Space Suit and Habitat Atmospheres

See also updates 11-20-14 and 11-26-14 and 1-1-15 at end.

Spacesuits are usually designed for pure oxygen “atmospheres“ supplied as breathing gas.  This is especially important to full pressure suit designs,  as lower pressures can be used with pure oxygen,  which eases the joint restriction design problem.  However,  long-term breathing of pure oxygen is detrimental to human health.  Plus,  it is a very serious fire hazard:  flammability and explosiveness are greatly amplified in pure oxygen at full atmospheric pressure. 

All of that taken together means that the habitation for astronauts (be it a ship,  a space station,  or a base on another planet) needs a suitable diluent gas.  Such atmospheres are thus inherently two (or more) gas systems.  We evolved in an atmosphere of 20.946 volume percent oxygen,  about 78% nitrogen,  and around 1% argon,  plus some other trace gases. 

Although sea level pressure is “the standard”,  there are many human communities living at altitudes where pressures are substantially lower.  Many large populations live near 10,000 feet elevation,  some near 15,000 feet,  and a very few approaching 20,000 feet.  Air pressure,  and oxygen partial pressure,  are both substantially lower at altitude. 

The pilots of aircraft are required to use supplemental oxygen when flying above certain altitudes.  The exact standards vary,  but the US Navy uses 5000 feet as the altitude above which oxygen must be used.  Most other authorities use 10,000 feet as the “trigger” for mandating oxygen.  Below those altitudes,  pilots seem to be fully functional.  Therefore,  atmospheric oxygen concentrations resembling those in the 5000-10,000 feet altitude range make pretty good criteria for the minimum life support requirements of any space suit design.  See also Figure 1.  These data were also used in “Fundamental Design Criteria for Alternative Spacesuit Approaches”,  posted this site 1-21-11.  

 Figure 1 -- Life Support Experiences

Suit Criteria

The Earth’s atmosphere has very variable humidity.  Dry air composition is 20.946 volume percent oxygen.  Water vapor acts to “displace” dry air,  so there is a slightly-lower oxygen partial pressure to breathe on very humid days versus very dry days,  at any altitude.  At human body temperature,  the steam tables show the partial pressure of saturated water vapor to be just about 47.06 mm mercury. 

Respiration depends upon the partial pressure of oxygen in the air we breathe,  not the volume percent.  This is because so many biochemical processes are driven by osmotic pressure differences. 

These two effects (water vapor displacement and partial pressure-driven osmosis) taken together say that the actual oxygen requirement,  when designing space suit and habitat atmospheres,  is the partial pressure of oxygen in the near-equilibrium saturated environment of the lungs.  This same water vapor displacement effect must be taken into account in selecting the pressure of the pure dry oxygen in the suit.  (There are other displacement effects,  but all are far less significant than this water vapor effect.)

The other factor to consider is leakage from the suit.  Suit pressures can be increased to compensate for losses that might (or might not) occur.  The “usual” factor seems to be 1.10 for leakage effects.  The suit oxygen pressure selection approach is shown in Figure 2,  Figure 3 presents the actual data.  This applies to any type of suit,  not just the full pressure suits that have been standard. 

Suit designs that supply pure oxygen at compression pressures between 147 mm mercury (10,000 feet) and 167 mm mercury (5000 feet) would thus seem to be quite adequate.  196 mm mercury corresponds to full sea level oxygen.  Those figures include the water vapor displacement effect,  but not the increase for leak compensation.  Using a factor of 1.10 as leak compensation,  the design suit pressures become 216 mm mercury for sea level-based,  187 mm mercury for 5000 ft-based,  and 161 mm mercury for 10,000 foot-based designs.  

Figure 2 – Selecting Pressure in a Pure Oxygen Suit

 Figure 3 – Pressures in a Pure Oxygen Suit of Any Type.

Getting from Suit to Habitat Atmosphere

Since we evolved to breathe them,  oxygen,  nitrogen,  and argon make pretty good choices for the gases in the atmosphere of any astronaut habitat.  This is important because interplanetary missions will require living in such habitats for years at a time.  Oxygen-nitrogen is a basic two-gas system.  We might add substantial argon as an additional diluent for reducing the risk of fire. 

The problem going from a two-gas system in the habitat to pure oxygen in the suit is a requirement to “blow off” the nitrogen (and any third diluent gas) dissolved in the blood,  without risking “the bends”.  This is “pre-breathe” time,  and it is quite significant with the atmospheres used in the ISS and associated spacesuits.  There is a tested criterion for this:  if habitat partial pressure of nitrogen ≤ 1.20 suit pure oxygen pressure,  then no pre-breathe is needed. 

