The Decline of NASP
NASP was one of Reagan’s programs, and for a time it seemed likely that it would not long survive the change in administrations after he left office in 1989- That fiscal year brought a high-water mark for the program, as its budget peaked at $320 million. During the spring of that year officials prepared budgets for FY 1991, which President George H. W Bush would send to Congress early in 1990. Military spending was already trending downward, and within the Pentagon, analyst David Chu recommended canceling all Defense Department spending for NASP. The new Secretary of Defense, Richard Cheney, accepted this proposal. With this, NASP appeared dead.
NASP had a new program manager, Robert Barthelemy, who had replaced Williams. Working through channels, he found support in the White House from Vice President Dan Quayle. Quayle chaired the National Space Council, which had been created by law in 1958 and that just then was active for the first time in a decade. He
X-30 concept of 1985. (NASA)
used it to rescue NASP. He led the Space Council to recommend proceeding with the program under a reduced but stable budget, and with a schedule slip. This plan won acceptance, giving the program leeway to face a new issue: excessive technical optimism.49
During 1984, amid the Copper Canyon activities, Tony duPont devised a conceptual configuration that evolved into the program’s baseline. It had a gross weight of 52,650 pounds, which included a 2,500-pound payload that it was to carry to polar orbit. Its weight of fuel was 28,450 pounds. The propellant mass fraction, the ratio of these quantities, then was 0.54.50
The fuel had low density and was bulky, demanding high weight for the tankage and airframe. To save weight, duPont’s concept had no landing gear. It lacked reserves of fuel; it was to reach orbit by burning its last drops. Once there it could not execute a controlled deorbit, for it lacked maneuvering rockets as well as fuel and oxidizer for them. DuPont also made no provision for a reserve of weight to accommodate normal increases during development.51
Williams’s colleagues addressed these deficiencies, although they continued to accept duPont’s optimism in the areas of vehicle drag and engine performance. The new concept had a gross weight of 80,000 pounds. Its engines gave a specific impulse of 1,400 seconds, averaged over the trajectory, which corresponded to a mean exhaust velocity of 45,000 feet per second. (That of the SSME was 453-5 seconds in vacuum, or 14,590 feet per second.) The effective velocity increase for the X-30 was calculated at 47,000 feet per second, with orbital velocity being 25,000 feet
per second; the difference represented loss due to drag. This version of the X-30 was designated the “government baseline” and went to the contractors for further study.52
The initial round of contract awards was announced in April 1986. Five airframe firms developed new conceptual designs, introducing their own estimates of drag and engine performance along with their own choices of materials. They gave the following weight estimates for the X-30:
Rockwell International McDonnell Douglas General Dynamics Boeing Lockheed
A subsequent downselection, in October 1987, eliminated the two heaviest concepts while retaining Rockwell, McDonnell Douglas, and General Dynamics for further work.53
What brought these weight increases? Much of the reason lay in a falloff in estimated engine performance, which fell as low as 1,070 seconds of averaged specific impulse. New estimates of drag pushed the required effective velocity increase during ascent to as much as
52,0 feet per second.
A 1989 technical review, sponsored by the National Research Council, showed what this meant. The chairman, Jack Kerrebrock, was an experienced propulsion specialist from MIT. His panel included other men of similar background: Seymour Bog – donoff of Princeton, Artur Mager of Marquardt, Frank Marble from Caltech. Their report stated that for the X-30 to reach orbit as a single stage,
“a fuel fraction of approximately 0.75 is required.”54
One gains insight by con – X-30 concept of 1990, which had grown considerably, sidering three hydrogen-fueled (U. s. Air Force)
rocket stages of NASA and calculating their values of propellant mass fraction if both their hydrogen and oxygen tanks were filled with NASP fuel. This was slush hydrogen, a slurry of the solid and liquid. The stages are the S-II and S-IVB of Apollo and the space shuttle’s external tank. Liquid hydrogen has 1/16 the density of liquid oxygen. With NASP slush having 1.16 times the density of liquid hydrogen,55 the propellant mass fractions are as follows:56
S-IVB, third stage of the Saturn V
S-II, second stage of the Saturn V
The S-II, which comes close to Kerrebrock’s value of 0.75, was an insulated shell that mounted five rocket engines. It withstood compressive loads along its length that resulted from the weight of the S-IVB and the Apollo moonship but did not require reinforcement to cope with major bending loads. It was constructed of aluminum alloy and lacked landing gear, thermal protection, wings, and a flight deck.
