials).  Those 3 don’t count.  The rest all show content with commercials.  As I scrolled through,  I counted 13 out of that 33 as showing commercials,  not content.  That’s an empirical estimate of 39.4% commercials in non-prime time!

So,  how does so much commercial time actually qualify as “broadcasting in the public interest”? 

Is there no law or regulation limiting this commercial content?  If not,  there should be! 

And I suspect there once upon a time there was at least a regulation,  which was quietly done away with,  long ago.  It was eliminated simply so that the advertisers could make more money!  Probably at the behest of the politicians those advertisers bought. 

What Trump Has Brought

I know that the great majority of my neighbors here in McLennan county voted for Donald Trump,  and for the other candidates of the Republican party that he dominates utterly,  in all the federal and state races.  I must wonder if you actually like what you have been getting.  I’m pretty sure it is not what you thought you were voting for.

The Economy

He promised you lower prices and prosperity.  What he has done with his tariffs inducing trade wars,  is to push what was a good economy to the brink of recession,  and start high inflation,  in less than 80 days.  This from the “expert businessman” who actually has 6 bankruptcies of record.  Look it up yourself!

The first symptom is the recent huge drop in the stock market.  Wall Street is not the economy,  but it is an early indicator,  as are the recent job losses and layoffs (besides the fired government workers,  which also makes job hunting more difficult for you).  The actual effect on the full economy lags by months.  The sharp rises in prices for just about everything due to tariffs will happen much faster,  but will persist well into the coming recession,  because of the supply disruptions the trade wars also cause.

You did notice the drops in your 401(k) accounts and investment portfolios,  did you not?  That’s just the beginning of this! 

Border Security

Trump promised both a closed border and mass deportations,  and he has begun doing exactly that.  But how he is going about this,  is the problem!  Adding more wall doesn’t help,  because all you need to get over a wall is a taller ladder.  You fix the immigration system instead,  which is a job Congress has failed to do,  since not long after the end of World War 2. 

It is the mass deportations without any due process that are the big problem!  Mass deportations of anyone not white,  regardless of whether they are legally here or not (or even a bona fide citizen)!  That is entirely illegal!  Not to mention unethical and totally immoral!

This is based on the demonstrably-false belief (and propaganda) that all immigrants are evil.  Trump’s minions that are in charge of ICE and the related agencies,  are betting that you will let them get away with these crimes,  because too many of you believed the propaganda lie about immigrants,  and so don’t care what happens to the deported. 

They are even defying court orders to return some of those they deported in error,  rather than trying to correct those wrongs.  “Commit any evil not to admit being wrong” seems to be the order of the day.

Social Safety Nets

Trump and the Republicans promised they would cut government (which they are doing),  but also that they would not damage your Social Security or Medicare.  Given the disparities between promise and action described above,  there is absolutely no reason to think your Social Security and Medicare are safe! 

They are in fact the largest portion of the budget:  the biggest,  fattest target!  The government workers who run them are already being fired.  So,  what do you think the ultimate result will really be?  It will start with the chaos of payments erratically not made to you,  or on your behalf!  Then it will get worse!

Operation of Your Federal Government

Trump and his minions (especially Musk) have been firing government workers at the FAA during a spate of airline crashes and near-misses.   They have been firing government workers at the IRS during tax season.   Just how does that serve you? 

Trump put a vaccine denier in charge of Health and Human Services when we are threatened with two possible pandemics (bird flu and measles),  and he and Trump have been firing heads of agencies that see to the safety of your foods and drugs. 

Funding has been cut for all kinds of medical and other scientific research.  That last includes criminal extortion:  universities are being forced to implement Trump-desired policies (many based on propaganda lies,  not fact),  or else lose their research funding.

This extends to pressuring the mainstream media outlets to say only positive things about him,  lest they get taken to court,  or in the case of PBS and NPR,  lose their federal funding.

Trump and his minions control the Executive Branch,  and they also control the Legislative Branch of your government because they utterly dominate the majority party in both houses of Congress.  They have demonstrable friends on the Supreme Court,  and several minions appointed to federal judgeships that will do their bidding instead of properly following the law.  The courts that still oppose them are now being openly defied. 

If you remember your civics classes,  consolidating all federal power into the Executive branch would be the very definition of dictatorship!  Now do you understand the “ways” that Trump spoke of vaguely,  in order to stay in office for an illegal third term (or more)?

You are about to lose your democracy!

Foreign Policy and Our Allies

Trump and his minions have already angered our NATO allies by apparently switching sides in the Ukraine-Russia war.  He so very clearly wants to give Putin a victory there,  when the Ukraine was actually NATO’s proxy army against Putin’s ambitions of European conquest.

Trump’s tariffs and trade wars only make that anger worse.  Our allies around the world,  not just NATO,  now consider us an unreliable ally at best,  and likely not an ally at all.  Damaging our alliances when we face four threatening adversaries (Russia,  China,  North Korea,  and Iran),  three of which are nuclear armed,  is doing our adversaries jobs for them. 

Doing your enemy’s work for him is very clearly “providing aid and comfort” to that enemy.  That is one of only two definitions of treason written into our Constitution.  Go read it for yourself!  It’s written in plain English,  not legalese.  You need no lawyer or judge to interpret it for you.

Treason is most certainly impeachable,  and it lies far outside any “Presidential immunity” intended by a Trump-friendly Supreme Court.

Protests

Here in the last few days,  there have been mass protests around the country against the policies of Trump and his minions.  There has even been a protest right here in Waco,  Texas!  Given the magnitude and severity of the problems described above,  why is that surprising?  Especially since Trump’s majority in the popular vote was quite slim nationally?  I hope these protests continue and grow,  despite the attempts to shackle the mainstream news media. 

Recommendations

I have two:  (1) join the protests if they continue (and I think they will),  and (2) contact your federal Senators and your House Representative. 

