The Xerus, a proposed suborbital RLV by XCOR Aerospace, will benefit from an incremental approach the company is taking. (credit: XCOR Aerospace)

Suborbital spaceflight: a road to orbit or a dead end?

<< page 1: the EZ way to orbit

No, well maybe, well actually yes

Henry Spencer is renowned for his encyclopedic knowledge of rockets and spacecraft, past and present. The regulars on the space newsgroups know they can count on him for clear and well-founded answers to questions on space topics ranging from the basic to the extremely technical and esoteric. To my question he answered: “no, well maybe, well actually yes”.

To the negative he notes:

It’s certainly true that getting into orbit is a much harder problem. In early-aviation terms, it’s the difference between barnstorming and flying the Atlantic. Many of the same technologies are applicable, but the numerical requirements are much more demanding. Even a company that has built and is operating a suborbital RLV will have to design a completely new vehicle for orbital operations, and the orbital vehicle will probably share only incidental bits of hardware with the suborbital one.

But then he says maybe there are benefits:

Bear in mind that a 100km apogee is not a fundamental limit on suborbital flight. When you start pushing for higher altitudes, and possibly also longer ground distances covered, the problems get harder and the disparity between suborbital and orbital shrinks. For example, you start needing real TPS; the TPS systems used for the first orbital vehicles (capsules) were simple derivatives of those used on suborbital vehicles (ICBM warheads). The question about this approach is whether the greater performance enlarges the suborbital market enough to pay the development costs of the hotter vehicles.

For the affirmative he offered four benefits from suborbital development:

1. Company credibility. This business [consisting of private startup launch companies] has always had a problem with convincing potential investors that you can actually deliver on something innovative, especially given the tendency of certain large companies and government agencies to exaggerate the difficulties. Delivering and operating a suborbital vehicle as promised will go a long way toward establishing technical credibility for later promises.
2. Development and operations experience. Granted that a suborbital vehicle is different from—and easier than—an orbital one, a team which has done the former will have a much better idea of how to do the latter right. Studies and viewgraphs are no substitute for experience.
3. Flight-test options. Being able to test orbital-vehicle systems in space on a suborbital vehicle will be a significant advantage. Some will be upgrades to the suborbital systems, while others will have to fly as payloads, but being able to try things out in their real operating environment—even briefly—will help a lot.
4. Regulatory experience. A company that has established to the FAA’s satisfaction that it can build and operate suborbital vehicles safely is going to have a rather easier time convincing the FAA that it can do orbital vehicles safely. Issues like reliability cannot be settled by reasonable numbers of flight tests; they are fundamentally a question of confidence in the company and its engineering process. Doing it once will make it much easier to do again, and will also greatly simplify convincing investors that you’ll be able to do it again.

With regard to the critics like Bond and Pike he says:

I think these folks are making a fundamental mistake. They’re assuming the traditional model of rocket development, where huge amounts of money are poured into building a system with absolute maximum performance, and into making “certain” that it will operate perfectly the first time. While this approach has been standard in the past, it is horribly expensive… and worse yet, it doesn’t actually work very well.

Notice that their objections are essentially on technical grounds, where none of the four points I make above is really technical. In the old model—government-funded development of artillery rockets technical problems dominate. In the harsh, cold real world that commercial rocket projects face today, the technical problems are not the hard ones.

Making markets

Mark Oakley is a Senior Engineer at Lockheed Martin’s Engineering Propulsion Laboratory and has worked on many rocket projects including the Atlas 5. Mark is also known for his Rocket Man Blog website where he writes about the challenges of designing launch vehicles and often reviews specific vehicles, including many of the X Prize vehicles. He believes that suborbitals will benefit efforts to reach orbit because:

Large companies are not very good at creating new markets. What they are good at is exploiting existing markets. As long as the launch vehicle market remains exclusively the province of large companies and governments, the existing uses for launch vehicles as satellite launchers and space station taxis will not be added to anytime soon.

Private commercial suborbital RLVs, on the other hand, are being developed to create new markets or exploit existing markets that the large companies are not servicing very well. Space tourism and suborbital missions are just two of these markets, but vehicles that can fly anywhere in the world in around two hours would open up new opportunities like fast passenger/package delivery or military bombers that could be based in the United States and quickly reach anywhere in the world.

He also believes there will be technical benefits:

The routine commercial use of suborbital vehicles would in turn contribute to the development of orbital vehicles by maturing the technologies needed for such vehicles and by creating the infrastructure needed to build and support them.

I then asked him, as an example, whether a company that first develops a 30,000-lb thrust engine and proves its reliability over hundreds of suborbital flights would not be in a better position to develop a 300,000-lb. engine for an orbital vehicle?

I have given some thought to your question and I have to say at first I thought yes, definitely. But after contemplating it for a while, I have to say I’m not sure. I think any experience with making a reusable engine would help in making another reusable engine, but I don’t think size necessarily matters. Large engines scale pretty well and the cost of designing/testing/building a 30k engine would not be orders of magnitude different from making a larger engine, although the smaller engine would definitely be cheaper. This smaller cost would allow you to spend more time (assuming the same budget) developing the smaller engine than the larger engine, but I don’t think developing a smaller engine is necessarily a prerequisite before you develop a larger engine. The main argument for developing the smaller engine first is that it can be done cheaper than starting with the larger engine and thus has a higher probability of actually being built.

Summary

So I found that some experts believe that suborbital RLVs will directly benefit the development of hardware for orbital RLVs but not everyone agrees. The disparity in scale of energy between the two regimes is so big that it’s difficult to point to a specific component on an orbital vehicle and say that it will definitely become more reliable and robust by first perfecting it for suborbital flight. Instead the argument is usually that suborbital vehicles offer the first step in an incremental chain of hardware development that eventually leads to hardware suitable for an orbital vehicle.

There is broad agreement, however, that low cost, reliable suborbital RLVs will create an infrastructure on which to build both orbital RLVs and a commercial human spaceflight industry. Such an infrastructure needs the development of many sectors. It needs engineers, technicians, and managers who know how to build vehicles that meet the demands of a market rather than those of government bureaucrats. It also needs people who know how to operate a space transportation system like an airline rather than like a research project. It will require profitable companies that reinvest profits into increasingly capable vehicles that eventually reach orbit. It will need a group of investors who are knowledgeable about commercial human spaceflight and who are rewarded for their investments on the short term as well as the long term.

The suborbital industry will also nurture and develop markets, especially space tourism, which will initially be too small to support the costs of developing orbital vehicles. By starting small, the vehicle companies can grow as the markets grow and can build each subsequent generation of vehicles from the profits made from the previous ones. This means that reaching space will be self-financing and not rely on the vagaries of government funding.

Finally, what is great about suborbital projects is that they are cheap enough to proceed regardless of what critics say. Real vehicles will be flying to 100km soon and we will find out empirically in subsequent years whether they were the first step on a road to orbit or whether they led to a dead-end at the shoreline to space.

Clark S. Lindsey is the founder and editor of HobbySpace.com and RLV News.

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