Category Rocketing into the Future

Conventional planes depend a lot on the atmosphere: air is required for the wings to provide lift, to enable surfaces like rudders and ailerons to provide control, and as a supply of oxygen for the engines. However, flying through air also produces drag, which makes going very fast at low altitudes difficult. The higher the altitude, the lower the air density and thus the lower the drag at a certain speed. As a result, it is possible to fly faster at higher altitudes. But at the same time the lower air density Umits the amount of available lift and thrust. Hence an aircraft’s overall maximum speed is linked to an optimal altitude. Above that altitude the maximum attainable speed drops, because the engines are less efficient and the plane may require to fly with a higher angle of attack in order to produce enough lift, which increases drag. Furthermore, below the ideal altitude the greater air density will increase drag and hence also limit the plane’s maximum velocity.

The use of rockets instead of airbreathing engines eliminates the reliance on the atmosphere for producing thrust. Without air obstructing the outflow of the exhaust jet, rockets can actually work more efficient in vacuum than within the atmosphere. With rocket engines, the maximum altitude of a plane is only limited by the lift it requires. And because lift is a function of the square of the velocity, rocket engines can accelerate the vehicle to such high speeds that even small wings can generate sufficient lift. Moreover, since rocket engines do not take in air from outside, they have no velocity constraints such as those that limit the speed at which airbreathing engines can be used. Unlike ramjets, rocket engines do not require a minimum flight velocity, and unlike gas turbine engines they aren’t constrained by a maximum velocity beyond which the inrushing air heats up the engine so much that turbine blades are damaged. Rocket engines thus allow high speeds at high altitudes; in principle even virtually unlimited speeds at unlimited altitudes. Rockets can thus even be used to propel an aircraft into orbit, turning it into a spaceplane!

At high altitudes planes do experience control problems, however: because the efficiency of an aerodynamic control surface depends on the density of the air, it drops as the altitude increases. At very high altitudes and in the vacuum of space a control surface is completely ineffective, no matter how large it is. As a result, the vertical stabilizer and rudder can no longer ensure that the nose remains pointing in the direction of flight, and the elevators and ailerons can no longer control pitch and roll. Small misalignments in the rocket engine’s thrust direction or weak residual aerodynamic forces on the wings and fuselage can cause a plane roll, pitch and yaw uncontrollably. If a plane falls in an uncontrolled orientation, maybe even sideways, the increasingly denser air could easily tear it apart. A solution is to equip the plane with reaction control thrusters: small rocket engines like those used on spacecraft for attitude control. When the aerodynamic control surfaces are rendered ineffective the thrusters can take over, pushing an aircraft around its axes to provide roll, pitch and yaw control.

In addition, the main rocket engine(s) can provide control during the powered phase of a flight by the use of gimbals. These enable a rocket engine to rotate in the horizontal and/or vertical direction in order to provide thrust vector control. When a rocket engine that is mounted in the rear of the fuselage is gimbaled up, its thrust will push the tail down. As the plane rotates around its center of mass, its nose will go up. Vertical gimballing of the engine thus provides pitch control. Likewise, rotating the nozzle in the horizontal direction provides yaw control. Roll control can be achieved if you have two rocket engines off-center from the rotational axis running from the nose to the tail of the plane, for instance one in each wing. In that case you can push one wing up and the other one down to make the plane roll. Rocket engine gimbals are pretty heavy however, and rocket planes are therefore normally controlled using aerodynamic control surfaces and small reaction control thrusters. The Space Shuttle Main Engines could gimbal to help to control the vehicle’s attitude during the ascent through the atmosphere. When watching footage of a Space Shuttle launch, look for the test-gimballing of these three engines shortly prior to ignition. The Solid Rocket Boosters could also vector their thrust, but these massive rockets were discarded about two minutes into the flight.

Although rocket engines enable a plane to fly both higher and faster than any conventional airbreathing vehicle, a downside is of course that rocket planes, apart from fuel, need to carry oxidizer with them. This adds weight, requires additional tanks and takes up more internal volume. An added complication is that while most jet aircraft carry part of their fuel in their wings, this is generally not possible for a rocket plane. That is because rocket engine turbopumps require the propellant tanks to be pressurized to at least several times that of the atmosphere at sea level in order to ensure an efficient flow without cavitation (the formation of bubbles). Generally only tanks that are more or less cylindrical in shape can contain such pressure while remaining relatively light (like a balloon), but the resulting bulky shapes are impossible to fit into the thin wings of a fast plane. If you look at a cut-away drawing of for instance the X-l or X-15 rocket planes, you will notice that indeed most of the available fuselage volume is taken up by propellant tanks.

