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© 1994 by Donald F. Robertson.



This article may be distributed at will, but only if it is not changed in any way, and only if the author's name, the copyright notice, the name of the journal it first appeared in, and this notice remain attached. In addition, this article may not be sold for money, or published for sale in any way, without the author's prior written permission.

This article originally appeared in very different form in Astronomy Magazine.

CHEAP SPACEFLIGHT PROMISED AGAIN: is single-stage-to-orbit the answer?


Donald F. Robertson

There is no obvious reason why traveling to orbit with a chemical rocket needs to be expensive. Chemical rocket fuel is cheap. Rockets, in theory, are simple devices. But spaceflight remains one of Western civilization's most expensive activities. Why?

The problems are twofold. First, our planet Earth happens to be the wrong size. It takes very close to the maximum possible performance that a chemical rocket can achieve just to lift a fuel tank, rocket engines, vehicle structure, and the diminishing quantity of fuel all the way to orbit.

The solution to that problem causes the second. The fuel tank is divided into segments, and each section of the tank is thrown away when it's fuel is exhausted. The vehicle's weight is kept as low as possible at each point in its flight. By staging a rocket, its performance is improved to the point where a useful payload can be delivered to orbit -- albeit usually just a few percent of the total vehicle weight.

Dividing the vehicle into stages, plus the need to nurse every last drop of performance out of the engines, vastly increases the complexity of the rocket. What should be a very simple machine becomes a temperamental gadget prone to failure. Modern launch vehicles require huge "standing armies" of expensive engineers and managers to tinker them into operation.

Besides staging, at least three other potential solutions exist to these problems. The performance of rocket engines can be increased to lift more weight with less fuel -- exemplified by the Space Shuttle Main Engines. But performance equals cost, and today's most efficient engines already yield fairly close to the best performance that chemical rockets can produce. Another idea is to reduce the weight and volume of the fuel that must be carried in the tank -- and in the process also reducing the weight of the tank itself. This is the approach of the National Aerospace Plane project. NASP hopes to replace the heaviest part of a rocket's propellant, the oxidizer, with atmospheric oxygen burnt in air-breathing engines. The size and weight of NASP's hydrogen fuel tank may also be reduced by using "slush hydrogen," where a percentage of the liquid hydrogen fuel is frozen to a solid, reducing its volume.

The third and perhaps the most promising approach is to reduce the weight and complexity of the entire vehicle as a system. This is being pursued by a new project sponsored by the Strategic Defense Initiative Organization. The program is called Single-Stage-to-Orbit by SDIO, or Delta Clipper by McDonnell Douglas, the prime contractor. [The project has since been re-named Clipper Graham, in recognition of an important proponent.]

The National Aerospace Plane, and other recent projects, have created a revolution in aerospace materials. Vast reductions in the weight of fuel tanks and other structures should now be possible, and can be applied to far more conventional launchers than NASP. Modern electronics also weigh far less than the avionics in current vehicles like the Shuttle. These technical revolutions may allow what has been considered largely impractical: a chemical Earth-to-orbit rocket without stages, a Single-Stage-To-Orbit vehicle, or SSTO.

A rocket without stages could be amazingly simple, making it inexpensive and reliable to operate. Ideally, it might be prepared for launch by a ground crew of a few engineers, or even just the pilots themselves. Ultimately, argue enthusiasts, operation of an SSTO should cost little more than a few times the price of its fuel, much like a modern airliner.

Those who remember the sad history of NASA's Space Shuttle wonder if they haven't heard these promises before.

* * *
Another innovation may prove far more important than anything involving the SSTO itself. The Strategic Defense Initiative Organization is experimenting not just with a new kind of launch vehicle, but with a different kind of management, one that many in the program see as a return to an older and better way of doing aerospace development.

SDIO has provided the project with $58.9 million for Phase-II development and testing. With that money -- and no more -- McDonnell Douglas must produce a single, working sub-scale vehicle within two years. This flight model will demonstrate high suborbital flight and airliner-like operations before the program has to approach Congress for larger amounts of money. The current funding comes out of SDIO's general research and development budget, and is not yet a line item subject to direct Congressional approval.

