Taking the initiative: SLI and the next generation (part 2)<< page 1: booster parachute recovery FlybackLying directly on the development path towards both Second and Third Generation systems, the third category of recovery techniques encompasses boosters that are either flown back to the vicinity of the launch site, or to a target area for semi-controlled recovery, or full recovery via runway landing. As such, it is certainly the most advanced, and also the most interesting, as it includes a significant divergence based on the architecture of the launch vehicle being considered. One concept involves the use of a vertical takeoff and landing vehicle (VTVL), performing a powered descent using the craft’s rocket engines to a gentle, tail-first touchdown reminiscent of the lunar landing, as well as most bad science fiction movies in the 1950s. To quote Robert Zubrin quoting Max Hunter, it is the technique “as God and Robert Heinlein would have wanted.” This was the approach used by the Strategic Defense Initiative Organization (SDIO) and McDonnell Douglas to startling effect in the DC-X experimental vehicle in the early 1990s.
Built as a proof of concept demonstrator for single-stage-to-orbit systems being contemplated by SDIO, the DC-X demonstrated not only the fact that VTVL was quite viable, but also what can be accomplished by a small organization with a focused, lean development approach. Under the auspices of SDIO, the DC-X was developed for $60 million and flown with a minimal ground crew. Prior to a landing accident caused by simple human negligence after control of the program was passed to NASA, the DC-X demonstrated not only the powered landing concept, but also key flight transition operations, as well a ground handling simplicity that allowed two flights in one day, even though it was a hydrogen/oxygen system. It is worth noting also, that the DC-X was powered by a set of four venerable RL-10 engines, whose proven record and long flight history allowed a degree of assurance that would have been impossible if the program required a new engine. Like SpaceX after it, the DC-X team reaffirmed the fact that the cost of vehicle development, while never cheap, is heavily influenced by the nature and priorities of the organization involved. While questions remain about the ultimate lift capacity of a VTVL system due to the amount of fuel consumed in achieving a safe landing, the concept itself is still evolving as Armadillo Aerospace and Masten Space Systems have embraced it for their own suborbital efforts. While VTVL was not the architecture chosen by NASA for its own very expensive, and ultimately failed SSTO program, the X-33, it seems likely the DC-X experiment at least has step-children. Specifically, it is the method outlined in a recent patent application by Jeff Bezos’s Blue Origin, except in this case, the first-stage booster would land on a platform at sea placed in the vehicle’s natural descent flight path. Blue Origin’s commitment to secrecy does nothing to shed much light on the subject, as well as whether this is intended as a serious operational proposal. If so, this would perhaps indicate a powered landing that sought to conserve fuel by occurring downrange, thus eliminating the fuel required for flying back to the launch site. What little has been released regarding Blue Origin’s Goddard test article looks very much like the DC-X, but the company has cautioned it may not be representative of the final product (see “Blue is a little less black”, The Space Review, February 22, 2010). Winged flybackIt is impossible to adequately discuss flyback boosters without considering how important they were to the people who designed and developed the shuttle system, and who sought to evolve and improve it over the years. For better or worse, the shuttle system has devoured that share of national treasure going to human space exploration over the last four decades, and to the extent that designers looked at reusable first stage options, they were looked at seriously, with consideration to the existing hardware, infrastructure, and operations. The fact that in 2011 Congress seems adamantly set on continuing to utilize much of the existing shuttle infrastructure for a new heavy-lift vehicle almost shouts that due consideration be given to those options for reusability that might still be incorporated within current demands and alleviate the crushing expense that is almost certain to follow. As we tend to forget, what eventually took shape as the partially-reusable shuttle system that has been part of our lives for the last 30 years was originally conceived as a fully-reusable two-stage-to-orbit launch vehicle. The system’s first stage would essentially be an enormous rocket plane; very large, piloted, and fully reusable. While in retrospect engineering such a vehicle in the first effort at creating a reusable launch system seems a very dubious, almost fantastic proposition, perhaps it was merely a matter of perspective. For a people that had just built the Saturn V and flown it to the Moon, achieving low Earth orbit with a set of wings and heat management system may have paled by comparison. Despite the challenges, the original, fully-reusable two-stage concept survived the first two rounds of proposals in the development of the Space Shuttle, and was only abandoned reluctantly when budget cuts in 1971 made it clear that simultaneous development both a reusable orbiter and a reusable booster to loft it was out of the question. As the shuttle program closes out, and all its success and failures are consigned to histories and museums, it bears emphasizing that the planners were true believers in full reusability. The budget-driven cutbacks forced a design that most grew to love, but very few wanted in the beginning.
