THE PERFECT SPACEPLANE

By the mid-1960s the age of the rocket fighter planes and experimental supersonic rocket aircraft had essentially ended, only some 30 years after it started. Jet engines could now also get a plane to high velocities and high altitudes in a short time, and most importantly were much more fuel-efficient than rocket engines. About the only advantage that rocket planes still had over jet aircraft was that they could operate at extreme altitudes and even in the vacuum of space.

In the 1970s the focus of spaceflight in the US turned to a vision of regular, easy and cheap access to low orbit, which seemed to mean good news for rocket plane development. The ideal launch vehicle would be completely reusable, reliable, safe, low-maintenance, efficient and require little work and time between flights (what aircraft operators call a short ‘turn-around’). This sounds just like the description of a regular airliner, and so designs to address these requirements often resembled aircraft.

Normally rocket propulsion would need to be incorporated, because jet engines do not work in space due to a lack of oxygen. But in the 1970s the technology to build a single-stage orbital rocket plane did not exist. NASA therefore sought a multi-stage design. It initially envisaged a combination of two rocket planes in which a massive winged booster would release a smaller vehicle at sufficient altitude and speed for it to insert itself into orbit. The booster would fly back to the launch site. In due course the spaceplane would also land and be prepared for another mission. This two-stage design meant less severe vehicle empty-weight minimization challenges, while both stages of this combination would be fully reusable. But a combination of technical and budgetary constraints mandated a compromise in which the winged first stage vehicle was replaced by a pair of reusable solid propellant rocket boosters, and the orbital spaceplane got an external propellant tank that would be discarded on each flight. This became the Space Shuttle that NASA flew from 1981 to 2011. An ideal spaceplane it was not, because its long launch preparations, complex maintenance, partial reusability and relative fragility led to high costs and high risks. It turned out to be far less cost effective and a much more dangerous means of gaining access to low orbit than its designers envisioned. By the mid-1980s there was a sense in the US, Europe, Russia and Japan that it was time for a fully reusable spaceplane with aircraft-like operations. To limit the propellant load, and thus the overall size and weight, the vehicle would have to combine airbreathing and rocket engines: using oxygen in the atmosphere as oxidizer as long as practical, prior to switching to rocket propulsion when the air density fell below the minimum level required to operate a jet engine. It was believed that materials, flight control and propulsion technology were sufficiently matured to make the development of such a space plane possible.

In a perfect world, a trip in an ideal spaceplane that adheres to the constraints of known physics and near-future technical possibilities could go something like this: On waking up at home you put on your simple flight overall, have a pleasant and relaxed breakfast, then get into your (flying?) car and head for the spaceport. Upon arrival you check in, but don’t need to put on a cumbersome spacesuit, you simply proceed to the plane. In contrast to the early days of spaceflight, there is no doctor checking your fitness, as the flight is extremely benign in terms of accelerations and shocks. Boarding the plane is similar to embarking a normal airliner. Soon after you have settled in, the vehicle starts to rumble down the runway. Just like any airliner, it takes off horizontally and initially ascends at a shallow angle. Apart from requiring a rather long runway there has been nothing extraordinary about the flight up to this point. Even when, two minutes after take-off, the pilot announces that the plane is going supersonic (faster than sound) you don’t notice anything peculiar other than that noise of the engines diminishes because the spaceplane is now outrunning its own sound waves in the air outside. A bit more than another two minutes later you are at an altitude of 12 km (8 miles) and flying at twice the speed of sound. Normal airliners don’t go higher than this, but your spaceplane keeps on climbing. Soon the sky goes dark and the stars become visible. The curvature of the horizon becomes noticeable, confirming the Earth to be a sphere rather than the flat surface it seems through the window of a normal airplane.

At over 28 km (17 miles) and a velocity exceeding five times the speed of sound the engines switch from their airbreathing to pure rocket propulsion mode, and the acceleration, which has been hardly noticeable, suddenly increases and pushes you deeper into your seat. The airplane has become a rocket spaceplane, independent of the oxygen and lift generating capabilities of the atmosphere. Sixteen minutes after take off you are 80 km (50 miles) high and accelerating more than three times more rapidly than a free-falling sky diver. At that moment the engines are shut down, and immediately all sensations of gravity and acceleration vanish and you feel yourself floating in your seat, held in place only by the safety harness. The spaceplane is now in an elliptical orbit around the Earth. You have not yet reached the highest point of your orbit, so you are still going up even although there is no sense of acceleration. Soon you cross the theoretical border of space at 100 km (62 miles) altitude. If you weren’t one already, you’re now officially an astronaut. At 400 km (240 miles) the spaceplane reaches the highest point and the rocket engines are reignited briefly for the boost required to achieve a nice circular orbit. By using small rocket engines, the spaceplane carefully maneuvers towards the space station that is your destination. A couple of hours later the vehicle docks at the large collection of cylindrical modules. You unstrap from your seat and simply float out of the cabin, through the docking tunnel into the space station. You stay there a couple of weeks to work in the biology experiments laboratory module.