If you do contact them,  point out the ongoing crimes,  and demand they take action against them,  lest they be complicit in them. 

I already have.  3 times now,  without any observable effect so far.  So,  I will continue.

Update 4-8-2025:  A slightly-edited version of this article ran as a board of contributors opinion piece in the Waco Tribune-Herald today.  

About Nuclear Pulse Propulsion

Depending upon the detail method chosen,  this kind of revolutionary propulsion could be in flight test within 5 years,  and flying in its initial form in 10 years.  That would be the 1950’s fission technology.  The other versions might perform better,  but lack the materials,  and the necessary detonation or containment technologies,  that are required even to build test devices.  They still might not be ready to fly in 50 to 100 years,  if fusion is involved,  according to some experts.

This article is based upon a NASA paper and two Wikipedia writeups about pulse propulsion,  plus George Dyson’s book “Project Orion” about his father’s work on the Orion project at General Atomics in San Diego,  CA,  in the 1950’s and early 1960’s.  That last begins with company R&D work leading up to their first government contract. 

Between those four sources,  a pretty good picture of the propulsion is available,  particularly the technologically-ready fission charge version originally pursued in the 1950’s and early 1960’s.  For readers wishing to pursue this further,  those references are:

AIAA paper 2000-3856 “Nuclear Pulse Propulsion - Orion and Beyond”,  by G. R. Schmidt,  J. A. Bonometti,  and P. J. Morton,  then from the NASA Marshall Space Flight Center,  Huntsville,  AL.

Wikipedia article “Nuclear Pulse Propulsion”,  as retrieved 2-26-2025,  and last edited January 2025. 

Wikipedia article “Project Orion (nuclear propulsion),  as retrieved 2-26-2025,  and last edited February 2025.

George Dyson,  “Project Orion – the True Story of the Atomic Spaceship”,  published 2002 by Henry Holt and Company,  New York City.  (A second edition is forthcoming very soon,  if not already available.)

The basic external detonation concept is depicted in the Figure 1 sketch below.  All figures are at the end of this article. 

The idea is to explode a fission bomb at a safe distance behind the vehicle,  which vaporizes a reaction mass,  and blows that reaction mass into the pusher plate at the rear of the vehicle.  This requires a sort of “shaped charge” technology for the fission device,  in order to increase the amount of reaction mass intercepted by the pusher plate.  There are shock absorbers between the pusher plate and the rest of the vehicle,  to smooth-out the high-gee “hits” into a nearly-continuous and almost-steady acceleration. 

Figure 2 gives some indication of the effects of vehicle size on this process.  Bigger mass is inherently a larger pusher plate dimension,  which then intercepts a larger fraction of the plasma blast created by vaporizing the reaction mass.  The bigger mass also reduces the high-gee “hit” from the explosion,  making the shock absorber design easier.

At the time this technique was pursued in the 1950’s and early 1960’s,  it was thought that the main environmental concern would be the radiation fallout in the atmosphere from a surface launch.  Being fractional-kiloton (KT) devices,  leading to low-KT devices once out of the atmosphere,  there is less fallout than one might otherwise suspect,  more or less comparable to one atmospheric test of a low-range megaton (MT) thermonuclear device.  See Figure 3.

The risk of electromagnetic pulse (EMP) effects was not really recognized until after the “Starfish Prime” megaton-range nuclear test,  that was conducted in space in 1962.  Some data about that are given in Figure 4.  This really restricts where and how one might surface-launch such a vehicle.  These vary inversely with the square of the distance,  and directly with yield.  Bear in mind that we simply do not have the necessary technological capabilities yet,  to build such vehicles out in space,  so surface launch is still the only means available in the short term.

Based on the data in the four references,  a rough approximation to the performance values one might expect are given in Figure 5.  These are quite remarkably high,  sufficient for sending crews pretty much anywhere within the solar system,  on relatively short trips.  Some of the fusion concepts,  if they can really be made to work,  might even be suitable for interstellar missions. 

It is this author’s opinion that we need to get over our fears about “nuclear things in near-Earth space”,  modify the Space Treaty to allow this kind of propulsion,  and simply “get on with the war”.  However,  finding massive new fissionable material resources is inherent to support the quantities of fissionable material that will be necessary.  If not on Earth,  then “out there” somewhere. 

As a conceptual example,  the one-way velocity requirement to go from low Earth orbit to rendezvous with asteroid 2024YR4 is about 5.9 km/s as depicted in Figure 6.  Back to low Earth orbit,  about 4 years later,  the same velocity requirement can decelerate one back to low orbit,  excluding all rendezvous and course correction requirements.  If one wanted to send a large expedition to explore mining this asteroid (to include bringing significant mass home),  using a pulse propulsion vehicle already based in low orbit,  the rough sizing of such a vehicle’s weight statement might look like the numbers given in Figure 7.  The contrast with a chemically-powered vehicle is quite stark.  But with pulse propulsion,  the basic message is to build it big!

Figure 1 – The Basic Concept of Nuclear Pulse Propulsion As It Was Originally Pursued

Figure 2 – “Bigger Is Better” For Nuclear Pulse Propulsion As Pursued in the !950’s and 1960's

Figure 3 – About the Risks From Surface-Launching a Nuclear Pulse Propulsion Vehicle

Figure 4 – About the “Starfish Prime” Nuclear Test That Revealed the Risks of EMP

Figure 5 – Assessment of Achievable Performance Vs. Size With Fission Pulse Propulsion

Figure 6 – Getting to Asteroid 2024YR4 At Its 2028 Close Approach

Figure 7 – How a Large Round-Trip 4-Year Mission Might Be Mounted to Asteroid 2024YR4

Trump Denial Syndrome

I found this image on LinkedIn,  and thought it was too funny (and too true!!!) not to repost here.  I have seen people labeled as having "Trump Derangement Syndrome",  and I been the target of that myself.  Sometimes this was shortened to the acronym TDS.