Elimination of the reusable flyback first stage generated a new series of studies to determine what should take its place. Concurrently, a further round of budget-driven changes identified potential development cost savings from building a smaller orbiter with external, rather than internal, fuel tanks. The corresponding tradeoff meant the fuel tank would be expendable, significantly increasing recurring flight costs, but rendering a more manageable orbiter development project. The final remaining major decision was whether to require an inline staging configuration or to place the orbiter into the now-familiar piggyback style parallel arrangement, which offered the advantage of confirming main engine ignition and satisfactory operation prior to liftoff. As planners worked through three primary options, the reusable booster concept endured in the form of pressure-fed liquid strap-ons for the parallel configuration, or a recoverable variation of the Saturn V first stage for the inline configuration. In retrospect, this interlude offered a real opportunity for establishing reusable liquid boosters as a viable option for standalone unmanned rockets forty years ago. Regrettably, however, with NASA policy fixated on the shuttle as the only acceptable future launch provider, any justification based on the added benefit of getting two systems for the price of one was never going to see the light of day. In the aftermath of Challenger, one might have expected liquid-fueled boosters to earn a second look as safer alternative to SRBs, but this was not the case. The Shuttle instead resumed flight as soon as possible with the redesigned Solid Rocket Motors, while also undertaking a costly and futile diversion with the Lockheed-Aerojet Advanced Solid Rocket Motor project, which burned through nearly a billion dollars before being abruptly cancelled. However, nearly a decade later, NASA was to take one last look at the reusable boosters many had wanted from the beginning. As part of a Shuttle Upgrades Feasibility study, NASA in 1998 asked Boeing and Lockheed Martin to study liquid-fueled flyback boosters as a replacement for the SRBs. Previous considerations of liquid-fueled boosters concentrated on utilizing the SSMEs as the power plant due to the fact that it was the only large reusable engine available. The shuttle engines, however, tended to constrain booster recovery to parachutes, because the large hydrogen fuel tanks they required were deemed incompatible with winged flyback. This time, however, the new liquid-fueled boosters would be powered by fully-reusable oxygen/kerosene engines. With smaller tanks to contain the denser kerosene fuel, it would be possible to install wings as well as air-breathing jet engines to power a return to a runway landing at KSC. Among the three engine options considered by both participants, the only one actually available was the Russian RD-180, which at the time would soon be flying on the Atlas III. The immediate availability of a suitable engine allowed for a serious, detailed proposal that concluded in part, “there are no technology breakthroughs required to design or manufacture the LFFB vehicle.” The breakthrough that was required, but was not forthcoming, however, was in the seemingly unrelenting commitment to ATK solid rocket boosters manufactured in Utah. Given the ASRM debacle, it seems anything other than the status quo stood little chance. This was doubly unfortunate, because the Lockheed Martin proposal’s conclusion went on to state, “the vehicle can provide a cost effective reusable alternative to conventional, expendable liquid or solid strap on booster systems for first and upper stages.” Once again, it was not to be. The 1998 Upgrades studies marked NASA’s apparent last gasp at incorporating LFFB’s into the shuttle system. Another flyback concept with direct relevance for an existing launcher began with Buzz Aldrin and the Atlas III rocket in the 1990s, but it is a story that may yet offer a number of surprises. The initial proposal, championed by Aldrin, involved placing the pressure- stabilized, thin-skinned Atlas III rocket inside a hollow aircraft shell to serve as the thrust structure for a reusable launch vehicle. Promoted through a company called Starcraft Boosters Inc, this reusable Atlas would serve as the basis for a family of small to medium-sized launchers. As envisioned, however, the winged Atlas did not offer promising results in terms of total payload capacity, and the idea went nowhere, although it was later updated to include the Atlas V configuration.
Regardless, the proposal centering on the Atlas strikes at the very center of one of the thorniest issues regarding reusable launch vehicle development, one that had been highlighted by numerous people with varying degree of frustration. As winners of the Evolved Expendable Launch Vehicle program, with a guaranteed customer in the US government, and even the faint threat of competition removed by their joint venture, neither Lockheed Martin nor ULA partner Boeing have had any real incentive to pursue reusable first stage options, no matter what the result of their own feasibility studies, until outside circumstances forced them to do so. That is, at least until SpaceX showed up and ruined the party. As far as lowering launch costs are concerned, SpaceX has gleefully let the genie out of the bottle, and it is not likely to climb back in. If SpaceX avoids a major launch mishap, it is going to be increasingly hard for ULA to justify Atlas and Delta launches at twice the price of a Falcon, no matter how covert they are. Moreover, if SpaceX actually succeeds in its effort to establish the Falcon 9 as even a partially reusable launch vehicle, and still lower prices, then ULA is going to need a lot more than three wishes. Perhaps with that in mind, Lockheed Martin, with little fanfare, has conducted several small-scale test flights of a vertical-launch, horizontal-landing winged rocket at Spaceport America in New Mexico. Apparently tests of the rather modest 2.4-meter-long (eight-foot-long) vehicle were mostly successful, although with one confirmed failure. In what is probably not an unrelated development, Aviation Week in April 2010 reported that the Air Force Space and Missile Systems developmental planning division had commissioned a study of flyback boosters. As reported, the program would include flight testing of a key “rocket back” maneuver as a prelude to a proposed development program through 2025, with final phaseout of EELVs in 2030. The plan, however, would depend on joint R&D with NASA of a new, large reusable hydrocarbon engine, along the lines of the RS-84, which was itself a product of the Space Launch Initiative. It is just this sort of engine that NASA and the Obama Administration wanted to study for the next five years before committing to full-scale development. If fully pursued, the reusable launch vehicle program would, in effect, set an expiration date on the expendable booster, at least for medium lift. Arriving as it would, only 32 short years after Boeing and Lockheed Martin concluded that a similar vehicle was achievable with technology existing at the time, one can only ponder what might have been. Harry Chapin knew: “she was going to be an actress, and I was gonna learn to fly.” In this case however, one tends to wonder who has really been acting. So what are we to make of the various proposals to integrate reusability into the space launch systems which comprise today’s offerings? Just because a proposal is made does not mean it makes sense. On the other hand, just because a proposal was not pursued doesn’t mean it did not make sense. The history of US launch vehicle development is highlighted by some spectacularly shortsighted decisions that imposed long-term costs. Once again, in the rush to field a heavy lift vehicle as soon as possible, it seems some would have us make the same mistake all over. Clearly there are a number of alternatives for incorporating First Generation reusability into whichever family of vehicles is selected. Failing to adequately consider the range of possibilities and find a way to make one work to lower launch costs is tantamount to giving up, not just on the concept of reusability, but on the realistic prospects for a sustainable program of exploration. This is not to say reusability is a panacea. The unique challenges of space will always demand expendable systems as the answers to some problems. Nevertheless, if history is any guide, whatever heavy-lift vehicle finally rises out of ongoing political storm,is likely to be with us for a long time to come, provided, of course, we can afford to keep it flying. Home |
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