Delta Clipper, then, is an evolutionary program. It will test a sub-scale model before committing to development of a full-sized vehicle, unlike the revolutionary Space Shuttle project that tried to produce an operational vehicle from scratch -- and failed to meet most of its operational goals.

The one-third size test vehicle is called the DC-X, and is to be close enough to the operational DC-Y that it can be scaled up with relatively little risk. Many advocates argue that the entire project should cost little more than $2 billion -- about one-fifth of what most launch vehicle development programs cost.

Lieutenant Colonel Pat Ladner, SDIO's program director for the project, strongly denied to Interavia Space Markets that a cost to completion for the DC-Y has been determined: "I refuse to talk about cost estimates. Because anybody [who] will tell you that they have a cost estimate on the SSTO, or they've got a good cost estimate on any other [aerospace research and development] program in this stage of the program, I think they're blowing smoke." After recent experience with the B-2 bomber, or NASA's Space Station, Congress should find this honesty refreshing.

In a management technique that is becoming increasingly common among American corporations, program staff is being kept very small to contain management costs, while the project contracts out for expertise. The Strategic Defense Initiative's "program office is two people," Dr. William Gaubatz, McDonnell Douglas' program manager for Delta Clipper, told Interavia Space Markets. About twenty-five other government employees at NASA and elsewhere are not paid from SSTO accounts. McDonnell Douglas itself has just fifty full time project employees, while engine contractor Pratt & Whitney has fifteen.

More importantly, the goals of the project have been kept strictly limited. Rather than try to produce a major technological advance -- like the National Aerospace Plane -- or create a giant launch vehicle to be all things for all people -- the Space Shuttle -- Delta Clipper is to be a medium launch vehicle, capable of just 9,000 kilograms to low Earth orbit (McDonnell Douglas appears to be aiming for 10,000 kilograms). Outside of the new materials, technology must remain as close as possible to off-the-shelf.

McDonnell Douglas' name for the vehicle reflects this simplicity, according to Gaubatz. "Delta" refers to McDonnell Douglas' current Delta launch vehicle, the smallest and simplest of the United States' major launchers; while "Clipper" was the name of a sixty-five meter 19th Century American sailing ship which traded throughout the world, but especially in Asia. (In 1854, the Clipper Flying Cloud established a sailing ship speed record that was only broken by a modern sailing vessel in 1989.) A Congressional source believes that the "DC" prefix for the SSTO vehicles is not an accident either, referring to McDonnell Douglas' early commercial airliners.

Delta Clipper is a small launcher shaped somewhat like an elongated Apollo reentry vehicle. In the project's prime innovation, it both takes off and lands vertically, using concepts and technology developed for the Apollo lunar lander. The key advantages of vertical take-off and landing are well known: such a launch vehicle does not carry heavy wings to orbit with every flight. The same powerful rockets that launch the vehicle when it is full of fuel and at its heaviest are used at low power to land the depleted rocket, removing the weight of a separate landing system. The ground system is much simpler, being little more than a cement pad; rail carriages or lorries, or an aircraft with fuel; and a crane. Delta Clipper can land at unprepared sites and in high winds, important for both emergency or military missions.

Pratt & Whitney's engines for the sub-orbital DC-X are based on the RL-10, the world's oldest high energy rocket. Despite a recent failure -- the RL-10's first -- it remains the world's most reliable engine. Four RL-10s, modified for sea-level operation, are mounted on each DC-X, said Jim Holloway, Pratt & Whitney's program manager for SSTO. The modifications involve much shorter nozzles and extended combustion chambers. The new sea-level RL-10s can be throttled from full thrust, to between fifty and thirty percent thrust; and they retain the old engine's restart capacity. Both these capabilities will be static tested, then "gang tested" as part of the propulsion system as a whole, and tested again during the vehicle's first flights.

The DC-X is steered by "a combination of throttling, the reaction control system of the vehicle, and a flap system," says Holloway. He expressed strong confidence that the DC-X's propulsion system can be developed within budget, since, "We are making use of a lot of existing hardware [and we] are not trying to get too exotic with at least the DC-X vehicle."