When it is time to go home, you board a docked spaceplane of the same type you came up with. It is not the exact same vehicle, because spaceplanes arrive and leave every couple of days transporting people, food, oxygen and experiments to the space station. After undocking, the vehicle slowly drifts away. A half-minute burst of a set of relatively small tail engines in the direction the spaceplane is orbiting, slows the plane down just enough to change its orbit from a circle into an ellipse. The highest point of the orbit is at the altitude of the space station and its lowest point penetrates the atmosphere. As the altitude of the vehicle drops, the aerodynamic drag from the thin atmosphere slows the plane even more. Half an hour after the de-orbit burn you still appear to be in orbit as if nothing had changed, but you are actually falling slowly back to Earth.

The plane was flying backward when firing its rocket engines to break out of its circular orbit, which is okay in the emptiness of space, but it’s not a healthy attitude for entering the thicker layers of the atmosphere. So the pilots rotate the spaceplane through 180 degrees to make it fly nose first. They also pull the nose up, to align its heat resistant belly with the wall of air it is about to encounter. This maneuvering is done using the small attitude control rocket thrusters, because in the near vacuum of space the rudders, elevators and ailerons on the wings and tail of the plane are totally ineffective. The heat shield protects the spaceplane from the extreme heat generated as it slams into the atmosphere. The edges of the wings glow red hot as they reach temperatures of 1,600 degrees Celsius (2,900 degrees Fahrenheit); greater than the melting point of steel. However, unlike the heat shields of the old space capsules the metallic shield on your spaceplane does not slowly burn up, and can thus be reused. It is also not as vulnerable as the old Space Shuttle’s thermal protection tiles, which were reusable but also rather fragile: if it had to, the spaceplane could fly through a hailstorm without damaging its heat shield. And to minimize the vehicle’s take-off weight no propellant was loaded as a reserve for the return flight, so the spaceplane glides back unpowered. As with the Space Shuttle, there is no option of aborting the landing and flying around to make another approach. This might appear to be risky, but actually it isn’t because if the return conditions weren’t perfect and for instance the weather at the airport was likely to be unfavorable, the spaceplane would have just waited in orbit or targeted another landing site well before the execution of the de-orbit burn. As usual the automatic flight system, supervised by the pilots, flies a perfect approach and landing. A bit shaky on your legs, because while you were in space your body adapted to weightlessness, you disembark from the plane. Even as you head home, the vehicle is being refueled, and after a short maintenance check it is declared ready for its next flight.

While this perfect reusable launcher does not yet exist, the above description is based on modern spaceplane concepts such as Skylon, currently under development

at the British company Reaction Engines Ltd. This machine only exists on paper, but realistically illustrates what a near-future orbital rocket plane may look like. What you experience as a passenger may not be too different from what it is like to fly in a high-performance jet aircraft, and while the spaceplane does superficially look like one, there is a big difference in the amount of propellant that it has to carry. While 40% of a large airliner’s take off weight may be fuel (51% in the case of the Concorde), a mixed-propulsion spaceplane such as Skylon will consist of 80% propellant. A pure rocket spaceplane that does not use any airbreathing engines at all would be over 90% propellant, a problem already accurately foreseen by rocket plane pioneer Max Yalier in the late 1920s. Such percentages are similar to those of existing expendable launchers but are much more difficult to attain for aircraft with wings, wheels and a cockpit, that must also be able to survive atmospheric re-entry. In addition, the liquid hydrogen that Skylon uses for fuel has a density that is much lower than that of the kerosene used by normal airplanes: where 1 Uter (0.3 gallons) of kerosene weighs 800 grams (1.80 pounds) the same volume of liquid hydrogen is 70 grams (0.15 pounds). The same amount of fuel thus takes much more room. The passenger cabin onboard a spaceplane will therefore be tiny compared to that of an airliner since most of the vehicle’s volume will need to be filled with fuel (as well as additional liquid oxygen to bum with the hydrogen during the rocket propelled flight phase).