But the real TDS is as listed in the illustration.  It is acquired by getting all your "information and news" from social media,  internet gathering-place websites,  and outfits like Fox News (infamous as right wing liars for some decades now).  

People come to believe lies that they hear constantly,  that has been known for centuries,  and underlies all propaganda operations.  

Rough/Soft-Field Space Landings

We have seen some tip-over failures among landers recently sent to the moon.  Looking at photos of these craft,  I see a pattern:  those that failed were too tall and narrow to reliably survive rough field landings in soft regolith on the moon.  There is a design criterion for that,  which worked well for Surveyor 1 and 3,  and the Apollo lunar landers,  1966-1972.  

There is much more to this than simple static stability,  per the classic high school physics problem!  There are the off-plumb angle effects of rough surfaces,  with both local slopes and obstructions.  There are the dynamic effects of nonzero vertical and horizontal speeds at the moment of touchdown.  There is also the issue of landing legs exerting too much pressure on the regolith and sinking-in.  That also leads to off-plumb angles,  as it will never occur symmetrically among the legs!   It also affects any subsequent takeoff,  as a sunken landing pad carries a load of regolith as extra weight,  and exerts a “tent stake” retardation.

The illustration shows all this empirical knowledge in one place.  Important are the minimum dimension s_min  across the polygon made by the centers of the landing pads,  and the height of the center of gravity h_cg.  The old criterion is shown,  which also helps address horizontal velocity limits at touchdown.  There is a lower limit on total pad area,  to avoid landing pads sinking into the regolith.  Tipping pads inward can help to “skitter-over” small rocks and protuberances.

The old criterion is more stringent than simple static stability on smooth level ground would indicate.  This is important because on the moon and Mars (and pretty much anywhere else with a solid surface),  the ground is neither smooth nor level.  There are plenty of obstructions,  and plenty of opportunities to dig a landing pad into the regolith,  and literally “trip” the craft into toppling over,  if it has any horizontal speed at touchdown.

For pressures exerted upon the regolith at touchdown,  there are empirical dynamic factors that get applied to the local weight,  which model the effects of small (but nonzero) vertical velocity a touchdown,  plus the possibility of one landing pad striking first.  These factors are only 1 at takeoff,  because the craft is not moving until it launches.

Overall Design Requirements

Details and Derivations

Basic static and dynamic stability is shown in the next illustration.  For the classic high school physics statics problem,  the weight vector “hanging” from the center of gravity must fall within the “footprint” defined by the landing pads,  or it will tip over.  Half the minimum footprint dimension,  and the center of gravity height,  lets one set up and solve a simple right triangle for the critical “out-of-plumb” angle that causes tip-over.  (For an odd number of legs,  go directly to the minimum half-dimension.)

If the effective local slope exceeds this critical angle,  the craft will topple!  Besides the general large-scale slope,  the small-scale roughness of the ground (or an obstruction under one landing pad) can make the local small-scale slope large indeed!  There is a table in the illustration of computed values from small critical angles at “tall-and-narrow”,  to very large critical angles at “short-and-squat”.  This large angle effect is exactly why the old design criterion of 1.5 < s_min/h_cg < 2 corresponds to “short-and-squat”.

Dynamic effects include both (1) horizontal speed at touchdown,  and (2) vertical speed at touchdown.  Nonzero horizontal speed brings with it the possibility of a landing pad on the leading side either striking a fixed obstruction,  or digging into the soft regolith and suddenly stopping.  Either way,  the retarding force is well below the center of gravity,  where the momentum vector is located,  thus creating an overturn torque.

Basically,  there is a radius from the center-of-gravity to the location of the blocked landing pad.  The craft rotates about the blockage point under the influence of the overturning torque,  which raises its center-of-gravity by the amount shown.  That radius gone vertical is the critical point:  any further and it topples over.  Thus,  conservation of energy says that if the horizontal kinetic energy exceeds the increase in potential energy,  it will topple over.

Static Stability and Dynamics Due to Horizontal Speed

Vertical speed at touchdown influences the transient pressure upon the regolith exerted by the pads.  If this transiently-higher-than-static pressure exceeds the failure bearing pressure of the regolith,  the pad sinks in!  Murphy’s Law says that will never,  ever happen evenly among the landing pads,  thus inherently acting to tip the vehicle out of plumb! 

Having a load of regolith atop a pad (or pads) is only an issue if the craft must later take off,  but bear in mind that Murphy’s Law also says those same regolith weights will be unevenly distributed among those pads.  That leads to attitude control problems,  in addition to the effects of simply being overweight at takeoff.

For low but non-zero impact speeds,  the usual empirical estimation practice in civil engineering is to double the static weight of the object,  as a measure of the transient force.  Add to that the strong likelihood of one pad striking first on rough ground,  and one should probably double that static weight again.  This is shown in the following illustration.

The average local static weight divided the total pad area is the static bearing pressure exerted upon the regolith below the pads.  “Factor 4 higher” (two factors of 2) would then be a good ballpark estimate of the transient pressure exerted underneath the pad that strikes first.  To avoid sinking in,  this transiently-exerted pressure must be less than the failure bearing pressure for that regolith. 

Problem:  we have little or no test data for the actual failure bearing pressure,  for any surfaces off of Earth. 

However,  we have observations from the moon and Mars that the regoliths there resemble the “soft fine dry sand” in Earthly sand dunes,  as long as any rocks embedded in that regolith do not touch each other.  Without touching,  those rocks simply cannot reinforce the strength of that regolith.  Essentially all of the surfaces of the moon,  and the vast majority of Mars,  meet that “soft fine dry sand” description,  as near as we know so far.

There are published strengths for various kinds of Earthly soils and surfaces,  used in foundation design.  These take the form of allowable pressures to support very long-term exposures,  so as to prevent soil settling under the foundations.  Those allowable pressures are usually about factor-2 lower than the actual failure pressure,  sometimes factor 2.5. 

