SSTO Concept Design Project Team
Section 6.8.7.
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SSTO to Moon vs LEO Staging Analysis

Adam London

OK, I've had a chance to run some more numbers. The following table presents the mass of H2 that needs to be lifted from earth for each unit mass of the vehicle that makes the trip to and from moon:

Full Aerobraking 2/3 Aerobraking Rocket Braking
EOI deltaV: 230 m/s 1000 m/s 3225 m/s
refuel in LEO only 1.02 1.44 6.67
refuel in LEO & LLO (keep H2)* 0.70 0.89 2.00
refuel in LEO & LLO (keep H2)+ 0.67 0.84 1.74

* keep H2 for TEI and EOI with you for lunar landing and ascent
+ leave H2 for TEI and EOI in LLO while you do lunar landing and ascent (pick it up when you drop off landing LOX in LLO on way out)

I didn't use another vehicle to provide the dropping off of fuel in various places, since I think we want to keep this overall system as simple as possible, and I really like the idea of being able to maintain everything on earth.

In the best case (lower left), using a single stage for whole trip, the fuel mass ratio on earth would have to be 93.92%, leaving about 6% for structure, payload, engines, TPS, etc. with T/W of engines at 50, and an initial acceleration of 1.2g, the engines need to be 2.4% of GLOW, leaving 3.6% for everything else. This may be just feasible, but I doubt it. I think that in order to make this with forseeable technology it would not be the workhorse we require. The margins are just too small.

On the other hand, if you stage in LEO, I think any of these options are feasible, with larger H2 numbers meaning you need a larger SSTO to deliver the whole thing to orbit. If we assume some kind of aerobraking, the difference between refueling in LEO only and refueling in LLO also is not very much. For a given moon vehicle size and aerobraking case, the mass that the SSTO has to deliver to LEO (referenced to the minimum size in each aerobraking case) goes as follows:

Full Aerobraking 2/3 Aerobraking Rocket Braking
EOI deltaV: 230 m/s 1000 m/s 3225 m/s
refuel in LEO only 1.21 1.33 2.80
refuel in LEO & LLO (keep H2)* 1.02 1.03 1.09
refuel in LEO & LLO (keep H2)+ 1.00 1.00 1.00

So the SSTO required to not have a LLO refueling post would only need to be 21% bigger if full aerobraking was used, and 33% bigger if 2/3 aerobraking was used.

It may make sense to design the vehicle for this option, and then once we are far along, and it makes sense to have a LLO refueling station, the passenger capability can be increased.

Therefore, I propose we investigate a staged in LEO system that initially only refuels in LEO, but is designed with the eventual goal of refueling in LLO orbit as well, which would then increase passenger capability significantly.

I'd like to pick an option for us to explore, and stick with it for a while, so that we can actually crunch some numbers and get a better idea of what we are looking at.

So if I don't hear any resounding "nay" votes, lets start with a SSTO delivering a moonship that only refuels its LOX in LEO. Throughout we will attempt to build in an expandability to add additional passengers per flight once a LLO refueling station is in existence.

SSTO Concept Design Project Team

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