The operational DC-Y is likely to use entirely new, multiple engines -- probably eight -- each about one-third of the size of a Space Shuttle Main Engine so that enough engines can be used to give an engine-out capability. Two concepts were under consideration: a "plug nozzle", or conventional bell-shaped nozzles like most modern rocket engines.

A plug nozzle is a blunt cone surrounded by a number of rockets; combustion products from the engines expand against the cone, creating thrust. Plug nozzles are light and compact, and since the combustion products are confined by the surrounding atmosphere the nozzle automatically compensates for the changing air pressure as the vehicle rises. Conventional nozzles can be optimized for only one air pressure, and operate below their theoretical performance through the rest of their flight. The key drawback of a plug nozzle is that it has never been used in an operational vehicle -- just the kind of risk the SSTO project wants to avoid -- although Holloway said plug nozzle engines use relatively conventional turbo-machinery.

While a formal decision had not been made at the time this article was written, reports in the aerospace press suggest the DC-Y program will go with conventional nozzles, to reduce program risk and because the performance difference isn't terribly great.

Even with a conventional nozzle, "the Y vehicle probably is going to reach a size where you want engines well above 100,000" pounds [45,000 kilograms], up to 150,000 pounds [68,000 kilograms] or more," says Holloway, referring to the engines' thrust. Existing engines are unlikely to be appropriate for the DC-Y: it would be difficult to uprate RL-10s to provide 45,000 kilograms thrust; Space Shuttle Main Engines are too powerful to provide the engine-out capability the project demands; and Saturn J-2s might be difficult to adapt to throttling. Although Holloway did not state this, that appears to mean that a new engine has to be developed whether the DC-Y uses a new plug nozzle, or conventional bell nozzles, eliminating some of the difference in risk between the two. Holloway expects the total cost of developing the DC-Y propulsion system to be "several hundred million dollars" independent of which kind of nozzle is used.

Once in orbit, Delta Clipper releases its payload from a standardized container. On the ground, this container is removed from the vehicle and replaced with a new one. Payload processing and installation is done outside of the vehicle to reduce launch delays attributable to the payload.

Unlike the Shuttle, Delta Clipper is very light on reentry, having burnt most of its fuel getting to orbit. A lighter vehicle skips lightly into the atmosphere and the DC-Y does not need the Shuttle's delicate heat protection tiles. Gaubatz said, "We have two types of insulation depending upon the thermal environment. The upper area [around the nose] which is hottest during [the nose-first] reentry is carbon-carbon material; the aft sections are a conventional titanium structure."

Delta Clipper's nose-first reentry is unusual for a vehicle that may use plug nozzle engines. Most plug-nozzle designs enter the atmosphere plug first. They use the same heat-protected structure that the rocket's exhaust expands against during launch to absorb the heat of reentry, avoiding the weight penalty of protecting the nose. Gaubatz said, "We evaluated that concept early on in Phase-I and rejected it [because] operationally we feel it is important to have a good flyback cross-range capability. So, we have some 1,600 nautical miles [3,000 kilometers] of cross-range that allows you to always be assured of being able to return to your operating base, divert to other bases, and land at other sites." Interavia Space Markets pointed out that past capsules, like Apollo, entered base first, and asked if a nose-first reentry was not a risk that should avoided. Gaubatz responded, "We have a history of some thirty years of flying this type of configuration." Was this experience obtained with military reentry vehicles? "Yes," says Gaubatz.

First flight of the sub-orbital DC-X is planned for 1993, and that for the DC-Y some four years later. DC-X will be test- flown "in the same context as an aircraft development," said Gaubatz. "Our first flights will [hover] analogous to taxi test [with an airplane]; then we'll fly . . . a lift-off, a hover, and a translation to the landing site. We'll repeat that maneuver at higher altitudes. Then we go into a roll-over maneuver which puts us in the nose-forward position to simulate a reentry trajectory [followed by] a rotation maneuver for landing." DC-X capabilities will slowly increase in altitude and payload, but probably stay sub-orbital. The DC-X test vehicle will not at first have a thermal protection system over its graphite - epoxy shell, which puts a limit on the velocity it can achieve.