These observations and surrogate data support the criteria and values given in the illustration.  These are ballpark-correct,  and should suffice well,  until better local information becomes available.  

Dynamic Effects Due to Vertical Speed

Observations and Recommendations:

The recent “short-and-squat” Firefly Aerospace lunar lander is upright and operating.  The two Intuitive Machines lunar landers were both “tall-and-narrow”,  and both ended up on their sides,  unable to function properly,  if at all.  See the pattern?  One has to wonder if the Intuitive Machines people actually knew about the old Apollo-era “short-and-squat” criterion.  That being from so long ago,  they may not have known.

Variants of SpaceX’s “Starship” vehicle are being seriously proposed as landers upon unprepared surfaces for both the moon and Mars.  That vehicle is inherently tall-and-narrow,  much like the “Falcon” booster cores they routinely fly back and recover.  However,  those “Falcon” cores have never landed upon anything but a flat, smooth,  reinforced-concrete landing pad,  or a flat,  smooth,  hard steel deck!  Those legs simply won’t work in rough,  soft dirt!  SpaceX has no experiences at all yet,  with rough-field and soft-field landings.  So far,  their projected “Starship” designs for the moon and Mars reflect that lack.

If these or any other companies want to land craft on planetary body surfaces off of Earth,  then they need to use the design criteria that I posted here.  This is old-time pencil-and-paper engineering stuff,  originally from the slide rule days.  You will not likely find this incorporated into most computer codes,  simply because we did not do it that way back then,  and the code writers may have been too young to know about any of this. 

I would be happy to consult about these issues.  Please contact me if you need help.  

Update 3-14-2025:  I found some dimensions for the Intuitive Machines "Athena" lander (over 4.5 m tall and only 1.57 m wide),  and also some information that the altimeter failed on both "Athena" and the first lander "Odysseus" (of similar dimensions).  The altimeter failure is why both machines touched down with very significant vertical and horizontal velocities!  How does it zero the speed at touchdown if it does not know its altitude?  It cannot!

For those dimensions and using a side view photo of an Athena mockup,  I was able to crudely approximate center of gravity height hcg as about 2.6 m,  and the minimum footprint "diameter" of a 6-leg hexagon as about 1.4 m.  Thus the min-s/hcg ratio is only about 0.5,  with a critical static tip-over angle of only about 14 degrees.  

I consider that angle just too low for a heavily cratered region on the moon.  The craft could touch down successfully at zero speeds,  and still tip over,  just because the local slope  near a crater rim was higher than 14 degrees!  Something quite common on the moon,  as we already know from Apollo.

Going further,  the radius from cg to the 2-pad leading contact point is about 2.7 m,  which at the critical 14 degree tip angle angle (radius gone vertical) says the cg height increase at tip-over is only about 0.09 m.  Using lunar gravity and that small height increase,  conservation of energy says the max survivable horizontal touchdown velocity is only about 0.5 m/s!  

Which more than likely is a limit that both machines failed,  and by a very large amount,  trying to land without an accurate altitude reading!  

But even with an altitude reading by which to nearly zero the vertical and horizontal speeds at touchdown,  the small critical static tip-over angle of 14 degrees may well be insufficient in lunar cratered terrain,  where local slopes can be much higher near crater rims.

"Short-and-squat" is just a more reliable lunar lander approach!  The numbers say so.  So does the prior experience decades ago.  You cannot argue with physics!  People are entitled to their opinions,  but no one is entitled to their own facts!

Upper-Stage Starship Failure, Test 8

The video on the SpaceX website is disabled to black.  I cannot watch it.

As for the two upper stage “Starship” failures in a row,  bear in mind that most flight vehicle failures in flight test are due to multiple things acting together.  Having only a single cause is unlikely in the extreme.

That being said,  after flight 7,  they made some changes to the vehicle for flight 8, which apparently  did not work.  Which strongly suggests that the real causes (plural!) were not the leaks in engine plumbing that they assumed after flight 7.

There was a significant vehicle design change after flight 6.  The vehicles for flights 7 and 8 were longer,  with larger propellant tanks.  Which raises the specter of some sort of slosh or other mode in the propellant tanks,  causing the excessive vibration,  and perhaps causing fatal leaks in weld joints of the lower tank aft bulkhead.  The larger propellant masses involved would likely amplify any such effect.  This is only speculation,  but it is a real possibility that they need to explore.

If it were me,  I'd re-fly the older Starship upper stage design on flight 9,  with the smaller tank volumes and shorter length.  If that design makes the ascent successfully,  when the “improved” design did not,  twice in a row,  that would pretty well “nail it” to the dynamics of the larger tanks vs smaller tanks.

If it's liquid-sloshing in the tanks that really is the source,  then change the baffles.  The tank structures in and adjacent to the aft bulkheads may need reinforcement to better resist the unanticipated loads.  They need to instrument these locations for possible effects,  probably to include in-tank camera views,  and some strain gages and pressure sensors.

That's not to say there might not also be failures in the engine plumbing,  too!  But whatever is going on there, is apparently being overwhelmed by something else they have not identified yet.

This sort of thing happens often in experimental flight test.  It shows up more frequently when you make too many changes to the vehicle between tests,  too early in the program.  In this particular case,  it may also trace to believing too strongly in computer code outputs,  by engineers who cannot detect a garbage-in/garbage-out problem,  because they have not done enough (or cannot do) “old-timey” pencil-and-paper design analysis. 

Same day update:  there was some sort of pogo-mode excessive low-frequency vibration that afflicted the first 2 Saturn-5 flights.  I do not remember what the cause and cure for this were,  but it did not afflict the next test flight,  which was manned.  That was Apollo 8 around the moon December 1968. 