So,  once a design suit pressure has been selected,  this factor of 1.2 should be used to determine the maximum partial pressure of nitrogen in the habitat atmosphere.  Suit and habitat share the same in-lung wet partial pressure of oxygen.  The in-lung wet and habitat dry atmospheres share the same total pressure.  If this a multiple-gas atmosphere in the habitat,  then we know its total pressure as the sum of the wet O₂ and N₂ (and Argon,  if any) partial pressures,  and can figure the volume percentage of oxygen in the dry mix as its partial pressure divided by the total.  That process logic is a part of Figure 4,  with the numbers as a function of suit pressure in Figure 5. 

As a two-gas system,  all the suit pressure designs under these criteria share the same habitat atmosphere oxygen percentage:  45%.  That sounds like a high fire risk,  unless low total pressure mitigates it.  These total pressures are much lower than standard atmospheric:  ranging from only 6.86 psia for the 15,000 foot-based design up to only 9.19 psia for the sea-level-based design. 

These total pressure and oxygen percent data and the oxygen partial pressure can be used to evaluate fire risks.  I have no ready criteria for this myself.  It can be tested experimentally in test chambers right here on Earth,  however.  Adding argon lowers the percentage oxygen (but not its partial pressure),  and raises habitat total pressure.  

 Figure 4 – Scaling Up from Suit to Habitat to Avoid Pre-Breathe Requirements

Figure 5 – Basic Two-Gas Habitat Atmospheres as a Function of Design Suit Pressure

Three-Gas System for Reducing Fire Risks?

I do not know of a pre-breathe criterion factor for argon.  One might assume the same 1.2 factor over suit oxygen pressure applies,  so that one can add substantial argon to the mix,  at a partial pressure equal to that of the nitrogen. 

On the other hand,  monatomic argon has a molecular weight near 39,  versus diatomic nitrogen’s molecular weight of 28.  If osmotic rates depend directly upon molecular weight,  then argon should be slower than nitrogen to dissolve into,  and out of,  the blood.  Then perhaps the pre-breathe criterion for argon should be nearer 0.86 than nitrogen’s 1.20.  We just don’t know.  But,  that assumption lowers the argon content,  which is in the safer direction.  Results are in Figure 6.

Figure 6 – Three-Gas Atmospheres as a Function of Design Suit Pressure

Results:

#1. Go with the Navy oxygen mask criterion of 5000 feet as slightly more conservative than the 10,000 foot criterion.   Your suit (whatever its design) is then at-minimum pure O₂ at 3.28 psia.  Factored up for leaks,  this is 3.61 psia.  Even full pressure suits can be somewhat supple at pressures this low. 

#2. For a two-gas habitat atmosphere,  use a pre-breathe factor of 1.20 for nitrogen as well-supported.  Use the in-lung wet suit oxygen partial pressure as the in-lung wet habitat partial pressure of oxygen.  Results are given in figure 5 for all the suit designs.  For the recommended 5000 foot-based suit design and the two-gas system,  your habitat atmosphere is 45.45% O₂ and 54.55% N₂,  at 6.13 psia,  with zero pre-breathe time required to don the spacesuit.  The 45% O2 is rather alarming as a fire hazard.  It is at best unclear whether the low pressure of 6.13 psia could mitigate this risk.  At worst,  this is simply too severe a fire hazard. 

#3. For a 3-gas habitat atmosphere,  we use all the same design factors,  plus we set the argon pre-breathe factor at about 0.86.  Results are given for all the suit designs in Figure 6.  For the 5000 foot-based design,  we are looking at a habitat atmosphere that is 32.68 volume % O2,  39.22% N2,  and 28.10% Ar,  at a total pressure of 8.16 psia,  again with zero pre-breathe time required to don the spacesuit.  Adding the third gas greatly reduced the objectionable high volume percent oxygen.  But it is unclear whether this is enough reduction. 

#4. It is entirely possible that more argon could be used,  once a pre-breathe limit factor has been established for it.  That would act to lower oxygen percentage and raise habitat total pressure,  but would not affect the suit design. 

#5. It might also be possible to add a fourth gas (preferably an inert one,  and one not linked to the biology of life as CO₂ is).    This would also lower oxygen percentage and increase habitat total pressure,  without changing the suit design.  One candidate might be helium,  which has been used in deep sea diving.  It takes longer to decompress than nitrogen in that application,  which means its pre-breathe factor is lower than nitrogen’s,  in spite of the low molecular weight. 

#6. Note that in the tables given in Figures 5 and 6,  that oxygen percentages are constant,  regardless of the suit design pressure.  Raising suit pressure will not cut oxygen percentage in the habitat,  without violating the pre-breathe factors that make EVA’s into practical operations. 

