Sunday, November 17, 2013

Payload Comparisons

Partly to determine how realistic my ballpark launch studies really are,  I looked at a payload fraction comparison among my two ramjet-assisted reusable systems,  the retired space shuttle,  and today’s commercial launchers as typified by Spacex’s expendable Falcon-9.  There are no data available yet for the reusable form of Falcon-9,  although I have left a place in the table for it,  when it becomes available. 

Payload Definitions

The payload fraction you calculate depends upon what you define to be the payload.  One-way deliveries of satellites require only the satellite plus a rather lightweight ascent shroud to be atop the rocket.  With men,  there must be a vehicle for them to ride.  As distinct from the satellite case,  with men there is “delivered payload”,  and there is “ultimate payload”.

“Ultimate payload” is the men,  suited,  and with life support supplies for the duration of the ride,  plus a safety margin in supplies.  Assuming conventional pressure suits,  good enough for an emergency walk in space,  I typically allow 400 pounds per man.  That’s just under 200 pounds for the man,  about 200 pounds for the suit,  and the rest some oxygen and drinking water.  If there is any luggage or cargo,  it goes into this definition.

“Delivered payload” is the ultimate payload plus the vehicle as it is delivered on-orbit.  This could be a capsule plus its service module (if any),  or it could be a spaceplane or lifting body vehicle. 

HTO/HL Study

In this article,  “HTO/HL” refers to the on-orbit delivery of a winged spaceplane with three people aboard,  using a ramjet-assisted horizontal takeoff system,  and horizontal landing with the returning spaceplane.  That is the recent article “HTO/HL Launch with Ramjet Assist” dated 11-6-13. 

Every booster item in that study (and there are multiple items) are intended to be fully reusable,  and with very long service lives.  In effect,  I traded the far-higher specific impulse of the airbreathing components for the larger inert mass fractions that can support long-life reusability.  Both structural beef and unpowered recovery items are included. 


“ZLL-IRR” refers to the on-orbit delivery of a small capsule with two astronauts aboard,  plus a small service module,  again using a ramjet-assisted launch system.  That study is “Manned Launch to LEO Using Ramjet Missile Technology”,  dated 10-27-13. 

This one is not vertical or horizontal launch,  but zero-length launch (ZLL) at a launcher angle,  like many surface-to-air missile systems.  The ramjet assist packages the initial rocket boosters within the ramjet engines themselves,  something called the integral rocket ramjet (IRR).  This is an operational missile technology,  available since approximately 1970.  Again,  the components are intend to be reusable with very long service lives,  with the exception of the service module,  and the replaceable heat shield and parachute systems on the capsule. 

Space Shuttle

I used data off the internet for this.  The launch weight was listed as 4,470,000 pounds.  The orbiter weight as delivered on orbit was 240,000 pounds.  Maximum crew was 7,  and the LEO cargo bay payload weight was listed as 53,600 pounds.  Just for a number,  I used the same 400 pound suit plus astronaut allowance,  in spite of knowing that shuttle EVA suits were closer to 300 pounds than 200.  7 astronauts at 400 pounds each added to the cargo for an ultimate payload of 56,400 pounds to LEO  in my comparison. 


I used data directly from Spacex’s web site for this.  LEO payload is listed as 13,150 kg,  and launch weight as 505,846 kg.  This would be for a  maximum satellite within a fairly lightweight shroud,  which shroud I ignored for this comparison.  I simply assumed a manned version of the Dragon capsule would gross out pretty close to the payload capacity of the Falcon-9 booster system.  It is listed as capable of crew sizes up to 7.  I used my same 400-pound allowance-per-crew for my estimates.  That makes delivered payload pretty close to 13,150 kg,  and ultimate payload pretty close to 1270 kg. 

Re-Usable Falcon

No data are yet available.  The concept calls for reduced payload to allow propellant residuals for powered landing of the first stage.  Experiments are currently underway to make that happen.  Whether the second stage can be recovered and re-used is an open question.    

Data Comparison

Ultimate payload fraction is simply ultimate payload divided by launch weight,  expressed here as a percentage.  Delivered payload fraction is delivered payload divided by launch weight,  also expressed as a percentage.  In the data table,  I have included notes about the propellant systems.  Hydrogen systems are well-known to reduce booster weights for a given payload.  LCH4 is liquid methane,  LH2 is liquid hydrogen,  and LOX is liquid oxygen. 

Reuse F9
ult frac
del frac

Concluding Remarks

If you look at what I define as ultimate payload,  all of the systems have fractional-percent ultimate payload fractions,  excepting the hydrogen-powered shuttle,  which is barely above one full percent.  I have to conclude that my ramjet-assisted system rough-outs are not too bad,  especially since I deliberately traded-off airbreather impulse gains for the larger weight allowances necessary for long-life reusability.  My systems appear to be competitive in terms of ultimate payload fraction with both the historical reusable shuttle,  and with modern low-cost expendable launchers as typified by Falcon-9. 

If you look at delivered payload fractions,  you get the same story,  just different numbers,  which will look more familiar to those who work with satellite launch scenarios.  All of the non-hydrogen systems fall in the 3% range,  excepting the hydrogen-powered shuttle,  which is closer to 5%.  Once again,  my ramjet-assisted reusable design studies look pretty reasonable,  and competitive. 

If I had designed for one-shot / throwaway systems,  my payload fractions (either definition) could have been much higher,  because I could have taken direct advantage of the higher airbreather impulse figures.  That’s not what I did,  I chose to design for reusability with ample weight allowances to support it,  and used the airbreather-conferred impulse increment to support that. 

It’s only an opinion,  but I think that may be the “best” way to add airbreather-assist,  at least for non-vertical launch scenarios.  The fundamental idea is to reduce launch costs with both reusability of the hardware,  and a far-smaller logistical tail drawing salaries. 

No comments:

Post a Comment