Update 3-13-2025:  I looked up the history of the Saturn-5 and POGO oscillations.  The unmanned test flight of Apollo-6 had severe POGO in the first stage,  doing enough damage to the second and third stages as to compromise the mission entirely.  Had it been manned,  this likely would have been an abort.  

There was severe-enough POGO in the Apollo-13 second stage as to cause a premature engine shutdown,  compensated by longer burns of the remaining 4 second stage engines,  and of the third stage engine.  These failures were overshadowed by the oxygen tank explosion later in the mission that led to the crippled craft aborting home without entering lunar orbit and doing the landing.  Man-rating the Titan-2 booster for Gemini was delayed because of POGO instabilities.  This was a rather common problem in a lot of rockets.

I will add this:  detecting this correctly requires the right instrumentation,  and people who know how to use it.  In the tactical solids I worked on,  combustion noise has frequencies of a few hundred Hertz,  of amplitude about 1-5% of the basic pressure level.  If it got to 10% or more of the pressure level,  we considered it "combustion instability" to be remedied.  Usually there was a discernible higher-amplitude instability signal at a frequency we could determine,  standing out from the lower-amplitude combustion noise at a "broad hash" of frequencies,  up to something approaching a significant fraction of a  MegaHertz.  

To see this behavior in the data,  you simply cannot use modern digital data acquisition!  That "pixelates" the signal too much to see what is going on.  Nobody wants to pay for digital systems capable of resolving those high frequencies.  You must use an analog tape recorder that uses FM data processing,  with a response of about 1 MegaHertz or higher.  You play that data back and display it in multiple plot formats until you can see what you need to see.  There are few today who actually have equipment like that anymore,  much less who know how to use it to diagnose combustion instabilities.  And yet,  POGO instability is said to result from combustion instability interacting with any or most of an abundance of longitudinal modes in assorted tanks and plumbing.

I had long experience with data from equipment like that,  diagnosing these kinds of phenomena.  While too old to be seeking regular employment,  I would be happy to consult in this area.  Please contact me if you need help. 

Update 4-8-2025:  I now think there may have been at least two problems unaddressed on Starship flights 7 and 8.  One is the POGO mode of thrust oscillations interacting with stage structural modes and perhaps organ pipe modes in the feed plumbing.  I still feel those thrust oscillations,  particularly in Raptor tests on the tower stand in McGregor.  You cannot hear them at only 5-10 Hz,  which is below the range of human hearing.  

The other is a radial or lateral slosh mode of the propellants in the tanks,  which at very low levels remaining (essentially down in the aft tank head end)  could possibly uncover the suction drains.  That last may have actually happened on at least one of the early Starship-only flight tests,  where one engine suffered a green flame during a landing burn at low propellant-remaining conditions,  indicative of burning the copper alloys in its construction.  Starved of methane by a suction problem,  that could easily happen,  putting more-or-less straight oxygen onto hot metal.

Separately,  I have since seen the videos from Flight test 8.  Seeing them changes none of my conclusions or suggestions. 

Dictatorship and Treason!

Update 3-05-2025:  Be aware as you read this,  that I see a different pattern than most of you out there.  That is because I look only at what the politicians and associated figures actually do,  I do not listen much at all to what they say. 

There is a great deal of truth to the old saw “you can always tell when a politician is lying:  his lips are moving”.   That is particularly true of Trump and his minions and sycophants.  The vast majority of everything Trump has ever said has proven untrue,  and his minions merely parrot him.

So I very strongly recommend that you look at what they do,  not what they say!  THAT track record of actions is what this article is all about.  And Trump’s speech before Congress does not change one single claim or conclusion in my article. 

------------------     

You are watching a slow-motion coup taking place to replace our democracy with a Trump dictatorship that cares nothing for the American people, just his billionaire cronies and giant corporate suck-ups. You are also watching Trump’s slow-motion treason right there in public for all to see, handing the Ukraine to Putin, after Putin could not conquer it in 3 years of war.

Coup to establish dictatorship

Under our Constitution, the three branches of government (the Executive, Congress, and the Judiciary) are supposed to hold each other in check.

Congress has been effectively dysfunctional for some decades now, unable even to pass a budget as required by the Constitution. Instead, we have seen only continuing resolution after continuing resolution, year after year, in violation of the law of the land. This comes from party power out-prioritizing the people’s business, in violation of their oaths of office (that violation ought to be a prosecutable crime), and Trump’s party now controls the vote in Congress.

In recent years, several courts have been packed with right-wing idealogues and Trump minions who will do what Trump says instead of following the law. The most egregious example is Trump-appointed federal judge Aileen Cannon in Florida, who gave Trump’s lawyers every delay and anything else they asked for, ultimately dismissing-entirely the classified documents case against him. This totally-egregious case would have been a slam-dunk conviction for any ordinary citizen! 

She’s not the only example: with the collusion of Senate Republicans, Trump packed the Supreme Court with three more “arch-conservative” justices, which in concert with the other “arch-conservatives” then ruled he is immune from prosecution for any “official act” as president! That decision was in violation of over two centuries of federal law and precedent. “Official act” is up to interpretation, and they who committed this travesty are the ones who will interpret what is an “official act”.

Since the beginning of our Republic, our only safeguards against a President trying to be a dictator were Congress and the Courts. With both of them rendered ineffective, who’s to stop him? A military counter-coup? Trump’s been cashiering all the high-ranking military officers who don’t rabidly support him. Now you know why!

Trump has been packing the Executive branch high offices with minions who will do his bidding instead of what is legal, or with choices who are so incompetent and unqualified as to render their agencies completely dysfunctional. That dysfunctionality is by design: a dysfunctional government looks bad compared to the strongman dictatorship Trump offers! The rest (minions who will do his bidding, even illegally) is how he will complete the consolidation of all federal power into the Executive branch, which is by definition a dictatorship!