Speculation:

Assume as a wild guess that the pre-breathe factor for helium is 0.5.  Assume also that the pre-breathe factor for argon is closer to that of nitrogen,  for about 1.0.  Under those conditions,  a hand calculation for the 5000 foot-based suit design gives a habitat atmosphere of 27.03% O₂,  32.43% N2,  27.03% Ar,  and 13.51%% He,  at 9.68 psia.  Earth-normal oxygen percentage is nearly 21%,  so surely this 4-gas atmosphere would be far less risky in terms of fire. 

Recommendations:

Determine reliable pre-breathe factors for argon and helium.

Identify other candidate gases that could be added to the mix,  and determine the pre-breathe factors for them. 

Final Note:

The atmosphere we evolved with (20.946% O₂ 78% N₂,  and 1% argon) at 14.7 sea level psia could be a habitat atmosphere as safe as here at home,  in terms of fire risk and any long-term medical risks.  The nitrogen pre-breathe factor of 1.2 says the spacesuit pure oxygen pressure would have to be about 9.55 psi in order to avoid pre-breathe time donning the suit. 

That’s impossibly high,  even for a full pressure suit.  Our astronauts have severe mobility restrictions at about half that pressure currently.  The ISS uses near Earth-normal air,  and hours of pre-breathe time,  to use a suit at about 4.85 psi.    

Something has to change,  or these EVA-practicality issues will stymie us.  

Update 11-20-14:

The preceding calculations and graphs were made under the assumption that we wanted a minimal suit pressure,  and that we calculated habit atmospheres to match the wet in-lung oxygen partial pressure of the suit  Only 3 and 4-gas atmospheres were considered (oxygen,  nitrogen,  and argon,  plus maybe helium).

For this update,  I looked at setting habitat atmospheric oxygen to something very near Earth-normal oxygen 20.946 volume % for fire protection,  and added two more noble gases to the mix,  resulting in a 6-gas mixture.  All the inert gases added to inert nitrogen are chemically-inert noble gases.  In order,  the diluents are nitrogen,  argon,  helium,  neon,  and krypton.  There are more noble gases available (xenon and radon),  but these are becoming radioactive items,  and are to be avoided. 

I used the same pre-breathe factor of 1.2 on the nitrogen that has been proven.  The other factors are speculative.  I used a factor of 1.0 for the argon,  and 0.5 for helium and the other noble gases.  This is an arbitrary selection,  but should be in the ballpark (so something like this mix will work.)  The last noble gas (krypton) I allowed to fall a tiny bit short of its max pre-breathe factor to balance the mix at 100%. 

I did not use exactly the Earth-normal oxygen percentage,  because I could not quite achieve mathematical closure with a 6-gas mix.  But since the next noble gases are radioactive,  I did not use them.  Instead I picked an oxygen percentage very close to a nominal 21 volume percent,  which should present exactly the same fire hazard as Earth-normal air.  My number was exactly 21.28% oxygen. 

I forced the wet in-lung oxygen partial pressure of the mix to match that from sea level Earth-normal air at 149.33 mm mercury.  Removing the water vapor but retaining the mixture total pressure gives this dry habitat atmosphere:

                Gas        Pp,  mm Hg         vol%

                O2          159.34                   21.28  compare to 20.946% for sea-level Earth-normal air

                N2          191.21                   25.54

                Ar           159.34                   21.28

                He          79.67                    10.64

                Ne          79.67                     10.64

                Kr            74.55                     10.62

                Dry tot  748.80                   100.

The habitat atmosphere works out to 0.9853 atm = 14.48 psia,  very close to Earth-normal air.  The difference is 5 diluent gases at significant concentrations,  instead of just one.  None of the 5 will have large concentrations (or osmotic pressures) in the blood.  Thus,  there should be little or no risk of “the bends” going straight from this atmosphere to the corresponding suit,  assuming my pre-breathe factors are anywhere close to right.  But,  it will take experiment to find out what the real factors are. 

In the wet-lung environment,  water vapor pressure displaces part of this dry atmosphere,  lowering the partial pressures.  I assume equilibrium water vapor pressure at body temperature.  Those adjusted data look like this:

                Gas                                        Pp, mmHg

                O2                                          149.33  same as sea-level Earth-normal air in the wet lungs

                N2                                          179.20

                Ar                                           149.33

                He                                            74.67

                Ne                                          74.67

                Kr                                            74.55

                Water vapor                       47.06

                Total                                      748.80  = 0.9853 atm = 14.48 psia

We want the pure-oxygen suit to match wet in-lung oxygen partial pressures,  so that tissue oxygenation and resistance to hypoxia issues is the same,  in the suit as in the habitat.  Because the suit is a 1-gas system,  you just add the water vapor partial pressure to the wet in-lung oxygen partial pressure to determine total suit pressure. 

                149.33 + 47.06 = 196.39 mm Hg = 0.2584 atm = 3.798 psia  

Even if you add a factor 1.10 increase to counter leakage,  the suit pressure is still only 216.03 mm Hg = 0.2842 atm = 4.177 psia.  That is far lower than current practice on the ISS. 

