Honolulu HI (SPX) Dec 04, 2008
It's an odd feature of aerospace history that many prototype aircraft that never went into production become "cultplanes". Some prominent examples are flying wings, Avro Arrow, B-70 Valkyrie, anything designed by the Nazis in 1945.
Cultplanes have their own Web sites run by amateur enthusiasts who are slightly dotty. These cultists take all the public relations hype from the designers as gospel truth and ignore all the potential problems. They argue that if their particular pet aircraft had been given a chance, history would have been changed: Canada would still have an aerospace industry, or supersonic airliners would be common, or the Nazis would rule the world.
Some of these sites have a strong paranoid slant, claiming that the Holy Plane was sabotaged by the Sinister Forces of U.S. Imperialism, International Communism, or the Established Aerospace Corporations. You can still see long-disproved conspiracy theories about the B-49 and the Arrow presented as historical facts on the Web.
We Space Cadets have our own "Cult Spacecraft". The blogosphere is full of ardent fans of ex-projects like X-20, X-30, DC-X, X-33, X-34, X-38 etc. who are convinced that their particular favorite would have been the key to cheap and reliable access to space if only the Sinister Forces hadn't killed it. These cultists are constantly calling for one of these dead projects to be revived as an alternative to the boring multistage expendable boosters we are still using.
This way lies madness - or at least irrelevance. Most of these cult programs were technically impossible. They often had severe political and management problems as well, but the main reasons for their failure were fundamental laws of physics, aerodynamics, and engineering that haven't changed today and never will. Trying to revive them is wasted effort that only makes the space advocacy community look technically illiterate and reduces its credibility.
So I have decided to expand an earlier article on this subject into a series that will explain in plain English the reasons why some of the more popular Cult Spacecraft would probably have been dead ends, even if they had received unlimited funding and had perfect management.
The obvious place to start is with X-20/Dyna-Soar. This little black spaceplane is not nearly as nutty as the other projects I will discuss in later articles, but it does have several cult web pages full of wishful thinking.
The Exospheric Bomber: Dyna-Soar had an unusually checkered development history. It had at least 4 different names (BoMi, ROBO, Dyna-Soar, and X-20), two prime contractors (Bell and Boeing), and a bewildering variety of launch vehicles.
It originally wasn't a spacecraft at all, but an alternative method of delivering a big thermonuclear bomb from the USA to the USSR. And it originally wasn't an orbital vehicle intended to travel in vacuum, but a "boost-glide" aircraft that was boosted to high altitude and speed by a disposable booster, then flew once around the Earth in the upper atmosphere where it was supported partly by wing lift and partly by centrifugal force.
The goal of this original BoMi (Bomber Missile) or ROBO (Rocket Bomber) was to attack the Soviet Union at altitudes and speeds higher than the Mach 3 B-70 bombers and Navajo cruise missiles under development at the same time. Speed/Altitude graphs for BoMi show that it was designed to attack the USSR over the South Pole, instead of the direct North Pole route.
The boost-glide concept poses formidable problems of navigation, bomb-aiming, and thermal protection that are not adequately addressed in the available sources. The vehicle would have left a hot plasma trail, attracting both radar- and IR-guided missiles like a magnet. Course changes would have been almost impossible. By 1958 it was clear that the plain ballistic reentry vehicle was cheaper, lighter, and more survivable than the "exospheric bomber" could every be - so BoMi was restructured as a research vehicle.
It was given the name Dyna-Soar from the phrase "dynamic soaring" which supposedly described its flight mode. Later it acquired the parallel designation X-20 to indicate its new role as a successor to the X-15 as a high-speed research vehicle.
In traditional X-plane fashion, it was planned to fly X-20 at gradually increasing speeds and altitudes, starting with B-52 drops at Edwards AFB and proceeding to suborbital flights from Cape Canaveral to Caribbean islands or Brazil. Later the X-20 would launch from Canaveral on a Titan IIIC, make one low partial orbit around the Earth, and land at Edwards AFB. Eventually a multi-orbit capability would be developed. This final version of X-20 would have retained the partly-fueled Titan Transtage for orbital maneuvering and retrofire.
Zen and the Art of Spacecraft Design: The requirements of even fractional-orbit spaceflight are so demanding that the X-20 turned into an ultra-minimalist design that just barely met the once-around mission requirements. Like the more recent X-38, it was not really a complete spacecraft in the sense of Gemini, Soyuz, or Apollo. There was no radar, computer, or maneuvering thrusters to change orbit. Attitude control was by hydrogen peroxide thrusters with a very limited fuel supply. And the life support system had very limited capacity, apparently only for three orbits in the final design.
A major reason for this very austere systems fit was the weight and space demands of the elaborate thermal control system needed to survive reentry. X-20 had no external thermal insulation like the Shuttle; it would have been covered with metal shingles made of rare elements like zirconium, molybdenum, columbium and tantalum.