The most egregious example of minions doing Trump’s bidding, even if illegal, is Elon Musk and the supposed Department of Government Efficiency (DOGE) that he commands. Musk was not elected by anyone, and he was not confirmed by the Senate. His supposed department was NOT created by Congress! All in violation of the Constitution!

Most of what Musk and his DOGE have done firing federal workers is illegal under the Constitution and two centuries of federal law. Yet NO ONE among the majority Republicans in Congress is stepping up to oppose this travesty! And only those courts not yet packed will oppose that.

Musk has revealed himself in public to have exactly the same bad character as Donald Trump: he is an arrogant, egotistical, conscience-less narcissist, bent only on increasing his own power and wealth by any means legal or illegal, and caring nothing for the American people, who will be severely damaged by all the resulting chaos. 

The treason issue

Vladimir Putin is the absolute dictator of Russia, make no mistake about that! His publicly-known goal is to reconstitute the old Soviet empire as the new Russian empire, and maybe more. He wants to dominate as much of Europe as he possibly can, by intimidation and invasion. That makes him the enemy of every country in NATO, make no mistake about that, either!

If he succeeds in Ukraine, he will next attack former Soviet countries that are now part of NATO. Which would trigger World War 3 in Europe if NATO were still intact! And Xi in China will invade Taiwan if Putin succeeds in Ukraine. That starts World War 3 in the Pacific!

Ukraine is not a member of NATO, nor has it received any official help from NATO, but it has been doing NATO’s job resisting the conquests of Putin for 3 years now! It has been doing this with direct help from the countries that make up NATO, including the US until now. And make no mistake, it is Putin who annexed the Crimea in 2014, and who invaded the rest of Ukraine 3 years ago. Putin’s forces have been committing war crimes against civilians in Ukraine ever since. Putin’s Russia is the enemy of us all, make NO mistake about that!

Meanwhile, Trump has been doing everything he can to undo the NATO alliance and our participation in that treaty. He has offended and insulted all of our allies. That is doing Putin’s job for him! Breaking apart NATO is something Putin has been unable to do for himself, and was what the old Soviet Union was unable to do during the decades of the Cold War.

Trump has suspended our aid to Ukraine, and staged a hostile confrontation with that White House meeting, as a set-up to make Zelenskyy out to be the bad guy. He did this to distract attention from the fact that he wants to give Ukraine to Putin as soon as he can. There was never any intention on Trump’s part to sign any deal for US aid, any acquisition of mineral resources notwithstanding.

Trump has also suspended all the economic sanctions on Putin’s Russia, so that their economy won’t crash so badly, and they might better win despite Ukraine still receiving aid from European countries. By handing victory over Ukraine to Putin, Trump is so very clearly doing Putin’s job for him, something Putin has been unable to do for himself in 3 years of war there.

And in the news today, Pete Hegseth, the Trump-appointed and Senate-confirmed Secretary of Defense, has suspended all offensive cyber actions against Russia, despite them increasing theirs against us! Defeating the US cyber offensive is another job Putin wants done. Trump and Hegseth have just done it for him!

Doing Putin’s job for him is (by definition) providing aid to an enemy. Which inherently also provides comfort to that enemy. We’ve already seen Russian officials celebrating Trump’s actions regarding Ukraine and NATO.

“Providing aid and comfort to the enemy” is one of only two definitions of treason, written directly into our Constitution. That document also provides the standard for conviction: two testifying witnesses to the actual act.

Millions have seen all of this happening before their very eyes on live television! That satisfies the standard for conviction. And yet I still see nobody using that word “treason” in public about the actions of Trump and his minions, despite how accurately and succinctly it characterizes them!

Final comments

Knowingly or not, the American people seem to have elected their very first dictator, who is already well along his way in creating that dictatorship! And your “elected dictator” is actively committing treason right in front of your very eyes! Who’s to stop him? A Congress dominated by the party he controls? The Department of Justice that he has packed with minions and weaponized? A military from which he is cashiering all the top officers who might oppose him?

All I can offer is that individually we are powerless, but together we might still have great power. Rise up and put a stop to this evil! Please!

Start by contacting your Representatives and Senators and let them know that (1) you see the dictatorship and the treason happening, and that (2) they are complicit in those crimes if they do not act to stop them.

I already did.

An overwhelming flood of such contacts might induce the non-minion Republicans to stand with the Democrats and try to stop this with impeachment.

Second Update 3-05-2025:

These are headlines for stories on the NBC News website as of 3-4-2025.  They prove exactly my point about Trump doing Putin’s job by damaging NATO. ---

From his perspective, Putin is now winning the yearslong struggle with the U.S.

The spat between Trump and Zelenskyy has dominated Russian TV coverage and a Kremlin spokesman said the West has begun to "lose its unity."

---

Trump's pause on Ukraine aid is like the U.S. switching sides in WWII, expert says

 

My take is that this just helps to further prove conclusively that Trump and his administration are working to destroy NATO while doing everything they can to help Putin,  including throwing victory in Ukraine to him.  Hurting your friends and helping the enemy is the VERY DEFINITION of TREASON,  of the “aid and comfort” type.  No one can argue with the facts of those actions,  and no words can obscure or defend those actions we have all seen. 

For me,  the remaining unexplained action out there is why no one is using that word “treason”,  when it is so obviously taking place in full view of everyone!  

Wake up!  Do something about it!  Act,  before you don’t have a free country anymore!

Spin Gravity Requirements

Artificial gravity is now known to be needed for long-duration space travel, because (1) decades of experience in orbiting space stations has shown that there are limits to the effectiveness of our countermeasures to microgravity disease, (2) there are many recently-discovered but initially-unanticipated ill effects with no countermeasures yet available at all, and (3) the sum of all that experience points to a time limit of about 400 days for exposure to weightlessness, and still have a decent chance to recover acceptably. That trend is in the wrong direction for planning to use long-duration weightless spaceflight. This is listed in Table 1.