How then did NASP offer an X-30 concept that constituted a true hypersonic airplane rather than a mere rocket stage? The answer lay in adding weight to the fuel, which boosted the propellant mass fraction. The I I «!*■■■. ІНІЦИНІН £¥ IduJ ІЇ FP£
vehicle was not to reach orbit entirely on slush – fueled scramjets but was to use a rocket for final ascent.
It used tanked oxygen— with nearly 14 times the density of slush hydrogen.
In addition, design requirements specified a tripropellant system that was to burn liquid methane during the early part of the flight.
This fuel had less energy than hydrogen, but it too added weight because it was relatively dense. The recommended mix called for 69 Evolution of the X-30. The government baseline of 1986 had percent hydrogen, 20 per – IsP ofJ1’40J° seconds’ delta’V “reach ^t?*7’?™***per
1 j second, and propellant mass fraction of 0.54. Its 1992 counter-
Cent Oxygen, and 11 percent part had less Isp, more drag, propellant mass fraction of 0.75, methane.57 and could not reach orbit. (NASP National Program Office)
In 1984, with optimism at its height, Cooper had asserted that the X-30 would be the size of an SR-71 and could be ready in three years. DuPont argued that his concept could lead to a “5-5-50” program by building a 50,000-pound vehicle in five years for $5 billion.58 Eight years later, in October 1990, the program had a new chosen configuration. It was rectangular in cross section, with flat sides. Three scramjet engines were to provide propulsion. Two small vertical stabilizers were at the rear, giving better stability than a single large one. A single rocket engine of approximately 60,000 pounds of thrust, integrated into the airframe, completed the layout. Other decisions selected the hot structure as the basic approach to thermal protection. The primary structure was to be of titanium-matrix composite, with insulated panels of carbon to radiate away the heat.59
This 1990 baseline design showed little resemblance to its 1984 ancestor. As revised in 1992, it no longer was to fly to a polar orbit but would take off on a due-east launch from Kennedy Space Center, thereby gaining some 1,340 feet per second of launch velocity. Its gross weight was quoted at 400,000 pounds, some 40 percent heavier than the General Dynamics weight that had been the heaviest acceptable in the 1987 downselect. Yet even then the 1992 concept was expected to fall short of orbit by some 3,000 feet per second. An uprated version, with a gross weight of at least 450,000 pounds, appeared necessary to reach orbital velocity. The prospective program budget came to $15 billion or more, with the time to first flight being eight to ten years.60
During 1992 both the Defense Science Board (DSB) and Congress’s General Accounting Office (GAO) conducted major program reviews. The immediate issue was whether to proceed as planned by making a commitment that would actually build and fly the X-30. Such a decision would take the program from its ongoing phase of research and study into a new phase of mainstream engineering development.
Both reviews focused on technology, but international issues were in the background, for the Cold War had just ended. The Soviet Union had collapsed in 1991, with communists falling from power while that nation dissolved into 15 constituent states. Germany had already reunified; the Berlin Wall had fallen, and the whole of Eastern Europe had won independence from Moscow. The western border of Russia now approximated that of 1648, at the end of the Thirty Years’ War. Two complete tiers of nominally independent nations now stood between Russia and the West.
These developments greatly diminished the military urgency of NASP, while the reviews’ conclusions gave further reason to reduce its priority. The GAO noted that program managers had established 38 technical milestones that were to be satisfied before proceeding to mainstream development. These covered the specific topics of X-30 design, propulsion, structures and materials, and use of slush hydrogen as a fuel. According to the contractors themselves, only 17 of those milestones—fewer than half—were to be achieved by September 1993. The situation was particularly worrisome in the critical area of structures and materials, for which only six of 19 milestones were slated for completion. The GAO therefore recommended delaying a commitment to mainstream development “until critical technologies are developed and demonstrated.”61
The DSB concurred, highlighting specific technical deficiencies. The most important involved the prediction of scramjet performance and of boundary-layer transition. In the latter, an initially laminar or smoothly flowing boundary layer becomes turbulent. This brings large increases in heat transfer and skin friction, a major source of drag. The locations of transition thus had to be known.
