Flying into space on wings of flame

[WWW Ed Note: This article is not authoritative. Some of the data about the Black Horse is inaccurate, and readers are encouraged to read with caution. In particular, several possible operational modes of the Aerial Propellant Transfer Spaceplane concept have been combined for dramatic appeal.]

Dateline: Edwards Air Force Base, California -- 5.20am, June 14th, 1996.

It's a small aircraft. Not much bigger than an F-16 fighter. Looking at its stubby nose and delta wings sweeping up at the tips, there's not much to suggest that this is the fastest aircraft ever built. Under the clear blue skies of Edwards Air Force Base, the upper surfaces look a dull grey, frosted with ice in patches that steam white vapour as the sun catches them. The snub nose is matt black, and shows the tell-tale scorch marks that are the only clue to this little aircraft's searing speed. This is Black Horse.

In a few minutes, it will be screaming heavenwards on a trail of fire, its powerful rocket engines burning hundreds of gallons of methane and liquid oxygen every second. Black Horse, for all that it looks like an aircraft, is a space-faring satellite launcher. Re-usable, economical, and flying as often as a fighter aircraft, Black Horse finally delivers on the $100 per kilo in orbit that the Space Shuttle promised when it was first suggested, but never came close to offering.

Satellites -- for communications, navigation, environmental monitoring -- will be vital to the technological society of the 21st century. But not unless they can be placed into orbit cheaply. That's why low-cost launchers are the subject of NASA's latest research programmes, and why they are currently are taking shape on the drawing boards of the world's largest aerospace companies. The space business is big business -- if we can find a way of avoiding the colossal waste of throwing away millions of dollars-worth of high-tech hardware at every launch. Black Horse and its streamlined competitors in the X-33 and X-34 research programmes are the answer.

With the fuelling pipes withdrawn, the pilot triggers the a pair of turbojets for the initial climb, and the stubby plane gathers speed. Soon it is a speck vanishing into the blue dawn sky. High above the base, Black Horse's pilot scans the sky for the waiting tanker. At 40,000 feet, a KC-135 is circling, its fat fuselage sloshing with chill liquid rocket fuel. This is the secret of Black Horse's tiny size. By taking off with the minimum amount of fuel aboard to reach an airborne rendezvous, the structural weight of the craft, its landing gear -- every part -- can be pared to the bone. The two aircraft meet in an aerial ballet, the little back delta homing in on the trailing fuelling boom. In a few minutes, Black Horse gulps 147,000 pounds of cryogenic fuel, and cuts loose from the tanker.

Throttling the rocket engines up to full power, the pilot points the nose skywards, and zooms into the blackness -- higher and faster than any plane has ever flown. Touching Mach 15 -- fifteen times the speed of sound -- Black Horse reaches 400,000 feet. If this was a satellite launching mission, the hypersonic aircraft would seek out another, its specially-modified fuel-carrying twin, for a final draught of liquid hydrogen before leaving the atmosphere entirely and reaching orbit. As it is only a test flight, the tricky zero-g rendezvous remains in the future. It is time to return to Earth.

Switching off the rockets, the pilot lifts the aircraft's nose to an angle of about 40 degrees, where it will remain as the craft "surfs" a searing wave of air to lower altitudes. The crackling roar of the air, and the vibration, builds up as the tenuous traces of atmosphere thicken around the plummeting aircraft. Soon the nose is cherry red, and the glow spreads along the leading edges of the wings. At mach 15, the air isn't an invisible, intangible fluid. It's a wall of flame with the power of a battering ram. In 1947, Chuck Yeager broke the sound barrier in the Bell X-1. Tomorrow's hypersonic aircraft will have to fly to and from the edge of space through the heat barrier. It's not going to be easy.

When Yeager broke the sound barrier, no one knew how an aircraft would behave "on the other side" -- faster than Mach 1. It turned out that the answer was not very differently from the way it behaves at slower speeds. Concorde now routinely flies at twice the speed of sound, as do hundreds of military aircraft every day. But in the hypersonic regime -- up there beyond mach 5 -- things really DO change. The characteristics of the air become very different as it reaches speeds of over 3500 mph. The heat begins to trigger chemical reactions never seen at slower speeds.