While neither the sub-orbital DC-X nor orbital DC-Y require a crew, both can carry a two-person module for missions requiring astronauts. "Our philosophy as well as [SDIO's] requirements," said Gaubatz, "is that this [will] evolve into a manned system. Alternatively, Gaubatz believes, "That same crew can [work] sitting in a virtual cockpit sitting on the ground at mission control." This involves a relatively new concept called "virtual reality", where astronauts work with a ground-based computer model of the spacecraft's cockpit. The model cockpit is connected by radio to the real cockpit in flight. Commands that the astronauts give to the virtual cockpit are duplicated in the real one, flying the vehicle as if the astronauts were actually on board.

* * *

If either the test, or the operational Delta Clipper, vehicles get built at government expense, could they be commercialized? Or could the project be completed as an entirely commercial venture?

Commercializing the DC-X test vehicle as a sounding rocket "is certainly a potential" said Gaubatz. Without being asked, Pratt & Whitney's Holloway independently brought up the same idea. The DC-X should lift a huge payload into a very high sub-orbital trajectory, with or without a human crew. Gaubatz said, "There are a large number of flights every year for sub-orbital payloads. The scientific community, NASA, and DoD [Department of Defense], have a lot of flights. . . . So, the interesting potential [exists] of having a totally reusable, sub-orbital system that could provide those same capabilities."

Gaubatz implied that it is too early to say whether the re-usable DC-X can compete financially with current, expendable solid fueled sounding rockets. But he added, "If you look through some of the brochures of current sub-orbital rockets, they are quite a totem pole of combinations of surplus solid motor stages. They're expensive, and you are at the mercy of the quality of the surplus stage that is given to you."

Further in the future, McDonnell Douglas clearly intends to commercialize any full sized Delta Clipper that results from the current research program. Gaubatz said, eighty to eighty-five percent of all civil payloads are 20,000 pounds or less. Some sixty percent of all the military payloads are in that same category." The company expects the vehicle "to replace the market that Delta has, and really the whole medium class market."

"It is in the interests of the government to help establish the United States as a very strong competitor in the world market," insists Gaubatz. It should also be in the interests of McDonnell Douglas. Interavia Space Markets asked how much money McDonnell Douglas might be willing to invest in Delta Clipper to gain and maintain a company leadership position in space transportation. Gaubatz declined to state clearly whether McDonnell Douglas will invest its own funds in the Delta Clipper, but strongly implied that they will not. He said his company had already invested $200 million in basic research and design tools -- which can be applied to many programs besides the Delta Clipper -- but was unlikely to spend more.

David Webb is not surprised. Webb heads the University-based International Hypersonic Research Institute and was a member of Congress' National Commission on Space. Developing Delta Clipper as a replacement for the Delta, says Webb, is "very sensible for McDonnell Douglas. God knows they need one. The only problem is, I don't see McDonnell Douglas putting up the money right now; they simply don't have it." McDonnell Douglas representative Anne McCauley declined to detail the performance of her company's space business, saying it is not separated out from other units, but she agreed with outside observers that space is one of the company's star performers. Other McDonnell Douglas units are said to be recovering from earlier production and financial problems, so the company's financial performance could be much improved in a few years' time. McDonnell Douglas might then be more willing to invest commercial funds in Delta Clipper, suggested several sources, if the government provided some form of market guarantees.

Pratt & Whitney's Jim Holloway said he "would not exclude" the possibility of commercial funding by his company. "It would be awfully easy for me to say, `Yeah, we would certainly do it,' but I think we'd have to have a cold hard look at it." He added, "The whole industry is pretty tight-fisted right now." But asked if he thought commercial development of Delta Clipper could be done, Holloway said, "That's correct."

* * *
In an era of unprecedented budget and social problems, why is the United States developing Delta Clipper? And why put the Strategic Defense Initiative Office in charge? What immediate SDIO requirement is Delta Clipper intended to fulfill?

The answer to that last question is far from obvious. A senior officer at SDIO, who did not want to be identified, admitted that there was none, although there could be many in the future.