The scramjet-performance problem arose because of basic limitations in the capabilities of ground-test facilities. The best of them could accommodate a complete engine, with inlet, combustor, and nozzle, but could conduct tests only below Mach 8. “Even at Mach 8,” the DSB declared, “the scramjet cycle is just beginning to be established and consequently, there is uncertainty associated with extrapolating the results into the higher Mach regime. At speeds above Mach 8, only small components of the scramjet can be tested.” This brought further uncertainty when predicting the performance of complete engines.
Boundary-layer transition to turbulence also demanded attention: “It is essential to understand the boundary-layer behavior at hypersonic speeds in order to ensure thermal survival of the airplane structure as designed, as well as to accurately predict the propulsion system performance and airplane drag. Excessive conservatism in boundary-layer predictions will lead to an overweight design incapable of achieving [single stage to orbit], while excessive optimism will lead to an airplane unable to survive in the hypersonic flight environment.”
The DSB also showed strong concern over issues of control in flight of the X – 30 and its engines. These were not simple matters of using ailerons or pushing throttles. The report stated that “controllability issues for NASP are so complex, so widely ranging in dynamics and frequency, and so interactive between technical disciplines as to have no parallels in aeronautical history…the most fundamental initial requirements for elementary aircraft control are not yet fully comprehended.” An onboard computer was to manage the vehicle and its engines in flight, but an understanding of the pertinent forces and moments “is still in an embryonic state.” Active cooling of the vehicle demanded a close understanding of boundary-layer transition. Active cooling of the engine called for resolution of “major uncertainties… connected with supersonic burning.” In approaching these issues, “very great uncertainties exist at a fundamental level.”
The DSB echoed the GAO in calling for extensive additional research before proceeding into mainstream development of the X-30:
We have concluded [that] fundamental uncertainties will continue to exist in at least four critical areas: boundary-layer transition; stability and controllability; propulsion performance; and structural and subsystem weight. Boundary-layer transition and scramjet performance cannot be validated in existing ground-test facilities, and the weight estimates have insufficient reserves for the inevitable growth attendant to material
allowables, fastening and joining, and detailed configuration issues________
Using optimistic assumptions on transition and scramjet performance, and the present weight estimates on material performance and active cooling, the vehicle design does not yet close; the velocity achieved is short of orbital requirements.62
Faced with the prospect that the flight trajectory of the X-30 would merely amount to a parabola, budget makers turned the curve of program funding into a parabola as well. The total budget had held at close to $250 million during FY 1990 and 1991, falling to $205 million in 1992. But in 1993 it took a sharp dip to $140 million. The NASP National Program Office tried to rescue the situation by proposing a six-year program with a budget of $2 billion, called Fiyflite, that was to conduct a series of unmanned flight tests. The Air Force responded with a new technical group, the Independent Review Team, that turned thumbs down on Hyflite and called instead for a “minimum” flight test program. Such an effort was to address the key problem of reducing uncertainties in scramjet performance at high Mach.
The National Program Office came back with a proposal for a new program called HySTP. Its budget request came to $400 million over five years, which would have continued the NASP effort at a level only slightly higher than its allocation of $60 million for FY 1994. Yet even this minimal program budget proved to be unavailable. In January 1995 the Air Force declined to approve the FiySTP budget and initiated the formal termination of the NASP program.63
In this fashion, NASP lived and died. Like SDI and the space station, one could view it as another in a series of exercises in Reaganesque optimism that fell short. Yet from the outset, supporters of NASP had emphasized that it was to make important contributions in such areas as propulsion, hypersonic aerodynamics, computational fluid dynamics, and materials. The program indeed did these things and thereby laid groundwork for further developments.