The space community finally
seems to be coming to terms with the fact that the President’s new space initiative requires the development of some kind of heavy-lift booster, one much more capable than the existing Delta 4H and Atlas 5H vehicles. But the question remains: what kind of booster?
A popular answer is: a Shuttle-Derived Vehicle (SDV), similar to the Shuttle-C and Shuttle-Z proposals of years gone by. This notion has a lot of superficial attractions — especially to those people who earn their living by serving in the great army of Shuttle support personnel. But if you study it a little, it becomes clear that this idea has severe drawbacks.
First, note that many SDV promoters are under-estimating the current cost of Shuttle launches by a factor of ~2. NASA has traditionally used a phony cost accounting system in which the salaries of civil service personnel and facilities support are not allocated to specific projects.
Sean O’Keefe has finally forced the universal adoption of full-cost accounting throughout NASA (about a decade after Dan Goldin ordered it). The Plan Bush budget chart now shows a wedge of about $5B/yr allocated to Shuttle.
An artist’s drawing of a Shuttle-C design in orbit
If we divide this by the recent maximum flight rate of 4/yr, we see that a Shuttle launch now officially costs about $1.2B — just about the value cited by the most outspoken Shuttle critics in the past..
There are a lot of different plans for SDVs that vary in different ways from the current Shuttle design. The only thing they all have in common is to get rid of the winged Orbiter and convert its huge dead weight into useful cargo capacity.
The underlying fallacy here is that most of the Shuttle’s problems are contained in the Orbiter, and that the rest of the stack can make a safe low-cost launcher. This is simply not the case. Recall that the two fatal accidents so far were caused by fundamental design defects in the SRBs and ET, not any deficiency in the Orbiter.
The simplest idea (let’s call it SDV-1) is to replace the Shuttle orbiter with a big cargo pod with the SSMEs mounted in the current position at the bottom. Unfortunately, the SSMEs are a major risk factor themselves. Despite 30 years of improvements, they still operate at the edge of safety, with persistent problems of material failure and constant risk of shutdown or explosion.
Even worse, the SSMEs are so expensive that some kind of recovery system is necessary that can bring them back from orbit. Even if this is a simple ballistic RV, it will still be very expensive to design.
It makes a lot more sense to substitute the new RS-68 engines designed for the Delta 4 family. This engine is basically the SSME with all of its advanced features designed back out again to reduce cost at the expense of a slightly lower performance.
These engines need not be recovered and can be mounted at the bottom of the ET with their thrust vectors aligned straight up. Then the cargo pod can be moved to the top of the ET to create a sensible (SDV-2) design that looks like a rocket instead of a flying oil refinery.
But SDV-2 still has a terrible problem — those solid boosters on the sides. These were a last-minute substitute forced onto the Shuttle designers by bean-counters at OMB to reduce the development cost. It was recognized at the time that they would greatly increase the operating costs of Shuttle.
The SRBs are often described as“reusable”, but this is nothing but propaganda. After every flight, they need to be dismantled, sent to Utah in pieces, remanufactured, sent to Florida, and laboriously reassembled.
This process clearly doesn’t save much (if any) money. Several boosters designed after Shuttle use similar SRBs, but none of them bothered to introduce the minor modifications needed to make them recoverable.
Besides consuming a huge amount of skilled labor, the SRBs are extremely dangerous to the ground crews and facilities at KSC because they are already fully fueled when they are assembled in the VAB.
Both Brazil and India have recently suffered fatal accidents due to accidental ignition of solid fuels. The prospect of an SRB or even a segment igniting inside the VAB is so scary that NASA does not use much of that huge building’s internal space to reduce the casualty list when this accident eventually happens.
Solid rockets traditionally have the advantage of simplicity, but the SRBs are not simple. Besides the complex system of joints, they have steerable nozzles driven by hydraulic power. That power comes from hydrazine-powered turbopumps that are the same model as the three APUs on each Orbiter that have been a constant source of worry, fires, and explosions.
Finally, the SRBs cannot be shut off in flight to allow the escape of a crew. SDV-2 could not be man-rated and we would have to develop a man-rated version of Delta 4 or Atlas 5 in addition to the SDV to carry crews up to LEO. (By “man-rating” I don’t mean some super-expensive program to increase reliability, but a simple fault detection system that triggers the escape tower on the ~1% of launches that fail.)