The need to carry oxidizer means that the flight time of a rocket plane is a lot less than that of a similar jet-propelled airplane that can fill all its tanks with fuel and take its oxygen from the air. Rocket planes are therefore primarily useful for missions that require only relatively short-duration rocket-propelled boosts. Rocket aircraft usually return in an unpowered glide, what pilots call a ‘deadstick landing’, because the rocket propellant will typically have been spent earlier in the flight.

Many experimental rocket planes have been dropped from carrier planes. This saves the propellant that would have otherwise been needed to accelerate the rocket plane to take-off speed and to fly it to the planned test area. The function of a carrier plane is thus similar to that of the first stage of a conventional launch system. The rocket aircraft also has a shorter climb to attain its target altitude, because it begins its independent flight at the altitude of its carrier. Finally, at high altitude the air pressure at the rocket nozzle’s exit is lower, enabling the exhaust gases to flow out
more freely and expand further than at sea level. With a nozzle optimized for these conditions, the result is a higher specific impulse and a higher efficiency without any changes in the rocket engine itself. All this combined, saves considerable propellant weight. For example, the starting weight of the air-launched X-15 (about which you will read more later on) was almost 43% fuel and oxidizer. In other words, the propellant weight was equal to ‘only’ three quarters of the rocket plane’s empty weight, but this was sufficient to enable the plane to climb to altitudes over 100 km (330,000 feet). A similar aircraft capable of doing that by taking off from the ground would have been about 70% of propellant: a propellant weight twice the vehicle’s empty weight. The air launch enabled the X-15 to be a much smaller aircraft with lower structural weight constraints. Of course, the price paid is a large carrier aircraft, although for many applications an existing, slightly modified bomber suffices.

Apart from directly saving on propellant, dropping a rocket plane in the air means that it does not need a robust and heavy undercarriage that can handle the weight of the fully loaded plane prior to taking off. Wheels or skids will still be required to land the vehicle, but these can be small and light because they will only need to support a nearly empty plane, since half of the initial weight was propellant and that has been consumed. Indeed, the undercarriage and structure of the X-15 was only designed to carry the plane on the ground with empty tanks. On one mission in 1959 a small fire in the rocket engine forced pilot Scott Crossfield to make an emergency landing. He was unable to dump all of the propellant before he

touched down, so he landed at a tremendous speed and the heavy load snapped the vehicle in two! The Southern California Soaring Society awarded Crossfield the ‘Order of the Streamlined Brick’ for this flight, as it had set a record for the shortest descent time from 38,000 feet (11.6 km) to the ground as a glider. Fittingly, the trophy was a streamlined brick mounted on a piece of mahogany.

If, like the X-l and the X-15, an air launched rocket plane is small enough it can be carried in the bomb bay or under a wing. When it is released it simply falls away from its mothership and can start its rocket engine once safely clear, so that there is little risk of a collision or damage to the carrier due to the rocket exhaust. A larger (space)plane will need to be put on top of its carrier, requiring it to have sufficiently large wings and initial thrust to quickly fly away without falling back down, yet not scorch its mothership with its super-hot exhaust. That this can be a risky procedure was demonstrated in 1966, when a unmanned experimental drone carried on top of an SR-71 suffered engine problems and struck the carrier’s tail immediately after separation. Both planes were lost, and although the Blackbird’s two crewmembers ejected and parachuted in the sea, one of them drowned.

The X-15 was what is called an ALHL: Air Launched & Horizontal Landing. There are a number of other possibilities, which aeronautical engineers designate with equally puzzling codes. Conventional aircraft are HTHL: Horizontal Take-off & Horizontal Landing. But if the thrust of the engines exceed its take-off weight, the plane could operate as a VTHL: Vertical Take-off & Horizontal Landing. At launch, the Space Shuttle System is a pure rocket and does not use its wings. Only on its way back to Earth does the Orbiter exploit its aerodynamics to fly and land like an airplane. One reason it takes off vertically is because it is attached to the huge External Tank and large Solid Rocket Boosters. It is hard to see how such a collection of bulky rocket stages and tanks could take off horizontally from a runway; imagine the enormous undercarriage required! Structurally, it is also much easier to design something that large and tall to be launched vertically. Just think of a long wooden pole: if you hold it vertically it will remain straight and you could put quite a load on top of it, but if you hold it horizontally its own weight may already be sufficient to bend or perhaps even break the pole.

