Crash course in rocket plane design

“Perfect as the wing of a bird may be, it will never enable the bird to fly if unsupported by the air. Facts are the air of science. Without them a man of science can never rise.” – Ivan Pavlov (1849-1936)

To be able to understand the possibilities, limitations, history and evolution of rocket planes, we must look at how they work. We start with the ‘rocket-’ part, then explore the ‘-plane’ element, and finally investigate the wonderful and dangerous things that happen when you combine them.

ROCKET ENGINES

The principle of a rocket engine is fairly simple: generate a gas at high pressure by burning propellant in an enclosed space and let it escape through a nozzle. The resulting thrust has nothing whatever to do with the rocket ‘pushing against the air’, but is purely a consequence of Isaac Newton’s famous principle: for every action there is an equal and opposite reaction. If you stand on a skateboard and throw rocks away, then you will move in the other direction: the ‘action’ is throwing rocks backward and the ‘reaction’ is you moving forward. Pushing the rock away also means pushing yourself away from the rock. Another good example is a fire hose: as lots of water spews out at high speed you feel the thrust trying to push you back. The hose sprays water in one direction, and in reaction the hose itself is pushed in the opposite direction. In essence this is a rocket engine working on water, and if you stood on the skateboard holding the fire hose, then you would have basically created a rocket propelled vehicle. Instead of throwing rocks, you would be throwing out water continuously. The principle that Newton derived works because the rocks and the water have mass, and the greater the mass and the higher the velocity at which you throw them away, then the higher will be the velocity that you achieve in the opposite direction. You can imagine that throwing something with a small mass relative to yourself, such as a feather, won’t have much effect. Throwing away a bowling ball with very little speed, essentially letting it fall out of your hands, will also not achieve much. Only if you throw away objects of substantial mass at a

M. van Pelt, Rocketing into the Future: The History and Technology of Rocket Planes, Springer Praxis Books, DOI 10.1007/978-1-4614-3200-5 2, © Springer Science+Business Media New York 2012

significant speed will the resulting thrust be enough to push you away on a skateboard. If the rock that you throw away is one-tenth of your own weight, then you will attain that proportion of the velocity at which the rock is flying out (ignoring the friction of the skateboard’s wheels with the ground). If you want to go faster, you can either throw out a larger rock at the same velocity, or the same rock at a higher velocity, or indeed a smaller rock at even higher speed.

The thrust of a rocket engine is measured in ‘Newton’ in the metric system, and in ‘pounds of thrust’ in the US. One Newton is the force that a 0.1 kg mass exerts on a floor on the Earth’s surface. Isaac Newton stated that a force (or thrust) is equal to mass times acceleration. On Earth, if you let something fall it will speed up by about 10 meters per second every second: i. e. after 1 second its velocity is 10 meters per second, after 2 seconds it is 20 meters per second, and so on. This means that on the Earth’s surface the gravitational acceleration is 10 meters per second per second (or to be more precise 9.81 meters per second per second). If you stand on your skateboard again, and every second you throw away a 1 kg rock at 10 meters per second, then you will be creating a thrust of 1 kg times 10 meters per second per second = 10 Newton.

In real rocket engines the necessary high pressures are generated by combustion. The resulting gas expands out through a nozzle at tremendous velocity, and because it has mass this results in a powerful thrust. It is basically a continuous explosion: a single explosion gives a short kick, a series of explosions provides a series of kicks, and continuous combustion and expansion yields a steady thrust. For combustion to occur, a fuel and an oxidizer are required. An oxidizer is a substance that contains the oxygen that makes things burn. In the engine of a car the fuel is gasoline and the oxidizer is ordinary air, which contains some 21 percent of oxygen. Rockets don’t use atmospheric air, but carry their own oxidizer.

In a liquid propellant rocket engine, the fuel and the oxidizer are in the form of liquids, for example alcohol and liquid oxygen. These are stored in separate tanks, from which they are fed into a combustion chamber using pressure in the tanks or powerful pumps. Such pumps are typically powered by a separate gas generator, in which some propellant (which can be the same as those used in the combustion chamber) is burned or decomposed to provide high-pressure gas. This gas is then fed

Rocket principle.

through a turbine that runs the pumps, and expelled through a separate exhaust; this arrangement is known as a turbopump. In more sophisticated engines some of the rocket’s propellants are burned in a pre-burner and then used to run the turbopumps. However, instead of being dumped directly, the exhausted gas is then injected into the main combustion chamber along with more propellant, in order to complete the combustion; this is called a ‘staged combustion cycle’. Turbopumps run at tremendous rates. For example, the turbines of the main engines of the Space Shuttle spin at 30,000 cycles per minute!

In the rocket combustion chamber the fuel and oxidizer are mixed and burned to create an extremely hot, high-pressure gas. This can only escape through an opening in the combustion chamber that is connected to the rocket nozzle. The gas flows out at high velocity through the nozzle, whose shape permits the gas to expand (and thus accelerate), and flow nicely in the right direction. For high-performance rocket engines the exhaust velocities are around 16,000 km per hour (10,000 miles per hour); much faster than throwing rocks! The nozzle is where the expanding gas exerts its forwards pressure, and the correct shape and length are critical in determining the achievable exhaust velocity. If the nozzle is too short or its shape

does not allow the exhaust to expand properly, then lots of energy that could be used for generating thrust is lost. The nozzle first converges to a narrow throat so that the velocity of the gas stream is increased, just as occurs when water is passed through a narrowing channel. At the throat it reaches Mach one (the gas mixture’s speed of sound) and creates a shock wave, after which the nozzle diverges to allow the high-pressure gas to expand and thereby flow out efficiently at speeds far beyond Mach one. The temperature of this gas stream can reach 3,000°C (5,400°F). The combus­tion chamber and the nozzle must be cooled to prevent them from melting. Their walls are often made hollow, so that rocket propellant can be pumped through in order to act as coolant before being burned inside the combustion chamber. It is also possible to make the nozzle so thick that it can be allowed to slowly erode during flight. These so – called ablative nozzles are relatively simple and cheap, but also heavy and obviously not reusable. They are usually applied in solid propellant boosters, which have no liquid propellants to use as coolants.

The efficiency of a rocket is indicated by its specific impulse, and is measured in seconds. It is one of the most important parameters in the Solid propellant rocket. equations that describe a rocket’s performance. A

specific impulse of 400 seconds means that with 1 kg of onboard propellant the rocket can generate 1 kg of thrust (i. e. 10 Newton) for 400 seconds. (In the US, 1 pound of thrust from 1 pound of propellant for 400 seconds). You can think of the specific impulse of a rocket engine as the number of seconds a certain amount its propellant is able to generate the thrust required to keep itself in the air.

The simplest rockets combine the fuel and oxidizer into a solid propellant like for instance gunpowder. A solid propellant rocket can be viewed as a pipe stuffed with propellant. The propellant grain is usually hollow in order to expose a large burning area, and hence a high thrust due to the high pressure and the resulting large amount of gas flowing out. Firework rockets are of this type. The main advantage of solid propellant rockets is that they are simple, because they do not require any pumps, pipes and valves. As a result, they can provide a lot of thrust for relatively low cost. A big disadvantage however, is that they normally cannot be reused: the throats and nozzles of these motors typically bum away during firing, as there are no liquids to act as coolant. Other very important disadvantages are that, in contrast to liquid propellant engines, solid propellant motors cannot be stopped and you cannot actively control the thmst. