The Space Review

 
EADS Astrium spaceplane design
Rather than evolve current suborbital spacecraft designs into point-to-point vehicles, it may be technologically less challenging to develop orbital RLVs first. (credit: EADS Astrium)

Point-to-point suborbital transportation: sounds good on paper, but…

In the past few years, there has been a resurgence in discussion of the use of rocket-powered vehicles for intercontinental passenger or cargo flights, for both military and commercial applications. The Air Force-DARPA “HOT EAGLE” and Marine Corps “SUSTAIN” projects, Virgin Galactic’s talk of potential use of SpaceShipTwo or derivatives for this role, and the nascent “V-Prize” for a suborbital flight from the East Coast of the US to Europe are the latest incarnations of an idea that goes back to the early science fiction of Heinlein and others. A recent article by Taylor Dinerman (“Point-to-point suborbital spaceflight and military logistics”, The Space Review, April 14, 2008) focused on military applications and their potential to pave the way for commercial use of rocket transports. Many see such vehicles as logical steps in a progression from suborbital tourist vehicles, to long-range transports, to eventual reusable orbital launch vehicles. But while a suborbital transport would intuitively seem to be easier to develop than an orbital vehicle, the reality is that it would be neither significantly easier nor significantly cheaper.

At first blush, it seems like a great idea, and a logical next step in the development of aerospace transportation capabilities. Depending on the actual range, the time of flight for a trip by an intercontinental rocket transport would be less than an hour. Certainly, there are high-value cargo and passengers for which this capability would be desirable, especially for military situations. Federal Express and UPS, among others, have shown how strong a market could be developed for the express delivery of goods around the world. And of course, such a vehicle—if developed by the military—could serve as a close analog of what the Boeing B-47, B-52, and KC-135 programs did in preparing the way for successful development of the 707 airliner and its cargo versions.

While a suborbital transport would intuitively seem to be easier to develop than an orbital vehicle, the reality is that it would be neither significantly easier nor significantly cheaper.

On closer inspection, however, there are some serious problems with the idea of point-to-point suborbital transports. Especially for military applications, there would be a number of operational difficulties. The idea of sending a rocket transport anywhere close to a battlefield situation seems dubious at best. The infrared signature of such a vehicle during reentry would make it easily detectable and quite vulnerable to surface to air missiles. Furthermore, operation in forward areas of a rather fragile, thin-skinned vehicle would make it particularly vulnerable to enemy fire. It would be exceedingly difficult to build a very weight-sensitive rocket transport that could absorb a lot of punishment.

Then, even if you were successful in landing it where you wanted it, the amount and types of propellant that would be required to fly it out again would be problematic. If you actually were operating in a forward area, the ready availability of the necessary refueling support would be unlikely. Carrying enough for the return trip would require a very much larger vehicle, which would in turn drive up weights of other subsystems such as the landing gear. Any requirement to preposition propellants and equipment would defeat the primary advantage (quick reaction) of a rocket transport. Designing the vehicle for limited flyback capability would only lessen, not eliminate, these issues.

Operation into military airfields well to the rear of combat zones would be more practical, with reduced threat levels and less difficulty in providing and maintaining the refueling and other support required. But then the question becomes how much materiel really needs to be at some rear echelon airbase within an hour? There are other ways to get critical cargo or personnel there within a day or so. Given these operational considerations, it would seem that rather than trying to predict the true military utility of new, unproven rocket transport vehicles a priori, some previous operational experience with such vehicles would be essential. And that remains one of the problems facing the entire NewSpace community: we still don’t have any rocket vehicles of comparable size or complexity operating on a routine, or even close to routine, basis. We are still guessing and conjecturing about what real reusable launch vehicles will end up looking like. It would seem to be imprudent to embark on such a major development program without some additional knowledge and experience.

Then there are the technical challenges. First and foremost, the velocity required for intercontinental ranges, and hence the “delta-V” that the vehicle would need to achieve, would be uncomfortably close to orbital velocities. For example, a flight of about 10,000 kilometers would require a delta-V of over 7,300 meters/second, which is already about 80% of that required to reach low Earth orbit. Thus, the mass ratio required would begin to approach that necessary for a single-stage-to-orbit vehicle. Likewise, the amount of propellant required would be significant. And while suitable propellants for such a vehicle (probably LOX/hydrogen or methane) are reasonably inexpensive, it would add up for a vehicle that must attain near-orbital velocities. Another consequence of these high velocities would be the need for a very robust thermal protection system (TPS), again very close to what would be required for an orbital vehicle. During my involvement with a hypersonic vehicle program in the late 1980s, the rule of thumb was that once you got over about 5,000 meters/second, the difference between that and an orbital reentry environment were small.

The bottom line is that a useful suborbital transport would require many of the same design features of an orbital vehicle. The TPS; the high-performance rocket engines; the amount of propellant that would need to be carried; the guidance, navigation, and control systems; as well as the launch support infrastructure would have it looking a whole lot like an orbital RLV. The cost per pound of cargo or payload, then, would be expected to be very close to that of an orbital RLV. Clearly, costs in the range of today’s $1,000–10,000/pound ($2,200–22,000/kilogram) to LEO (the lower figure being an estimate of what the Russians could hit if they really wanted to) would make a suborbital transport prohibitively expensive. Even the oft-touted $100/pound ($220/kilogram) does not compare at all favorably with the $5 to $10 that it now costs to ship a pound halfway around the globe via FedEx.

It’s a compelling idea: that the development of a suborbital, military rocket transport could provide a springboard for a similar commercial derivative, and ultimately for the development of low-cost orbital RLV’s. However, it seems that “antipodal rocket transports” carrying large quantities of goods and people around the globe are likely to remain in the mythical “helicopter in every garage” future that has yet to materialize. Even for military applications, the costs and complexities of operating such vehicles seem likely to rule them out. Any military market would undoubtedly remain small: just look at how many latest-generation jet fighters are procured nowadays. And even if military requirements could be found to justify the development and operational costs of a rocket transport, it would be unlikely that any surplus or derivative vehicles would be attractive for commercial operators. You don’t see any C-5’s or C-17’s in commercial service. Much has changed since thousands of C-47’s were snapped up by private air carriers after WW2.

Given the fact that we already have pretty effective ways of moving things from point A to point B around the globe, and given the level of performance that would be required for a useful suborbital transport, it doesn’t appear that rocket transports offer a particularly helpful stepping stone on the way to orbital RLVs.

Unfortunately, there don’t appear to be any technological “magic bullets” that would alter this analysis. Yes, there is room for improvement over current TPS technologies, but even that wouldn’t alter the fact that an orbital-class system would be needed. Robust, reliable, and reusable rocket engines are certainly necessary, but we do already have pretty good engines such the Pratt & Whitney RL10, with ongoing work on derivatives and upgrades. In the area of structures, composites are routinely held up as offering a way to make major strides in vehicle mass ratios, but we are already pretty darn good at making lightweight propellant tanks out of aluminum.

Speed is expensive, whether you are talking about a military transport or a high-end sports car. We certainly have had the capability to build supersonic transports for decades, but only the Concorde saw (heavily subsidized) commercial service; none have ever been developed for military applications. Given the fact that we already have pretty effective ways of moving things from point A to point B around the globe, and given the level of performance that would be required for a useful suborbital transport, it doesn’t appear that rocket transports offer a particularly helpful stepping stone on the way to orbital RLVs. For the foreseeable future at least, it seems that military programs in this direction would be unlikely to spur private investment in the development of derivative commercial vehicles.

Perhaps it would be better then to skip over the suborbital concepts, for now at least, and to focus any military RLV development work, and related commercial developments, on reusable orbital transports. A well-conceived military effort aimed at a reusable orbital transportation capability would be much more likely to lead to commercially viable derivative vehicles. The capability to get payloads and people into orbit, quickly and at low cost, would really provide a major jump over our current aerospace transport capabilities. After more than 50 years of spaceflight, we still don’t have that for military or civilian uses. It just might be that development of and operational experience with orbital RLV’s needs to pave the way for any future, point-to-point rocket transports, rather than the other way around!


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