This metal skin was highly vulnerable to oxidation and needed a silicon overcoating that might well have been as fragile as the Shuttle tiles. It also had a high thermal conductivity; reentry heat flowed freely through the "Thermal Protection System" which only protected itself, not the spacecraft structure underneath.
To cope with this heat conduction, the wing, fuselage, and landing gear structure of the X-20 was made of somewhat less exotic alloys and allowed to heat up freely. But the pilot and internal systems still needed protection. This was provided by a 3-stage active cooling system:
Stage One was a layer of insulation to delay heat propagation into the spacecraft interior.
Stage Two consisted of a double cabin wall enclosing a layer of water which was stabilized by a gelling agent. This "water-wall" would boil off during reentry.
Stage Three was a traditional aircraft glycol cooling loop that dumped heat into a flow of liquid hydrogen from a huge spherical tank that took up about 20% of the X-20's fuselage. The hydrogen was then burned with liquid oxygen in APUs that provided hydraulic power for the aerodynamic control surfaces.
This thermal control system was extremely wasteful of mass and internal volume. Attempts to give X-20 some minimal operational capability were severely limited by the available space. Every cult web page about Dyna-Soar shows a cross-section of an "X-20X" 5-seat space station crew ferry configuration - but if you look closely you see that the escape rocket has been removed and the passengers are crammed in so close that ejection is impossible.
Active cooling also placed a strict limit on the orbital stay time of the X-20, since the LH2 would have been boiling off continuously in space. This system could never have been used on the Space Shuttle orbiters as some have suggested. The LH2 tank alone would have taken up most of the cargo bay.
A supposed advantage of a robust TPS is that the vehicle is reusable without elaborate and labor-intensive refurbishment. But the surviving Dyna-Soar documents indicate that reusability was only a long-term goal of the program. Boeing built mockups of a conventional production line that would supply new X-20s for every mission. Ironically, the competing Gemini capsule did demonstrate reusability when Gemini 2 was reflown as Gemini B during the MOL program.
The Curse of the Bomber Generals: X-20 was managed by the U.S. Air Force whose generals at that time were almost all former WWII bomber pilots. Through the whole history of space travel, pilot-astronauts and pilot-managers have shown a visceral dislike for ballistic RVs and a psychological need for traditional stick-and-rudder controls. In the Dyna-Soar this syndrome produced truly insane results.
The whole basis of the program was piloted reentry, but it was a very strange sort of piloting. The X-20 astronaut would have sat in front of a circular display with a grid of lines indicating various bank angles and angles of attack. Below the display was a row of ten buttons bearing the names of various US Air Force bases. (X-20's wire-brush landing gear would only work on concrete runways, not dry lake beds or dirt strips.)
Before reentry, the pilot pressed a button to choose a runway. After that, a computer would project onto the display a pip indicating the flight angles needed to reach the selected runway, and a second pip indicating the actual angles currently being flown. The pilot had only to keep the two pips together like a human servomotor.
Not content with this, the USAF actually insisted that the pilot should "fly" the booster during the ascent to orbit, again robotically following instructions presented by a computer while subjected to the crushing g-force and intense vibration of a rocket launch! Studies of this absurd guidance mode were actually continued after X-20 was cancelled.
The designers at Boeing pandered to pilot prejudice even more with a proposal for "Synergetic Orbital Plane Change." In this maneuver, Dyna-Soar would have made a retro burn to drop its perigee into the atmosphere, rolled onto its side and used aerodynamic lift to make the plane change, then lifted itself back into stable orbit with another burn.
The author of this idea seems to have forgotten that Dyna-Soar didn't have a rocket engine - all its delta-vee was in the partly-fueled Titan III transtage. With the flimsy and explosive transtage still attached, this maneuver is quite impossible. This doesn't stop some cultists from citing it even today.
Even the overall airplane-like configuration of the vehicle was fundamentally wrong and obsolete by 1963:
1) A winged configuration is about three times as heavy as an equivalent semi-ballistic capsule, and therefore costs three times as much to launch. The one-man X-20 was operationally less capable than the two-man Gemini, yet it required the expensive 4-stage Titan IIIC instead of Gemini's cheap 2-stage Titan II. A major element of this weight was a huge abort rocket (based on the Minuteman I Stage 3) that had to be carted all the way to orbit.
2) A winged spaceplane located on top of a booster is an aerodynamically unstable configuration, akin to an arrow with the feathers at the front. The guidance packages and thrust-vector controls on 1960 rockets were not capable of steering the stack in the lower atmosphere. Huge tailfins were designed for all the proposed boosters to maintain static stability. (This problem cropped up again in the X-37B program and resulted in a big payload shroud being added.)