In fact many people seem to assume that we will be using separate cargo and personnel boosters in Plan Bush. This is another current fad that makes no sense. It would increase the expense and vastly complicate launch scheduling. It’s tough enough to get one rocket off on schedule, much less two of completely different designs. The history of the Gemini-to-Agena docking program is a good example of the problems involved.
So the SRBs need to be replaced. NASA knows this perfectly well. The most common safety upgrade proposed for the Shuttles over their long life was to replace the SRBs with liquid boosters. This change was never made because NASA never could scrape up the money to develop a suitable engine.
But this is no longer a valid objection. We now have a cheap high-performance kerosene engine in the form of the Glushko RD-180 used on the Atlas 5. Its development cost has already been paid by the old Soviet Union and the USAF. Although it is of Russian origin, production and support facilities are being developed in the USA.
So replace the SRBs with strap-on Atlas 5-based boosters. This SDV-3 will be much safer because it is not fueled until it reaches the pad. We can keep it inside the VAB for most of its prep time and reduce the weather and salt exposure that has caused so much trouble with Shuttle.
Now all that is left of Shuttle is a modified External Tank. But this has its problems also. The absurd brown Styrofoam exterior has obviously got to go. And the production cost of each ET has ballooned up to over $60M — more than a baseline EELV booster complete with engines and avionics!
an earlier heavy lift program
The reason here is the obsolete labor-intensive methods of manufacture, combined with a complex structure needed to absorb the forces generated by the side-mounted engines. Clearly a cheap SDV-4 needs new lightweight first-stage tankage designed to be as simple as possible and fabricated mostly by robots instead of people. The Delta 4 tankage might be a good starting point.
Also, it is not clear to me that the parallel-staging concept, with the core engines burning hydrogen all the way up from ground level, is any better than the Saturn configuration with LH2 reserved for upper stages.
Like the SRBs, this was a last-minute compromise forced on the Shuttle designers for political reasons. Engine weight is reduced, but the core engines must have a compromise nozzle design which reduces their performance, and more tankage weight needs to uselessly be boosted all the way to orbit.
The side-booster configuration sacrifices an important reliability and safety feature. Since the fuel load is distributed in several separate tanks, any engine that fails kills the mission even if it is shut down safely. The monolithic stage can burn all its fuel in the surviving engines and usually reach orbit.
So for SDV-5 we will adopt a first stage based on Atlas V technology, and a second stage based on Delta 4. With a little care it will still fit on the old Saturn V launch facilities. It will be cheap, reliable, and crews can escape from a launch failure with the good old escape tower.
The result of this little exercise in back-of-the-envelope engineering shows the complete absurdity of a SDV. The only workable members of the family are SDV-5 or SDV-4, and these are not really “Shuttle-Derived Vehicles” because they no longer retain any element of the Shuttle.
These designs are really EELV-Derived Vehicles. The EELV program has provided NASA with a true second generation of chemical rocket technology, free of many of the bad features that earlier space boosters inherited from the military missiles of the 1950s.
It would be absurd not to use it. Why should we perpetuate the 1960s technology of the Shuttle, which has proven too complex, unreliable and expensive even for short missions in LEO?
There are two hidden reasons that the SDV idea gets so much attention. One is employment. There will be strong political resistance to cutting back the huge Shuttle workforce. To Congress, NASA is a middle-class entitlement program and the large amount of money spent on Shuttle salaries is actually a large part of its political strength.
The other reason is the “Not Invented Here” syndrome. The EELV program was entirely managed by the Air Force and has provided the final proof that NASA’s own booster design group at MSFC is unnecessary as well as incompetent. Naturally these people will fight against the adoption of blue-suit technology that would make them redundant.
The bigger question here is whether even EELV technology can make launches cheap enough that the ambitious goals of Plan Bush can be achieved at a reasonable cost. I’m not sure that this is the case. But I am sure than any booster based on the failed technology of the Space Shuttle will certainly be too expensive and dangerous to be the basis of a viable manned Moon or Mars program.