Lander said that it is too early to talk about applications for Delta Clipper. "That is like having a three-month-old child in your arms and talking about how he is going to win the Olympics."

The SDIO source said his agency "was asked to do this" by Vice President Danforth Quayle's National Space Council and others. SDIO was chosen because it is the one government agency involved in research and development of space hardware that is not already working on a launch vehicle. NASA has their Shuttle; the Air Force Space Division is advocating the National Launch System; Wright-Patterson Air Force Base has the National Aerospace Plane, said the source, and the Council wanted an "un-biased" analysis.

Inter-office memos and other documents written by individuals who helped to sell the SSTO project to the Administration, and provided to Interavia Space Markets on the condition their origin not be identified, strongly suggest that the Vice President himself is pushing the project. One individual wrote that SSTO is the only new launch vehicle project that could provide flight results early in any second term for President George Bush, before Quayle starts any campaign of his own for the Presidency. The source who provided the documents confirmed that rumors to this effect are current on Capital Hill, but also said, "SSTO proponents are trying to inject this argument into the Vice President's camp." The Vice President's office referred all questions to the National Space Council; representatives of the Council refused to comment for publication.

The Strategic Defense Initiative Organization does not "intend to develop this vehicle," said the SDIO source, referring to the operational DC-Y. "I think our mission with SSTO is to get it to a point, at the end of Phase-II when we fly something [the DC-X] and demonstrate the [short] turnaround, to where people will accept the fact that it can be done." Then, "The interest would shift either to NASA or to the Air Force."

That may not a good idea if David Webb is right. He agreed that it was politically improbable for Delta Clipper to come to fruition within the Strategic Defense project: "Congress won't buy [launch vehicle development by SDIO] because they are afraid they are buying SDI through the back door." But as for moving it to another Agency: "That is going to be a very difficult thing because the `not invented here' syndrome is going to come into action." Agreeing with the SDIO source, Webb added, Delta Clipper "is not going to get any help from [anyone at] NASA because it will directly compete with [NASA Langley's] Personal Launch System and [NASA Marshall's] Advanced Manned Launch System." If NASA is ever to continue Delta Clipper, Webb believes, "The Administration is basically going to have to order NASA to build it."

With the Space Shuttle now clearly out of favor within the Administration, that is not as inconceivable as it was a few years ago. But NASA is unlikely to want to take on a project that originated outside of the agency, especially while they are cutting their own programs. The day before I wrote these words, NASA Administrator Richard Truly handed his resignation to President Bush, reportedly over some of these very issues.

The Bush Administration's goal in supporting the project, as Gaubatz sees it, is to "have in place key elements of [a new] space [transportation] infrastructure" by the end of the Administration's tenure. "The government looks at this as a basic infrastructure development, in the same way that highways and railroads were built, to establish -- firmly establish -- access to space." He added, "We compete with the French [Arianespace] and the Chinese, and the Japanese are coming on, and the Soviets are going to be a new" competitor whatever form that country ultimately takes.

Congress is not very sympathetic to such arguments, and has refused to appropriate significant funds for new launch vehicle development. A Congressional staffer who deals with launch vehicle issues, and who did not want to be identified, said he "does not think there is a great deal of awareness of the program" among members of Congress. Pointing out that the SSTO project is funded "at a relatively low level at this point," the source believes, "If it becomes a serious program, then yes, it is likely to attract some attention."

And then? "The record shows that Congress has not been enthusiastic about separate launch development efforts for SDIO, particularly [in so far] as these were perceived to be connected to some early deployment option for space based strategic defenses," the source said agreeing with Webb. "Based upon that record," if, in two years, Delta Clipper is proposed "as some crash development program for space-based strategic defenses, anybody's guess would be that it would run into some trouble."

On the other hand, added the source, "Congress has always given SDIO wide latitude to spend the amounts that are appropriated for the organization. To the various directors of SDIO, it may have looked as though Congress has micro-managed the program, but I'd say far from it compared with any other program. They've been extremely free to spend money as they see fit. With some exceptions with regard to the broad guidance of the program, that remains the case."

