Suborbital spaceflight: a road to orbit or a dead end?
by Clark S. Lindsey
|Many in the aerospace world found little of significance in the SS1 or in the dozens of other suborbital RLV projects currently in development.|
In addition, from the famous rocket equation you can determine for a given propulsion efficiency what percentage of a rocket’s initial mass must be fuel to reach a given speed. For a single stage suborbital X Prize-type RLV, the fuel will take up about half the initial mass. For a single stage orbital RLV, the fuel will take up about 90% of the initial mass. The remaining 10% of the mass that makes it to orbit includes the structure, the avionics, the recovery system to deorbit, reenter and land, and all the other essentials for operating the ship plus, finally, the payload. Staging improves these percentages, but it’s still a tough haul.
For the critics these numbers say it all. The scale of difficulty to reach orbit is so much greater than it is to reach suborbital space, a suborbital vehicle can contribute very little to the development of hardware for orbital launchers.
Let’s look, however, at the space transportation challenge in greater detail. A list of all the phases of flight that a reusable orbital launcher must successfully transit is truly daunting. Such a vehicle will takeoff, either horizontally or vertically depending on the design, and accelerate to supersonic and then hypersonic speeds. It will transition from ground level atmospheric density to hard vacuum. If it’s a multi-stage vehicle it will need to deal with the dramatics of staging at high speed and the return of the boosters. It must reach and maintain orbit and maneuver in the space environment. If crewed, it must contain a life support system. It will fire a deorbit thruster, reenter the atmosphere and undergo very high surface temperatures. It must decelerate from hypersonic to supersonic to subsonic, and then land at a specific spaceport, or at least within a designated area. Practical, low cost access to space requires that the vehicle repeat all of this in as short a time as possible. The first generation commercial RLVs will probably achieve weekly turnaround and later generations will carry out daily flights. They must, of course, do all of this safely and reliably.
The critics say in effect that it is more sensible to go directly to a vehicle that can do all of this rather than focusing first on the lower level phases. This doesn’t sound like a sensible approach to me but I’m not an aerospace authority. So I decided to contact a small sample of experts in and out of the suborbital industry and see what they say. I report below on the responses to the following question:
Could you list the top 3 or 4 reasons why you think (or don’t think) that the development of commercial suborbital RLVs will contribute directly or indirectly to the development of orbital RLVs?
Pat Bahn has been a champion of suborbital RLV development for many years, long before it became a topic on the pages of Newsweek and the Wall Street Journal. His company TGV Rockets, which has recently shifted to hiring mode, plans to build a large suborbital vehicle aimed particularly at the remote sensing and reconnaissance markets. He relates suborbitals to the development of the personal computer:
My basic feeling is that suborbital has implications for orbital development in the same way that the 8-bit micro-computer from Altair had implications to the mainframe computing industry.
Micro-computers served to train whole legions of students on binary arithmetic, Boolean algebra, logic, programming, hardware design, board design, product maintenance, meanwhile creating an entire industry that, with internally generated R&D, developed billions of dollars of new technology ultimately surpassing the mainframes.
|Suborbital has implications for orbital development in the same way that the 8-bit micro-computer from Altair had implications to the mainframe computing industry.|
He goes on to note that the huge mainframes were for a long time not threatened technically by microcomputers but eventually the mainframes went away almost entirely, to be replaced by server farms of commodity boxes, and small data warehouses, with a little climate control and surveillance cameras. Similarly, suborbitals will never affect the technical base of the orbital ELV, but it will become better in whole new ways and change the rules of the game.
He then lists the ways that the game will change:
Len Cormier has been in the launch vehicle design business since the 1950s. He is currently head of the company Tour2Space and leads the PanAero X Prize team. He also consults with TGV and helped to design their vehicle. However, despite his work with suborbital projects, his heart is really in orbit:
I am not a strong fan of suborbital, since 100 km at relatively low speed is a place to avoid when trying to get to orbit. Suborbital—low-delta-vee suborbital—is not likely to contribute much technically to an orbital space transport.
Nevertheless he pursues suborbital projects for the following reasons:
Dan DeLong is Chief Engineer of XCOR Aerospace and one of the founders of the company. It was Dan’s Long-EZ, in fact, that became the EZ-Rocket. Dan is more optimistic about the hardware benefits of suborbitals and why they can evolve into orbital systems. First, they allow for incremental and frequent testing. The test flights can focus on one new thing per flight and safe recovery is easier at low altitudes. Over time the evolution of the vehicle designs grow until orbit is reached.
|The oft-quoted 25X energy requirement for orbital flight is a misleading number. Attaining high vehicle energy is largely a function of mass ratio and specific impulse.|
He points out that incremental development is the normal course for new technologies. The first airplanes were barely able to carry the pilot at speeds slower than other contemporary forms of transportation, but nobody thought design should stop until transatlantic flight was viable. He notes that previous programs began with suborbital tests: just as Mercury Redstone sent Alan Shepard and Gus Grissom on suborbital flights before the Mercury Atlas orbital flights, so will low-cost, robust commercial suborbital flight operations lead to low-cost commercial orbital flights.
He then addresses the energy scale objection:
The oft-quoted 25X energy requirement for orbital flight is a misleading number. Attaining high vehicle energy is largely a function of mass ratio and specific impulse. If the energy requirements can’t be met, adding stages has been the traditional way to get the job done. Similarly, many aspects of an orbital reentry thermal protection system (TPS) can be tested on a suborbital vehicle. Real-world operations in rain, repeated heat/cool cycles, and maintainability aspects of TPS can be tested suborbitally. Certainly, the peak temperature profile will not be modeled, but this is not the only requirement for robust TPS.
With regard to infrastructure capabilities, he says that a company developing a suborbital will benefit from the training gained by first tackling suborbital flight. Such teams then have the best chance to move on to orbital systems. Reusable vehicle developers get practice designing materials and systems for the space environment. Furthermore, by developing real hardware, as they did with the EZ-Rocket, they can offer much greater confidence in predicting the costs of the next generation system such as the Xerus.
Finally, he notes that in fact a suborbital RLV can be part of an orbital system:
An operational suborbital RLV will make a good reusable first stage for an ELV upper stage. This flyback booster will cut the cost of getting near-term satellites into orbit. One might argue that this IS an orbital vehicle, performing an orbital mission.