Conclusions:

#1. It is quite feasible to have habitat atmosphere at very nearly Earth-normal pressure and Earth-normal oxygen percentages,  and yet avoid high nitrogen concentrations,  by going to a multi-gas mixture.

#2. It is very likely feasible to do this in a way that avoids the long pre-breathe times associated with the high nitrogen concentration in Earth-normal air.

#3. If you do it as a multi-gas system,  there is no need for a high-pressure space suit,  just to lower (but not eliminate) the pre-breathe times using Earth-normal air. 

#4. The lowered suit pressure requirement makes full pressure suits more supple,  and makes mechanical counterpressure suits immediately feasible with compression levels already easily achieved.

Final Note: 

My pre-breathe factors for all but nitrogen are probably only ballpark,  but certainly not exactly correct.  The research to quantify these factors correctly can be done on the ground.  Thus there is no excuse not to research this,  the payoff is simply too attractive. 

Update 11-26-14:

Once again,  my pre-breathe limit factors for all but nitrogen are guesses requiring experiment before application!  I have heard that the US Navy knows the pre-breathe factors for at least some of the noble gases that I proposed using here,  and has experience with actual use of multiple diluent gases,  exactly as proposed here.  The pre-breathe factor of 1.2 for nitrogen is in fact a US Navy value.

While not conclusive as presented here,  this does make my analysis seem at least ballpark correct.

Use the Navy's factors,  and this becomes possible as a 4-, 5-,  or at most 6-gas system.  You will be at no more at risk of fire than here on Earth with 21% by volume oxygen,  and you can use a lower-pressure,  more-supple pressure suit in the vicinity of at most 4 psia (for a sea-level-equivalent oxygenation design plus leak margin).

And,  best of all,  you need no long pre-breathe times to avoid the bends.  That is completely unlike current practice on the ISS.

Or even better than a supple full pressure suit,  you can use a really supple mechanical counterpressure suit at the same low pressure.  Such a low-pressure suit is technologically feasible right now,  and has been since 1968,  in one form or another.  3 to 4 psia compression has been well demonstrated as feasible in that approach,  with two disparate technologies.  

There is simply no excuse not to try all of this out.  The payoff,  in terms of basic astronaut mobility,  and in terms of effective human exploration capability,  is just too great.  Not to mention enabling feasible and practical designs for habitats and bases on other celestial bodies of all kinds.  And,  enabling far more effective assembly and repair operations by humans in space!

Any space agency,  public or private,  that is really interested in manned travel beyond cis-lunar space should be looking very seriously at the issues and possibilities raised here.

As for the rest of us,  we can safely conclude that agencies not looking very seriously at these same issues and possibilities,  are simply not yet serious about manned travel beyond cis-lunar space.

Update 1-1-15:

I have run across documents about what the Navy pre-breathe factor work really involved.  They looked at decompression needs coming from higher-than-1-atm pressures,  which is for deep sea diving.  What they found for that regime was that the prebreathe factor of 1.2 applies to the sum of all the diluent gas partial pressures:  sum Pp diluents / low total pressure = 1.2.  A lot of experiments were conducted to confirm this.

The low-pressure regime has not been explored experimentally yet.  When decompressing from near 1 atm to low total pressures in a pure oxygen suit,  the question is this:  does the prebreathe factor still apply to the sum of all diluent gas pressures taken together,  or can you use a separate factor for each diluent gas,  or does it correlate as something in-between these extremes?  That would be a good topic for someone research experimentally.  

However,  the Navy result is suggestive that using multiple diluent gases does you no real good,  because the factor of 1.2 applies to the sum of diluent partial pressures,  not those pressures individually.  That runs counter to what I explored above.

If that is indeed the case,  then the decompression problem revolves around this choice:  either (1) you choose a habitat atmosphere (the usual choice is Earth-normal sea level air) and design suits to match it that require no pre-breathe time,  or (2) you figure out what suits you really want,  and use the pre-breathe factor criterion to design your habitat atmosphere to be compatible.  That bounds the problem,  and the answer lies between those extremes,  very sensitive to the assumptions you actually make.