3) A manual runway landing requires big windows in the front of the vehicle, where reentry heating is intense. X-20 addressed this problem with a detachable metal heat shield covering the windows. If this failed to jettison, the unfortunate pilot would have no forward view at all.
4) The pilot is seated in the correct position to take the launch g-forces "eyeballs-in", but during reentry he is oriented in the unfavorable "eyeballs-down" direction. This is tolerable during a normal shallow low-g lifting reentry, but not in many launch abort scenarios where the initial entry angle is very steep.
This "black zone" problem seems not to have been discovered until the program was well under way. Robert Godwin's book reproduces a report of April 1958 which states that the Dyna-Soar pilot would be subjected to a peak g-load of 22 gravities during the worst-case launch abort. This is about the same as in a Soyuz abort - but in a Soyuz the crew is correctly oriented eyeballs-down.
Was McNamara Right? When Secretary of Defense Robert Strange McNamara and his staff of "Whiz Kids" subjected X-20 to a series of reviews in 1963, it was revealed as more expensive, less capable, and less safe than the existing semi-ballistic spacecraft, Gemini. By the time X-20 would actually fly in 1967-69, it would be an embarrassing anachronism: a spacecraft with Mercury-like capability flying alongside Apollo.
Furthermore, McNamara and at least some members of the Dyna-Soar review panels were aware of the rapid progress being made by top-secret unmanned military spacecraft such as CORONA and GRAB. These programs were already far more capable in 1963 than any operational variant of X-20 might have become circa 1975. Declassified documents indicate that discussions of Dyna-Soar usually ended up discussing whether any manned military space program was justifiable.
McNamara's decision to axe Dyna-Soar in favor of a military version of Gemini was really a compromise, designed to keep the Defense Department involved in manned space at minimum cost. But "Blue Gemini" evolved into Manned Orbiting Laboratory (KH-10 DORIAN), an expendable one-shot manned spy station.
MOL was absurdly more expensive than the parallel KH-9 unmanned system and was cancelled in turn. The Soviets actually did deploy the Almaz manned military space stations which proved beyond doubt that there is no useful military role for men in space.
Dyna-Soar cultists have argued that McNamara was wrong in evaluating the vehicle as an operational military spacecraft, since it was intended as a pure X-vehicle to develop and evaluate the spaceplane concept. It is politically incorrect to defend the man who gave us the F-111B, the joint service belt buckle, and the Second Indochina War - but I'll put my neck on the chopping block and claim that McNamara was correct - at least from his perspective in 1963.
The whole purpose of experimental aircraft is to develop technology for future operational vehicles - but all the new technologies being developed for X-20 look like losers.
But from the perspective of 45 years later, I wish that McNamara had made the wrong decision and allowed X-20 to continue. The program would have been a waste of money in the short term, but it might have saved both the USAF and NASA much more money and agony in the long term.
The Dyna-Soar configuration was close to that eventually adopted for the Space Shuttle Orbiter, much closer than the unmanned ASSET and X-23A vehicles that were the only "winged" RVs actually test-flown in the 1960s. Like the earlier X-planes, X-20 would have carried a huge package of electronics to record aerodynamic data during reentry. A similar package was carried by the first Shuttle Columbia in 1981 - about a decade too late.
If X-20 had actually flown in the late 1960s, that instrumentation would have supplied the Shuttle's designers with a priceless data base. They would not have been forced to rely completely on wind-tunnel data and primitive computer models and would have produced a better design. For instance, the Orbiters carry two tons of lead blocks in their noses to compensate for an error in aerodynamic models, and X-20 data might well have prevented this mistake.
It is even possible that X-20 would have convinced NASA and USAF management that a winged vehicle was the wrong way to go - that the operational problems and parasitic weight were just too crippling to be worth the supposed advantages of high cross-range capability and runway landing.
Many people came to realize this unpleasant truth during the test flights of Columbia in 1981-82, by which time it was politically impossible to abandon the Shuttle or cut it back to an X-program. Since then, spaceplanes have been rejected in favor of semi-ballistic capsules every time they have competed for the same mission, most recently in the Hermes/ARD, Kliper/ACTS, and OSP/Orion programs.
So this is my own alternate aerospace history fantasy: If there had been a few X-20 flights, and the data base from these flights had been available in 1971, it is possible that we could have avoided the whole Space Shuttle fiasco? Might we have stuck with capsules instead of getting lost in a blind alley? Could Dyna-Soar have saved us 14 dead astronauts and over $100B of wasted money? Could we have spent the last 35 years doing something useful in space, if that useless little black spaceplane had actually flown?
Jeffrey F. Bell is a former space scientist, recovering pro-space activist and amateur aerospace historian.
For further reading: Richard Godwin, DYNA-SOAR (Apogee Press, 2003).
Dyna-Soar at Wiki
Rocket Science News at Space-Travel.Com
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