It's all about energy. Below Mach 1, the kinetic energy of the air flowing around an aircraft is pretty small relative to the energy stored as temperature and pressure: in supersonic flight, the ratio of the two kinds of energy is about 1. But as the speed climbs higher, the kinetic energy of the air becomes much larger than any other kind of energy, and dominates its behaviour. Bring air to a standstill at Mach 6, and it'll heat up to over 2000F. And no matter how you design an aircraft -- one with wings at least -- in some places, like the leading edges of the wings, some air will be slammed to a stop, dumping its heat into the materials covering the aircraft. Managing the energy of the searing flow around the bird is the hypersonic aircraft designer's main job: directing the flows to slow the air as little as possible; choosing materials capable of withstanding temperatures of up to 3000C.

Even back in the 1950s, the engineers building the craft flown by pilots with the Right Stuff ran up against the heat demons. Less than ten years after Yeager's record run in the Bell X-1, a new generation of X-planes was being designed to probe the edge of space at up to six times the speed of sound. NASA's North American X-15 still holds the records for a powered aircraft: it reached Mach 6.72 (4520 mph), and soared to a highest altitude of 67 miles. Since space begins "officially" at 50 miles high, eight of the twelve X-15 pilots - Bob White, Joe Walker, Jack McCay, Bob Rushworth, Joe Engle, Pete Knight, Bill Dana and Mike Adams - qualified as astronauts during the test programme.

The X-15 was designed - on the back of a single sheet of paper at a conference - in 1954: only eight years after the sound barrier was broken; fifty years after the Wright Brothers first flight; and three years before Sputnik. Yet it was designed to reach Mach 6 and 250,000 feet. Since the X-1, several other rocket powered research craft had advanced through Mach 2, but the X-15 was heading for uncharted territory. Exhaustive tests had shown that Inconel-X, a nickel-based alloy developed to be used inside jet engines, could keep up its structural strength out beyond 750C. Enough, the designers calculated, to reach Mach 7.

A slim spike, with the smallest, stubbiest straight wings that would allow control at slow speeds, the X-15 was basically a rocket with a cockpit. It's long narrow shape was like that of a rifle bullet - except the X-15 flew twice as fast as the fastest rifle bullets.

98% of the atmosphere is below you at 100,000 feet. The sky above is the black of space. Since the X-15 was designed to reach an altitude over 2 1/2 time higher, it was to be the first "re-entry vehicle": the first craft to fly into space and then return to the atmosphere. And as they worked out what flight paths the X-15 should follow in its fiery return to Earth, its designers ran up against one of the fundamental problems of hypersonic flight into orbit and back.

It reared its head as the engineers tried to choose the best ratio of the lift produced by the aircraft's wings to the drag - the force required to push the air aside - caused by its shape. For years, to fly ever faster, engineers strove to minimise an aircraft's drag. To fly into space, surely the ratio should be as large as possible? But as the engineers modelled the X-15's re-entry into the atmosphere, they discovered a BIG problem. Fly to a landing using its wings like a conventional aircraft, and The X-15 would be pummelled to pieces by the air in the first few seconds of its re-entry. The drag, which created friction, was the only way to sap the energy of the hypersonic craft, slowing it for a landing.

The ideal shape for a re-entry vehicle was a blunt cone, with a lift to drag ratio of between 0.1 and 0.3. It had been proved by classified studies of the best shapes for ballistic missile warheads, which had to follow an accurate path through the atmosphere. Soon after the X-15 began flying, Max Faget, the designer of the Mercury space capsule, adopted this shape for his craft.

To fly up almost into orbit, the X-15 needed to be a slim arrow with lifting wings; to fly back it ought to be a stubby cone with no wings at all. How to compromise?

The answer, it turned out was not to point the sleek nose of the X-15 at its landing ground as it re-entered the atmosphere, but to hold the nose high, at up to 40 degrees above the direction it was travelling, so the entire belly of the craft met the air head on. That way, the drag was much higher, and the heat loads were spread out over a broad area. That compromise, scratched out by pencil and clockwork calculator in 1954, is still the basis for the design of every craft intended to fly into orbit and return.

Eventually, a specially modified X-15 with strap-on hydrogen tanks and an additional ramjet engine was to speed to Mach 6.7. Even with its Inconel structure, the vehicle couldn't take the heat. It was painted with a special polymer coating which would slowly burn off, carrying away heat - and the pilot had a cover over one side windscreen which he would jettison just before landing, so that he could see through the otherwise soot-covered windows. The X-15's early 1960s successor, however, was designed to fly to orbit on board a Titan missile, and fly back to a landing on a dry lakebed. And it was to do it over and over again, with a re-usable thermal protection system Sounds familiar?