There is context to consider surrounding this choice and the assumptions.  As pointed out above,  full sea level air pressure is not really necessary.  NASA is working on full pressure suit designs with more dexterous joints and gloves,  even at higher suit pressures (in the 8-9 psia range).  NASA cannot accept oxygen concentrations beyond about 30%,  as being too much of a fire hazard.  With MCP suits,  existing technologies would only support about 0.25 to 0.30 atm compression.  MCP is inherently more dexterous,  and far more fault-tolerant of punctures and rips,  offers the possibility of being machine-washable,  but is far more difficult and time-consuming to don.  Those are just the facts of life with which we must deal.

habitat-first approach with full Earth-normal air:

Assume Earth-normal air as "synthetic air":  0.209 atm oxygen and 0.791 atm nitrogen.  Using a pre-breathe factor of 1.2 on the nitrogen partial pressure gives a required suit pressure of 0.659 atm pure oxygen.  That high a suit pressure (9.69 psia) presents extreme challenges designing-in glove dexterity in full pressure suits.  That high a compression level also rules out using MCP without major technological breakthroughs in materials.  Assuming 10% loss from leaks says your min allowable suit pressure is 0.599 atm.  With suit oxygen pressures in the 0.6 atm range,  there are medical worries about exposure to too much oxygen,  if exposure times are long.  On the ISS,  we use essentially Earth-normal air.  Accordingly,  we have long decompression pre-breathe times,  because current suit pressures are not that high.

suit-first approach using only what is needed for oxygen in the suit:

Assume your suit pressure (full pressure suit or MCP) is 0.209 atm pure oxygen,  same as Earth-normal air.  10% for leaks gives you 0.190 atm,  corresponding to the oxygen content in Earth-normal air at about 3000 feet above sea level.  That's just fine.  Using the factor of 1.2 on the design suit pressure of 0.209 atm  sets the habitat atmosphere nitrogen at 0.251 atm.  Using max allowable 30% oxygen sets the oxygen pressure at 0.108 atm,  and the habitat "air" pressure at 0.359 atm.  That level of oxygenation is still just too low,  being equivalent to Earth air at about 17,000 feet elevation.

compromise with reduced-pressure but otherwise earth-normal air in habitat:

In flying,  pilots must use oxygen above 10,000 feet elevation,  below that altitude,  they do not have to use it,  which presumes adequate oxygenation,  something proven by long experience.  The air pressure at 10,000 feet is 0.6878 atm. Assuming synthetic air at 20.9% oxygen,  that's 0.1438 atm oxygen and 0.5440 atm nitrogen.  That would be adequate as habitat air.  Using the pre-breathe factor on that nitrogen,  we get a pure-oxygen suit pressure of 0.4533 atm (6.66 psia).  That's still a considerable design challenge for dexterous full pressure suit (although NASA is working on glove designs for 8-9 psia).  It still rules-out using MCP without decompression time.  We are still very limited,  and would need decompression pre-breathe times to use MCP technology as it exists.  But this possibly could eliminate pre-breathe requirements with existing full pressure suits,  especially if refitted with more dexterous gloves.

same reduced-pressure habitat but with max oxygen allowable for fire safety:

Require the same oxygen as Earth-normal air at 10,000 feet:  0.1438 atm oxygen.  At 30% oxygen,  the habitat pressure is 0.4793 atm,  and the nitrogen is the other 70% at 0.3355 atm.  Using the pre-breathe factor on that nitrogen gets you a suit pressure of 0.2796 atm (4.11 psia).  That falls to 0.2542 atm with 10% leakage,  still quite adequate.  With suit pressures near 4 psia,  the new more-dexterous glove designs for full pressure suits will be far more effective,  and the compression levels are achievable with MCP.  Both kinds could be used with no decompression pre-breathe time requirement.

conclusions and recommendations:

What this does is bound the problem and show feasibility by making more appropriate initial assumptions.  The water-vapor displacement effect inside the lungs has not been taken into account,  and must be,  in any final design.  But those results should not be very far from these.  I'd guess the habitat and suit pressures to be just slightly higher than these bounds.

The "feasibility" shown here is threefold:  (1) we can dispense with the decompression pre-breathe times by using a significantly lowered habitat pressure,  and with the oxygen content still within known fire safety criteria,  (2) we can immediately add existing materials technology-based MCP suit designs to the mix of things available to the astronauts,  and (3) the more dexterous glove designs being developed for the full pressure suits will be even more effective at reduced suit pressures.  Those are all very favorable outcomes,  and they are also incredible enabling factors for astronauts to be far more effective when working in vacuum.

The only thing currently going on is the more-dexterous glove design for full pressure suits being done by NASA.  No experimentation is going on with habitat atmospheres on the ISS,  it's close to Earth-normal air almost by default.  MCP suits are still not seriously funded.

I see some things here that should be going on,  if we ever expect to explore other bodies like Mars more effectively.

Related Articles on this Site:

2-15-16    Suits and Atmospheres for Space (the latest!!)

1-15-16   Astronaut Facing Drowning Points Out Need for Better Suit

11-17-14    Space Suit and Habitat Atmospheres

2-11-14   On Orbit Repair and Assembly Facility

1-21-11   Fundamental Design Criteria for Alternative Space Suit Approaches

A Really Nice Airliner

On Wednesday 10-29-14,  I had the privilege of working on EAA’s Ford tri-motor NC8407 (built in 1929),  as part of a team of volunteers,  doing a 100-hour inspection.  This was at the McGregor,  Texas,  airport,  and I did this just after completing the annual inspection on my own plane (a Cessna 170B,  N2794D,  see "Super Red-Letter Event",  posted 1-18-14 on this site). 