This difference between airplanes and spaceplanes is not so much a matter of their operating altitudes, rather it is the result of their vastly different velocities. Airliners fly at about 950 km per hour (590 miles per hour), whereas to achieve a low orbit a spaceplane will have to achieve a velocity of about 7.8 km per second (4.8 miles per second), which translates to 28,000 km per hour (17,400 miles per hour)! This means the spaceplane’s velocity needs to be about 30 times greater than that of the average airliner. Furthermore, the energy needed to attain a given velocity increases with the square of the flight speed. This means a spaceplane needs some 30 x 30 = 90 times more energy than an airliner of the same weight. This energy must be gained by the engines converting the chemical energy of the propellant into kinetic (movement) energy. And this simplified calculation does not take into account the aerodynamic drag during the climb out of the atmosphere, which also increases quadratically with velocity.

When a spaceplane gets above the atmosphere and reaches orbital velocity, it can circle the Earth without any further need to burn propellant. An airliner however, needs to continuously compensate the drag of the atmosphere it is flying through to maintain its velocity. It does that using its engines, which consume fuel during the entire trip of often thousands of kilometers. This is why airliners in reality do not fly with 90 times less propellant than a spaceplane would require, which you would expect if taking into account speed alone. Nevertheless, whereas airliner designers achieve an optimum in terms of velocity, amount of propellant, cabin volume, cargo weight and ultimately cost, spaceplane designers are pretty much stuck with the need to cram as much propellant as possible into their vehicle, and hopefully in the end have some weight capacity left for the cargo that needs to be taken into orbit, which is, after all, the whole reason for the spaceplane’s flight! In other words, the margin between success and failure is very small: if the spaceplane tank structure proves to be a little heavier than anticipated or the rocket engine yields just 1% less thrust than foreseen, you may end up with a very fast but useless suborbital rocket plane with zero payload, rather than a satellite-launching, money-making, orbital spaceplane.

This narrow margin, plus frustration with the Space Shuttle in terms of costs and risks, has made the world’s space agencies and industries developing launchers extremely cautious with regard to spaceplanes. Since the development of the Space Shuttle hundreds of concepts for spaceplane (and other types of reusable launchers) have come and gone. Some hardware was build and tested, and some designs even got as far as flying a sub-scale test vehicle. But none of them has yet resulted in an operational vehicle, largely because the step to develop a full-scale spaceplane was deemed to be too risky and too expensive, and the benefits could not be sufficiently guaranteed.

In 2005 I attended an international conference on spaceplanes and hypersonic systems in Italy, where scientists and engineers from the US, Europe, China, Russia and Japan, and even Australia, India, South Korea and Saudi Arabia presented developments on exciting sounding topics such as pulsed detonation propulsion and aerospike engines, as well as highly specialized issues like ‘Fluctuations of Mass Flux and Hydrogen Concentration in Supersonic Mixing’ or ‘Pseudo-Shock Wave Produced by Backpressure in Straight and Diverging Rectangular Ducts’. It seemed to me that half the world was involved in spaceplane technology development, and there was certainly no lack of concepts. However, at the time of writing none of the designs for large prototypes, let alone operational spaceplanes, have moved beyond the drawing board.

Whereas up until the 1970s advances in rocket plane technology were often soon incorporated in new experimental planes and high-altitude rocket interceptors, now engineers seem to be stuck in their laboratories, able to fly their innovations only on small-scale test models. This is due to the extreme complexity and enormous cost of developing modern high-performance airplanes in general and space launch vehicles in particular. In the Second World War it took the famous P-51 Mustang fighter only six months to progress from the conceptual design to its first flight, with just another 19 months until it entered combat service. At the time the US government purchased them at $50,000 per airplane, the equivalent of $600,000 today. The US Air Force’s latest F-22 Raptor fighter took over 20 years to advance from concept to operational fighter at a development cost of $65 bilhon. Each of these sophisticated planes costs some $143 million: 238 times more than a Mustang! A future spaceplane will have a lot more in common with modern aircraft like the F-22 than with the nuts-and-bolts Mustang. There is no easy and cheap way to develop an operational spaceplane, so the technical and financial risks will be high. Hence, the benefits must also be high and more or less guaranteed.

Looking into the near future, it is clear that the preference for expendable launch vehicles is ongoing. Various new throw-away launchers or updates of existing ones are under development by a number of agencies, while research on reusable launch vehicles is continuing at a very slow pace with much lower levels of funding.