No, it wasn't the Space Shuttle, but the X-20 Dynasoar. The DynaSoar was an aircraft before its time. It was to have been built from the most advanced "super-alloys", then under development for the next generation of higher-temperature jet engines. Its designers, at Boeing, had to invent from scratch ways of rolling and forming these hard, brittle materials. The vehicle's blunt nose cap, which would bear the brunt of the flaming slipstream, was to be made from a carefully-cast ceramic reinforced with hundreds of metres of carefully-grown pure sapphire crystal fibres. Inside, the pilot would be protected from the heat by walls filled with a glass fibre "wick" soaked in water, which would slowly evaporate. The heat inside the bays of the craft would be so intense that no tyres could survive the ordeal of re-entry: instead, DynaSoars was to slide to a stop on the salty lake-bed on skids covered with brushes made of superalloy. But DynaSoar got caught up in political squabbles between the Pentagon and NASA, and eventually, with its skeleton two-thirds built, DynaSoar was made extinct by Defence Secretary Macnamara in 1964. Everyone forgot about America's FIRST Aerospace Plane.

Fifteen years later, the Space Shuttle finally flew. NASA had been working on its design since 1966. So complex were the problems of flight into orbit and back that the Shuttle had been through literally hundreds of designs before the blunt, boxy brick we now see got into orbit. Sleek pointed vehicles with short straight wings like the X-15; triangular gliders like DynaSoar; boosters that flew back to base and boosters that were thrown away - even an ingenious package of three identical triangular winged rockets called the Triamese: sandwiched together, the outer pair performed the function of boosters, leaving the centre craft to fly into orbit and return. Eventually, NASA settled on the Shuttle we now have: abandoning the goal of a fully re-usable system in favour of cheaper and simpler throw-away boosters; and settling for an appallingly complex system of tiles for thermal protection. The refurbishment of the water-proof tile coating which prevents them being damaged by rain is one of the most time consuming operations in the hundred days of work it takes 10,000 people to return the Shuttle to the launch pad after a flight. Not surprisingly, the Shuttle did not become the Space Truck NASA promised, able to put cargoes in orbit for less than $50 per pound. At nearly $1bn a launch, the world needs something cheaper. And, finally, it looks as through we are going to get it.

In the late 1980s, America began the National Aerospace Plane program: the most ambitious aircraft development effort in history. NASP was a large aircraft - the size of a DC-9 airliner - able to take off under its own power from a runway, fly into orbit at over Mach 25, and land after its mission requiring nothing more than re-fuelling before it took off again. After billions of dollars had been spent, the over-ambitious NASP was cancelled. But America's aerospace companies - all of whom took part - had learnt a lot about designing hypersonic aircraft and engines. Today, that knowledge is being put to good use.

NASA is sponsoring the X-33 programme, to design a re-usable rocket. Three proposals have been entered: Rockwell, with a "son-of shuttle", Lockheed, with a blunted triangular "lifting body", which does away with wings, generating its lift purely by the shape of the fuselage; and McDonnell Douglas with a conical rocket which re-enters the atmosphere nose first and then rotates to cushion its landing using its rockets. The X-34 is also on its way: it is a small winged rocket, developed by Rockwell and Orbital Sciences Corp., which is bringing the X-15 concept right up to date.

All these craft have one thing in common. They are built from advanced lightweight composite materials. The searing heat of re-entry is absorbed by ceramics and carbon-fibre reinforced carbon panels. Without the NASP programme, none of these materials would exist. NASP also proved to aerospace engineers (most of whom probably knew already) that it's a bad idea to try and develop a totally new and untried engine - NASPs hydrogen-burning supersonic ramjet 1F- and a new generation of aircraft at the same time. The X-33 contenders and the X-34 all use good old fashioned rockets (Lockheed's is a bit different, but that's another story).

Black Horse, too, is designed with simplicity in mind. It's engines are advanced versions of a tried and tested Russian rocket motor; it is built from materials that people can make relatively cheaply and know how to use in aircraft. Above all, the legacy of NASP is that the hypersonic route to that stars has to be a simple, well-mapped one, not a giant leap into the unknown.

High above Edwards, a dot appears in the sky. Barrelling from side to side to slow down, the grey delta plummets through the cold air, its panels popping as they radiate away the last remnants of the searing heat of re-entry. Swooping low over the runway, the pilot lifts the nose and lowers the landing gear in a single swift movement, and with a puff of blue smoke and a squeal of tyres, Black Horse is home......for today.

Copyright 1995, Focus Magazine