On Saturday 11-1-14,  I got to take a ride in that same Ford tri-motor.  It was a really nice ride,  and very comfortable.  The seat pitch allowed one to stretch one’s legs,  something ignored by airlines today.  We flew around the Waco,  Texas,  area for about 20 minutes,  at about 90 mph,  at around 3000 feet.  Very comfortable.  

The pilots that morning were Jeff Skiles and Ashley Messenger.  Yes,  it was THE Jeff Skiles,  Capt. Sullenberger’s copilot during the “Miracle on the Hudson” water landing.  I got to talk extensively with both of them. 

I found a good image of this aircraft overall,  and a unique image of the engine gauges on the outboard engine.  You read them by looking out the window at them.  Enjoy

EAA Ford Tri-Motor NC8407

3-Gauge Instrument Cluster on No. 1 (Left) Engine

Two Commercial Spaceflight Disasters in One Week

Two commercial launch failures within days is a lot for a fledgling industry to take.  Especially with political critics that are quite undeserved.  One of these failures relates to commercial manned flights,  that being Virgin Galactic's SpaceShip Two.  The other relates to unmanned cargo and satellite launch,  that being Orbital Sciences' Antares rocket and Cygnus cargo spacecraft.  

Update 11-3-14:  with a rocket-powered vehicle,  the first suspect is always the engine when there is a problem.  That appears to be the case with the Orbital Sciences launch failure.  

But with the Virgin Atlantic spaceplane,  preliminary NTSB comments reported this morning indicate there was a problem with the re-entry feathering system,  not the engine.  This one is going to be very interesting when the report is done.  

In any event,  enjoy the rocket technology discussions that follow.

Update 11-5-14:  Published news releases as of today indicate that Orbital thinks their launch disaster was caused by a turbopump failure.  They also no longer intend to use those engines.  The best new information about the Spaceship Two disaster suggests that the two pilots were literally ejected into the air as the cabin broke up around them,  at about Mach 1,  near 50,000 feet.  These reports indicate they were so ejected without pressure suits or supplemental oxygen.  One managed to deploy his parachute and lived,  the other didn't and died.  

Update 4-15-15:  There is no news yet from the NTSB regarding Spaceship Two that has made public release.  However,  regarding the Antares explosion,  it does appear that the turbopump bearings were essentially destroyed in one of the refurbished Russian engines,  probably from foreign object damage.  

Orbital (builder of the vehicle) and Aerojet Rocketdyne (refurbisher of the engines) are at odds over this.  similar to Ford and Firestone Tires a few years ago.  At issue is where the foreign objects that got into the turbopump came from.  Whose fault was it that such debris got in there?  

If it were in the engine to begin with,  how did it escape quality control at Aerojet Rocketdyne?  If it came from the vehicle tanks or piping,  was there not an inlet screen in Orbital's design to catch such stuff?  Both possibilities are rather hard to believe.  

We may or may not ever hear in public which is true.  But there's not a lot of other possibilities for that kind of failure.  

Hybrid vs Solid Rocket Technology

Hybrid rockets “done right” have safety advantages over solid propellant rockets,  if the fuel is just that:  fuel,  without any oxidizer in its formulation.  That can be a rubber,  a plastic,  or even a wax,  and it can have solid powders dispersed in it,  just not oxidizer powders. 

That kind of hybrid ceases combustion when you stop the liquid oxidizer flow.  That makes the vehicle abortable,  since you can shut off the engine,  as with liquid rockets.  Solids burn like dynamite sticks:  once lit,  they burn to completion,  no choice.

Solid propellants are typically a rubber or rubber-like binder loaded with particulate solids,  solids that include the solid oxidizer as powder.  Solid oxidizers are typically ammonium perchlorate or ammonium nitrate these days.  There are some others. 

Solid propellants,  even those that are seriously fuel-rich,  all burn with two physical effects controlling their burn rate.  The prime factor is an inherent burn rate of exposed surface that depends upon chamber pressure per a power function with an exponent under 1,  usually well under 1.  The secondary factor is often called “erosive burning”,  and shows up as a burn rate enhancement (added term) in the presence of hot gas scrubbing along the exposed surface at high speed. 

All exposed solid surface quickly produces massflow,  because there is very rapid surface flame spread from lit areas to unlit areas.  The most common problem encountered with solid rockets is the presence of cracks or voids in the propellant charge,  because these very quickly (in milliseconds) add large quantities of burning surface,  producing massive increases in motor massflow.