This relative freedom may help Delta Clipper get established as an on-going program with its own constituency, although in today's budget environment that strategy no longer works as well as it once did.

A second Congressional staffer, in the House, who strongly supports the program, responded. "Here's the point: if you demonstrate something that works ninety percent, or [even] eighty percent, of the way [to orbit] in 1993, for a total expenditure by that point [including Phase-I] of on the order of $70 million at the outside," than Delta Clipper becomes a serious proposition. "That final twenty percent of the performance is what generates most of the cost," said the House source, but achieving that first "eighty percent enables you to get all the rest of that money by demonstrating early and quickly" that it can be done.

With both NASP and the National Launch System facing funding problems, is there room for a new government-developed launch vehicle? Gaubatz said he does not see NASP and Delta Clipper as competing. "We would look to NASP as a long-term investment by the country . . . to remain competitive. As new materials [are] developed on NASP, we could use those as block upgrades" for the DC-Y In other words, NASP is a research and development program while Delta Clipper can be seen as the first application of the technology being developed in the NASP program. Asked a similar question, David Webb said with great reluctance that if Congress is forced to choose between NASP and Delta Clipper, "We should go with the tried and true; with the rocket engine [Delta Clipper rather than air-breathing NASP]. I would hate to see that choice, because I think we should [continue to] drive the technology the way it has been driven by NASP."

In light of the Space Shuttle experience, Interavia Space Markets asked Gaubatz how he will convince members of Congress and space scientists that another try at inexpensive, reusable spaceflight should have a higher claim on limited funds than applications launched on existing vehicles. Applications, after all, are needed to create the markets that commercialized launch vehicles will serve. Gaubatz admitted: "A lot of the gray-heads say, `Gee, I think I heard this pitch twenty years ago [with the Space Shuttle], what's going to be different?'"

For the first time, Gaubatz said answering his own question, "We can now build a Single-Stage-To-Orbit vehicle. That means you have [just] one vehicle to fly to orbit and back, maintain, and develop. Space Shuttle ended up as a multi-stage vehicle. . . . The care and feeding and maintainability of [the orbiter, Solid Rocket Boosters, External Tank, and the integrated vehicle] results in a large amount of costs." A SSTO vehicle avoids all of these expenses simply by being a single vehicle which does not have to be integrated with anything else.

The Shuttle had many requirements ranking higher than operations cost and reliability; Delta Clipper does not, added the House source. Keeping low cost operations the primary requirement of the program should make achieving that goal more likely.

Scientists who, at first blush, would rather build a useful spacecraft with $58.9 million than spend it on another try cheap spaceflight, should support the project anyway, says Gaubatz. "You want to do a little investment for a future payoff. The future payoff to the scientific community is an enormously reduced cost of access to space, and the ability for [scientists] to be regular passengers, and have regular schedules for scientific payloads that they can afford to take into space" themselves. Asked about claims by individuals who sold the project to the government that a SSTO could be refueled in orbit then use its standard vertical landing technique to land on Earth's moon, Gaubatz said, "That's true. You spend a lot of energy just getting off the Earth." The same propulsion system is more than powerful enough "to go on to the moon . . . land on the moon, fly back, and land on Earth." Gaubatz believes that Delta Clipper gives "the Space Exploration Initiative an enormously flexible vehicle that . . . could become a[n all- purpose] utility vehicle" throughout the Earth-moon system.

Gaubatz enthused that if he can really achieve an airliner type operation, "you get down to where costs of operation approach those of the propellant that you need for each flight. There is nothing that is inherent in the vehicle design or the ground operations and flight operations that leads" him to feel that cannot be achieved. Delta Clipper is "a revolutionary space transportation system [and a return to] a very rapid paced space program." Webb agrees: "If we can get [a Single-Stage-To-Orbit vehicle] out of SDI and built, it would simply take over the world space market."

It is becoming harder to convince people outside of the aerospace industry -- who must ultimately pay for new launcher projects -- that this perennial promise can be fulfilled. Someday soon, it had better be, or Americans will stop paying for the "cheap and easy" rail road to orbit that only seems to get more expensive.


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