 

There is only one set of physics that we have available for the production of artificial gravity: centripetal acceleration a_centr (that some call centrifugal force), produced by spinning around some center at some spin radius R_spin, and at some spin rate N. Those 3 variables are related as shown in Figure 1 below. For purposes of units conversion, be aware that there are 3.2808333 feet per meter. Everything else anyone might want to know is already in the figure.

The very first question that comes to mind is “how much artificial gravity is enough?”, meaning what rim gee do we need to supply? The right answer is very unclear, since humans have never ever experimented with partial gee in rotating space stations in Earth orbit. We only have experience with 1 full gee down here on Earth, and at 0 gee (weightlessness) in orbit. There is nothing, no experiences at all, in between those two extremes! 

This is further complicated by time limitations, and by short-duration exposures of crews to high mission gees for the transients of rocket braking, entry deceleration, and so forth. We went into our roughly 5 decades of 0-gee orbiting space station experiences, expecting to find muscle atrophy and bone density loss, and looking for countermeasures to both problems. 

Figure 1 – Using Spin for Artificial Gravity

We found a lot more adverse effects than initially expected, and have found no countermeasures yet for most of those. The countermeasures for muscle atrophy and bone density loss are exercises and drugs as expected, but we have found definite time limitations, even for those:  about 400 days max exposure time to weightlessness. And those experiences are limited further to entry aerobraking gees in the 3 to 4 gee range, returning to Earth from low orbit. 

The only other actual-experience data we have at all, are from the Apollo lunar missions. Those crews were fully Earth-fit at launch, and exposed to weightlessness for no more than about 2 weeks during their missions. They experienced very near 11 gees during entry braking, in those free returns from the moon. They did just fine, enduring those gee levels at that level of fitness. 

The net result is that for partial-gee spin gravity, be it lunar at 0.165 gee, or Martian at 0.382 gee, we must observe the same limitations as for weightlessness exposures, meaning no more than 4 gee transient exposures, after any long exposures to low gravity. There is no data to support any other conclusion! Further, the total mission duration must match the weightless max mission duration limit of about 400 days, again because there are no data to support any other conclusion. 

Violate either limit, and we must supply about 1 full gee’s worth of spin gravity, in order to be fully Earth-fit, and so able to take a transient of 11+ gees during the mission. Further, interruptions in artificial gravity likely should not exceed the 2 weeks demonstrated during Apollo. This is summarized in Figure 2.  

Figure 2 – What Artificial Gee Level Do We Need?

There are only 3 variables interacting to provide artificial gravity: rim gee, spin radius, and spin rate. Mathematically, any two determine the third, per the first figure. 

The limits on supplied gee were just discussed above: either weightlessness for no more than 400 days and exposures to no more than 4 transient gees, or else full 1 gee artificial gravity, able to endure 11+ gees transient exposure, and likely limited to no more than 2-week-long interruptions with weightlessness. 

The limitations on spin rate and spin radius are shown in Figure 3, to the extent that any of this is known, and supported by any actual experiences or data. There is a long-standing but anecdotal-in-nature perception that 3 to 5 rpm, or really maybe only 3 to 4 rpm, is tolerable for essentially steady-state exposures to spin rate. The 5 rpm figure, coupled with partial gee, is the genesis of the centrifuge design depicted in the 1968 movie “2001,  A Space Odyssey”. None of this is supported by hard test data, though. Those tests were never run because of the presumption that the countermeasures for weightlessness would allow weightless long-term space travel. That has proven to be an error, as this article indicates.

Much more recently, NASA has funded some efforts at academic institutions to investigate some aspects of the limitations to spin gravity. The most notable of these (Reference 1) determined that with sufficient individualized acclimatization training, many persons could tolerate spin rates up to perhaps 20 rpm, in terms of the “cross-Coriolis” effect. That is the sudden-onset (and often severe) nausea induced by sudden head movements out of the spin plane. This was work done by the University of Colorado Boulder in cooperation with Arizona State University, and funded by NASA.

There are other possible effects, and those have yet to be tested. One possibly serious effect is the gee gradient along the spin radius,  which induces a significant difference in gees as felt at the head, versus the “rim gees” felt at the toes, while standing. This is also indicated in the figure. There are two serious effects to be anticipated from the gee gradient: (1) blood pooling in the legs which could lead to fainting if a so-called “gee suit” is not worn, and (2) long term weakening of the heart or vascular system, from the heart not having to work so hard, pumping blood back up from the feet to the heart at lower average gee. These are unexplored risks at this time. 

There is also a sort of practical lower limit on spin radius. The numbers quickly get entirely ridiculous if the spin radius does not exceed about 2 or 3 man-heights. The more-or-less-average height of a standing human is about 1.65 m (65 inches, or about 5 feet 5 inches). That puts the practical geometric minimum spin radius (exclusive of health effects) at around 3 to 5 meters.

The upper limit on spin radius depends entirely upon what might be practical to build. That is an entirely separate topic, not covered here. Just be aware that bigger is always more expensive.

As for higher spin rates,  that depends upon how long a training and acclimatization interval one can afford, but there is still an ultimate limit, of about 20 rpm after about 40-50 days of training, as depicted by the plot in Figure 4 below,  obtained from Ref.1. For lower spin rates, those training intervals are shorter, as depicted, but there was still training needed at 5 rpm. 

Figure 3 – What Are the Limits on Spin Rate and Spin Radius?

However, it would be wise to use increased intervals and decreased spin rates, versus those shown in the figure, as the same reference indicated a large confidence interval, meaning a lot of scatter in the data. Even so, there were no spin rates higher than 20 rpm that proved trainable at all. Anecdotally, something like 3 or 4 rpm may need no training at all, but that is below the range of rpm that was tested, in the cross-Coriolis study.

Figure 4 – There Are Actual Experiments for the Cross-Coriolis Effects

Conclusions

There are known, and still unknown, limits on the spin gravity design problem. What we know, or can surmise, follows. Each of the 3 variables needs to fall within the appropriate limits, or else the design must be deemed infeasible. These are also briefly summarized in Table 2 below.