This very quickly drives the chamber pressure too high,  overpressuring the case,  and causing a violent explosion.  It typically happens without warning symptoms,  and very quickly (on a few milliseconds time scale).  This is due to the very nonlinear mathematics of what determines the chamber pressure,  which is a balance between pressure-driven surface massflow generation,  and pressure-driven nozzle massflow capability.

The second most common failure in solid motors is a case insulation failure,  leading to a burn-through,  followed quickly (usually just under a second) by the case exploding.  The third most common failure is a type of combustion instability wherein the oscillations cause higher average pressures from an enhanced burn rate,  due to the erosive-burning effect.  These can lead to over-pressurization explosions on a time scale of a few seconds,  but not always.

That kind of combustion instability has an unstable positive feedback of combustion energy released from tiny combustion eddies directly into the oscillations.  It occurs when some natural gas vibration mode in the geometry is close to the small-scale combustion eddy frequencies.

A very distant fourth most common failure is a piece of propellant or other debris coming loose inside the motor,  and getting blown into the nozzle,  where it obstructs the throat,  causing the motor to virtually-instantaneously explode like a pipe bomb. 

The most common problem in the hybrids is a kind of combustion instability that causes very rough operation and vibrations,  but usually without the unstable feedback of the solid.  This is inherent due to the gross unmixedness resulting from generating the fuel from only the erosive-burning effect of the hot port flow (oxidizer plus fuel from upstream plus still-combusting fuel and oxidizer). 

There are localized fuel-rich and fuel-lean pockets of gas.  As fuel and oxidizer suddenly combine in the very turbulent mixing,  these pockets explode.  The bigger the “exploding vortices”,  the stronger the thrust and pressure oscillations.  But because there is no feedback into a chamber-pressure burn rate effect,  the instability doesn’t grow catastrophically out-of-control within seconds,  the way it does in solids. 

The other two fatal hybrid failure modes are the same as solids:  case insulation failure leading to burn-through,  and debris blocking the throat.  Update 11-3-14:  hybrids with no oxidizer in their fuel grains are pretty much immune to the effects of grain cracks or voids.  There is no pressure effect burn rate in these freshly-exposed spaces,  and the massflow scrubbing effect just doesn't reach there hardly at all.  Fuel burnout "tailoff" characteristics can be affected somewhat,  though.  

Historically,  hybrids have suffered design impracticality from very low effective burn rates (more properly,  “regression rates”).  The historical “fixes” for this are (1) multiport grain designs that reduce the effective thickness to be burned,  and (2) adding low amounts of oxidizer powder to the fuel grain formulation. 

In recent years,  a third option has become available:  fuel formulations that liquefy (literally melt) before they pyrolyze into combustion.  That third option is the best.  The multiport option leads to low volumetric efficiency.  Adding solid oxidizer is absolutely the worst,  by far.

Adding oxidizer,  even in amounts too low to sustain combustion with the liquid oxidizer shut off,  converts the hybrid “fuel” into a fuel-rich conventional solid propellant.  That negates the very most important safety advantage of being a hybrid.  Now there is fast surface flamespread,  drive by the inherent burn rate vs pressure effect of a solid,  into cracks and voids.  This restores the same vulnerability to motor explosion as any other solid.  

If you add too much solid oxidizer,  you cannot stop the motor by cutting off the liquid oxidizer.  This negates the other advantage of a hybrid,  abortability. 

A Word About Liquid Rockets

Liquids have three of the very same failure modes that afflict solids and hybrids:  burn-through,  debris blocking nozzle throats,  and combustion instability.  The failure of regenerative cooling (not insulation) is what causes the burn-through.   Combustion instability in liquids takes many forms,  all being related to ignition of mixing eddies that are rich or lean in fuel. 

The unique problem with liquids is turbopump failures,  since nearly all liquid systems have them. (Update 11-3-14:  some hybrids can also have pumped liquid oxidizers,  others are pressure-fed.)  These components are very highly stressed,  and exposed to very extreme conditions of heat and cold simultaneously.  It is not surprising that they fail.  When they do,  there is almost always some kind of explosion. 

Another failure mode known since the V-2 missile of World War 2 is starting at too high a propellant flow rate.  This will literally blow the engine apart,  virtually instantaneously.  It does take a very finite time to spool turbomachinery up from low thrust settings to high thrust settings.  Further,  there are serious limitations on just what the minimum thrust of any particular engine design can be. 

SpaceShip Two’s Loss 10-31-14:

The available data-to-date are spotty at best.  We will have to wait for the NTSB investigation to complete,  before the truth and all the facts come out.  That can take over a year. 

The news photos I have seen indicates that SpaceShip Two dropped from White Knight Two successfully,  and then successfully ignited its rocket engine.  Then there was an explosion. Update 11-3-14:  not an explosion,  a mid-air breakup,  according to very preliminary comments made by the NTSB.  

