Sorry I don’t have quite as pithy title this time. My creative juices only go so far some days, and I wanted to leave some for the actual article. Nothing more Dilbertesque than spending so long trying to come up with a clever article that you never get around to your point…
Also sorry that I wasn’t able to finish this up sooner. We’ve been really busy the past few days trying to wrap up the igniter development and qualification at work to the point where we can start rocket engine testing. There’s also been a few lovely little flamewars on the SFF’s Space Arena BBS that I willingly allowed myself to be dragged into.
That said, I want to comment on a commonly held belief in the space enthusiast crowd. That belief is that in order to do anything important in space, you need a Heavy Lift Vehicle. Something big, and cool like a Saturn V. While I think there may become a time where the cislunar economy develops to the point where there is sufficient traffic to justify Heavy Lift Vehicles, I don’t think we’re anywhere near that point at the present.
One of the common reasons why people tend to think HLVs are neccessary for manned
lunar travel is that they seem to think that the only alternative to HLVs for lunar missions is to do extensive on-orbit assembly. They then point to the experience with the ISS and use that to show that they think on-orbit assembly is just too expensive and difficult. There are a couple of problems with this argument:
- Most of the mass required in LEO for a lunar trip is the propellants. If you count lander propellants too, you’re probably close to 75-85% of the total mass in LEO. Propellants are easily divisible into smaller segments, and don’t have to be launched at the same time as the rest of the vehicle.
Large-scale transfer and storage of cryogenic propellants is still a poorly developed field, but one that is absolutely critical for the development of a spacefaring society. Unless you develop the capability to refuel in microgravity (whether that takes place in LEO, GEO, GTO, LLO, or at the L-1 LaGrange point doesn’t matter), it will be difficult or impossible to reuse lunar transfer vehicles, and it isn’t really possible to have a meaningful cislunar economy if you throw away your vehicle after every flight.
What this means is that most of the “on-orbit assembly” is entirely in the form of docking and transfering propellants, not astronauts doing spacewalks connecting cables and turning bolts.
- Most lunar missions, and almost all manned lunar missions have natural “break points” anyway. What I mean by this, is that there are natural places where you are probably going to want to split the spacecraft anyhow. For instance, in the Apollo program, there was a Command Module, a Service Module, and a Lunar Excursion Module (which itself had a descent and ascent portion). Every mission flown required a rendezvous and docking maneuver anyhow to mate with the LEM in the proper translunar injection configuration. This is a natural break point. The LEM didn’t have to be designed to be launched on the same vehicle as the CM or the CSM.
Future lunar missions are likely to differ quite a bit from the methods taken in Apollo, but that doesn’t mean that they don’t also have logical break points. If one wants to reuse their cislunar architecture, some good break points might be staging at L1 or in a Lunar orbit. The lander probably can and probably ought to be a separate spacecraft from the transfer stage, which implies that there isn’t any particular reason why it should be launched at the same time as the transfer vehicle. In fact, there’s no good reason why the lander even needs to be flown on the same lunar transfer vehicle as the passengers or cargo.
- As can be seen from those previous points, almost all of the “on-orbit assembly” is going to be of the rendezvous and docking form. We’ve been doing this since the time when most engineering was done with a sliderule. How hard can that really still be using modern technology?
- Even on-orbit assembly of the ISS sort doesn’t have to forever remain difficult or impossible. A lot of the cost of ISS assembly labor stems from the high cost of getting into space in the first place. If that can be lowered, and made more frequent, the relative costs and difficulties of on-orbit assembly can be greatly reduced.
There are some operational drawbacks to an earth-orbit rendezvous architecture, that I’ll probably delve into in a later post. However, as one can see from some of the points I’ve made already, there are some very good reasons for not going with an HLV. A quick summary is that:
- Going with an HLV discourages the development of technologies and procedures that will be needed anyway before affordable cislunar travel is possible
- Going with an HLV will ensure that payloads are designed intentionally for the HLV, and not for anything smaller or cheaper
- Going with an HLV requires you to actually develop an HLV, which in spite of Griffen’s crack about already having an HLV, will actually take significant time and money to field (more on that one later)
- Going with an HLV means that you can’t take advantage of reductions in launch prices that occur as the private sector gets moving
- Since NASA will probably try to design most of their payloads to be launchable only on an HLV (to justify its existance), any launch failure of the system will ground the whole program
- Going with an HLV will require substantial capital costs for upgrading launch infrastructure
- Going with an HLV will mean that NASA can’t benefit from lower prices due to other customers paying their share of the amortization of the flight systems
- Going with an HLV means that NASA is spending taxpayer money to compete with existing and planned commercial vehicles in spite of laws forbidding it to do so
- Going with an HLV derived from the Shuttle perpetuates that wasteful and expensive bureaucracy associated with that failed program, and insures that NASA will continue to be seen as a make-work scheme for rocket nerds
I think that’s enough dead horse beating for today.
Latest posts by Jonathan Goff (see all)
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