Level of gee supplied

Until we actually know better, the gee level to be supplied can be either 1 full gee, or a lower value, including zero gee. At present, there is nothing known in-between those limits, so that partial gee cannot be supposed as any different from weightlessness, in order to take a conservative approach with respect to health risks. Zero or any level of partial gee is OK, if (1) the transient gee exposure does not exceed about 4 gees, and (2) the low-gee exposure does not exceed about 400 days. Note the “both-and” coupling of the two limits! 

But if the transient gee exposure does exceed 4 gees or the mission exposure time exceeds about 400 days, the only currently-supportable choice is supplying near 1 full gee, with a demonstrated capability of resisting transient 11 gee exposures. There is also a relevant-but-different time limit: no more than about a 2-week zero-gee transient interruption to the near-1-full-gee artificial gravity. That was demonstrated during Apollo, but may actually be longer. We just do not know. Note the “either-or” coupling of these limits. That reflects the necessary conservatism, relative to the “both-and” coupling of the limits to the low gee case.

               Spin rate limits

It is known from Ref. 1 that for the cross-Coriolis problem, training can raise the max spin rate tolerable, to (at the very most) about 20 rpm, with something like at least 40-50 days training. There is enough scatter in their data to justify reducing this to about 15 rpm with an increase to about 60 days’ training. Lower spin rates require shorter training, down to about 5 rpm with very little training. However, this is the one and only effect so far evaluated, and it was tested in short-term centrifuge tests. Therefore, this limit is probably subject to future revision. Further, the training had to be very individualized: there is no general rule-of-thumb to use! That is likely to be expensive training!

Longer term, there may be other limitations, we just do not know “for sure”. Anecdotally, the no-training threshold may be nearer 3-to-4 rpm (since 5 rpm needed training), for very long-term (essentially steady-state) rotation rate exposures. No one knows for sure, as the experiments have yet to be done. 

About 4 rpm max (or maybe a more conservative 3 rpm) is recommended by this author for the no-training threshold, until we know better. There is no minimum spin rate limit that we know about, or can imagine.

               Spin radius limits

These are still unexplored experimentally, as best this author can tell. It shows up in the radial gee gradient, which causes head-to-toe gee differences, which could then cause any of a variety of health effects. They could show up as blood pooling in the legs, or in weakening of the heart from having to work so much less pumping blood up from the toes back to the heart,  at lower average gee. No one yet knows for sure. 

From a practical geometry standpoint, spin radii probably ought to be at least 2 to 3 times the average height of a standing person, or about 3 to 5 meters minimum. The unexplored health effects may well increase that. Nobody yet knows for sure. A wild guess says keep the head-to-toe gee difference under about 0.1 to 0.2 gees. That limits the gee gradient down the spin radius to something around 0.1 gees per meter of spin radius maximum.

The maximum spin radius is determined by the practicality and expense of what can be built, more than anything else, so far as we know. Those are not health risk issues. Accommodating large spin radii in smaller vehicle designs will be rather challenging to say the least. That much is certain.

Final remarks

For manned interplanetary voyages, there are many vehicle design requirements that are also necessary for unmanned missions. These include protection from microgravity diseases (the topic here), protection from radiation exposures, protection from excessive heat and cold, and protection from meteoroid impacts. Those last two also apply to unmanned vehicle designs. None of those others (besides spin gravity) are addressed in this article. 

However, spin gravity and some or all of those other protections, are integral to the design of interplanetary vehicles in general, as a part of the larger topic of mission and vehicle design approaches to make interplanetary travel less difficult and dangerous. The vehicle designer must worry about all these things.

One part of that is the reduction of the departure velocity requirement from low Earth orbit by means of a reusable space tug-assist, the subject of Refs. 2 and 3. Another part of that is the application of the lessons of history regarding getting from early exploration to being truly ready to plant permanent settlements. That is the topic of Ref. 4. These things all go together, but taken all at once, the article would be too big to be posted, or to be presented as a paper. That does suggest a book, and not a small one. 

References:

#1. Bretl and Clark, “Improved Feasibility of Astronaut Short-Radius Artificial Gravity Through a 50-Day Incremental, Personalized, Vestibular Acclimation Protocol”, a paper published in 2020 as open-access by Nature Partner Journals, in npj/microgravity as npj Microgravity (2020) 6:22 ; located at https://doi.org/10.1038/s41526-020-00112-w; work done by University of Colorado Boulder, in cooperation with Arizona State University Biodesign Institute, and supported by NASA.

#2. G. W. Johnson, “Tug-Assisted Arrivals and Departures”, posted 1 December 2024, at http://exrocketman.blogspot.com. 

#3. G. W. Johnson, “SpaceX’s Starship As a Space Tug”, posted 2 January 2025, at http://exrocketman.blogspot.com (update added for evaluating Centaur stages as tugs).

#4. G. W. Johnson, “Exploring Mars Is Not Settling Mars”, posted 1 February 2025, at http://exrocketman.blogspot.com.

For references that are articles posted on this site, all you need in order to find them without scrolling down (the hard way) is to jot down the date of posting and the title. Then use the archive tool, left side of page. Click on the year, then the month, then the title if need be (such as if there was more than one posting that month). Anything posted here is freely available by simple copy-and-paste. 

There are lists sorted by topic of many of my technical articles posted on the “exrocketman” site. Those were posted 21 October 2021 in an article title titled “Lists of Some Articles By Topic Area”. I try to keep it updated, but the very latest articles may not yet be added to it. This includes in the radiation risk topic list my best take on NASA’s own radiation protection data and (older) exposure limits, with 5 October 2018 “Space Radiation Risks: GCR vs SFE”, and 2 May 2012 “Space Radiation Risks”. Both reference the same NASA site, and identify it.