A vertical launch also ensures that the Space Shuttle clears the denser part of the atmosphere as soon as possible, to limit aerodynamic drag and therefore the amount of propellant needed to achieve orbital speed: spending less time in the atmosphere and not requiring wings which create lift and also drag generally means that vertical take-off launchers need less propellant to get into orbit than a spaceplane taking off horizontally. Even if a rocket propelled spaceplane were to raise its nose sharply for a near-vertical climb immediately after takeoff, it would still use considerably more propellant than a vertical take-off vehicle because it must fire its rocket motor for a longer time due to its less efficient trajectory. But if the spaceplane combines jet engines with rocket propulsion, and spends a considerable time efficiently building up speed using its airbreathing propulsion within the atmosphere, this can more than compensate for the energy required to overcome aerodynamic drag. Moreover, a horizontally flying spaceplane can use its wings to stay in the air, while a vertically launched machine has only its thrust to keep it from falling back to Earth, and thus needs a more powerful engine. A winged launcher taking off horizontally can gently build up speed, while a vertical take-off vehicle needs to accelerate rapidly because otherwise the energy that it loses due to gravity will be too great (costing too much propellant to compensate). The result is that for a HTHL vehicle the acceleration during launch can be low, which is especially beneficial for passenger transport. Such accelerations are called ‘G’ forces. Standing on the ground you experience one G, resulting in your normal weight. In a fast accelerating sports car, the horizontal force with which you are pushed back into your chair may be 0.7 G, i. e. equivalent to 70% of normal gravity. A descending elevator initially causes a bit less than 1 G, making you feel lighter, but once the elevator achieves a constant velocity, only the normal 1 G gravity force remains. And while it is slowing down, you feel a bit more than 1 G. In a free fall you have no weight, i. e. 0 G. This is the situation in orbit, which is merely a continuous free fall around the Earth.

Another benefit of a HTHL is that it potentially has better abort characteristics: a horizontally flying plane that loses thrust can continue underpowered, even glide if necessary (provided it has sufficient speed). A vertically launched vehicle will simply fall out the sky unless it has a sufficient number of redundant engines (‘engine-out’ capability), but this imposes high weight and cost penalties. The sudden shut down of an engine in the first few seconds of a vertical launch is thus also likely to have catastrophic results. However a spaceplane starting horizontally may be able to stop before the end of the runway, or fly around on reduced thrust for an emergency landing in the same way as a normal multi-engined aircraft. The possibility to fly at less than full power also means that HTHL spaceplanes can be test flown, progressively increasing speed and altitude on successive missions. In contrast, the VTHL Space Shuttle’s first powered flight had to be a full-blown orbital mission.

In contrast to vertical take-off vehicles, HTHL planes can, at least in principle, use existing runways and airports for take-off and landing (of which trillions of dollars worth of infrastructure already exists, spread all over the planet); launchers leaving the ground vertically need dedicated launch platforms and towers. On the other hand, horizontal take-off exposes a plane to failure modes which do not apply to vertically launched machines, such as collisions with obstacles or blown tires (for instance, out of the nineteen SR-71 Blackbirds that were lost in accidents, four were as a result of tire failures during take-off, and the only Concorde crash was caused by coming into contact with debris on the runway).

A Vertical Take-off & Vertical Landing (VTVL) vehicle is in principle possible, but generally does not make much sense for a winged plane that is optimized to fly horizontally. The Harrier ‘jump jet’ was equipped to take-off and land vertically in order to be able to operate from small clearings on a battlefield, but that capability adds a lot of complexity to the design of the plane and its engine (which uses four rotating nozzles), and was purely to satisfy the aircraft’s military requirements. There are concepts for VTVL launch vehicles, but these do not have wings and can thus not be considered rocket planes.

If you have wings on your vehicle, it is generally best to exploit them as much as possible and thereby minimize complexity and the amount of thrust required. Rocket planes are therefore usually HTHL or ALHL vehicles. But HTVL does not really

An AV-8B Harrier lands vertically on an aircraft carrier [US Navy].

make much sense, since if you have the wings and sufficient runway to take off horizontally, it is hard to justify the need for a vertical landing when you get back.

So with this technical background in mind, let’s now see how rocket planes have evolved since they first became a serious business shortly before the Second World War.