Photos of the wreckage tell me that the engine compartment was violently destroyed,  also blowing away the tail booms of Spaceship Two.  Without the tails,  the fuselage and its stub wings just become a tumbling ballistic projectile to the desert floor. 

The tail booms,  and that which is recognizable as fuselage wreckage,  appear to have crashed in different places on the desert.  I did recognize the fuselage aft pressure bulkhead in some photos of the wreckage.  There was nothing left aft of it.  Update 11-3-14:  more published photos from more view angles show that there was nothing left on either side of this bulkhead.  

Some accounts I have seen claim CNN as a source for the assertion that the engine shut down,  and then exploded upon a restart attempt.  I do not know anything about that,  myself. 

Nothing I have found among open sources on the internet would indicate that solid oxidizer was a part of the original HTPB fuel grain formulation,  or the new plastic grain formulation on the lost flight.  That’s not to say there wasn’t any,  but nothing released publicly indicates it. 

Everything I saw says fuel-only grain plus liquid nitrous oxide (N2O).  The only change seemed to be the switch from HTPB rubber to a plastic for the binder.  That occurred when Virgin took the motor design in-house,  away from Sierra Nevada,  who had done the HTPB version.

That leaves motor case burn-throughs or debris plugging the nozzle,  from the failure modes listed above.  There is also the possibility that the restart attempt was transiently just too violent an event,  and simply split the motor case open. 

Whatever happened,  the event was violent enough to suddenly “disperse” the entire engine compartment from the aft fuselage,  since the fuselage debris in the desert terminates at the aft pressure bulkhead.  Update 11-3-14:  with a midair breakup,  it was broadside air pressures that broke up and dispersed everything.  It doesn't take an internal explosion to do that,  just a loss of attitude control.  

This same violent event was enough to separate both articulated tail booms from the aircraft.  They can be quite clearly seen about a vehicle length away on each side in the news still photo that was taken a split second after the explosion. 

Pieces of the motor case are going to be hard to find in all of that desert.  It may be impossible to find enough to reassemble it,  to determine if there was a burn-through. 

If there was some kind of mixing baffle inside the motor to help with the low-grade instability of hybrids due to poor mixing,  then it might have come loose and blocked the nozzle.  It might be vulnerable during an ignition transient.  If it can be found,  extreme distortion of the part might suggest this possibility.

Finding out whether restart attempts might be too violent will require motor restart testing on the ground.  Virgin and Scaled Composites certainly have a lot to investigate.  I would suggest doing any such restart tests in a revetment of some kind,  as a ground crew safety measure. 

Update 11-3-14:  With the NTSB focusing up the the feathering system causing a mid-air breakup,  the engine issue revises to confirming that it was operating correctly.  

Orbital Sciences Antares/Cygnus Loss

I have seen news video footage of this loss enough times to know that something was wrong from liftoff.   The engine plumes of kerosene-liquid oxygen engines are very brilliant,  enough to cause camera “image bloom” all the time.  Yet in this launch,  that “apparent fireball” was just too bright. 

A few vehicle lengths off the launch pad I saw the vehicle slowing to a stop,  followed by the first explosion.  That first explosion would have been the range safety self-destruct charge ripping apart the first stage.  The rest of it blew up as multiple explosions upon impact very near the launch pad.

I do not know,  but I suspect either a turbopump or chamber failure leading to a loss of incandescent gas around the engine at vehicle rear.  After a few seconds overheating,  the engine blew apart,  killing thrust,  and precipitating self-destruct.  These are refurbished 40+ year-old Russian-made engines! 

Orbital Sciences is using these Russian-made engines as the only thing available to them in the right size range.  These were originally built in Russia over 4 decades ago,  and were stored most of those years in a barn of some kind in Siberia.  Aerojet Rocketdyne refurbished these for Orbital Sciences.

This says more about corporate (and government) policies regarding offshoring overseas critical US capabilities,  than it does anything about the engines themselves.  If “we” had not frittered-away this country’s rocket engine manufacturing capabilities,  Orbital would not have been forced to use refurbished 40+ year old foreign made units.  Period.  Somebody needs their butt thoroughly kicked over this issue!

From what I can find on the internet,  one of these same engines blew up in a ground test a while back.  I could not locate anything explaining that test failure,  but then,  I am no internet expert.  However,  that’s enough to cast some doubt on this engine.

Conclusions:

Orbital has to get to the bottom of this.  They will need Aerojet’s help.  The rest of us need to give them the “wiggle room” to get that job done.  Critics stirring up trouble in the halls of congress are not needed,  and can go stuff their unhelpful “suggestions” where the sun doesn’t shine!

Same goes for Virgin’s fatal problem with SpaceShip Two:  they need “wiggle room” and time to get to the bottom of what happened. 

I can only wish both outfits well,  and may God speed their investigations.  I would gladly help either or both of them.

GW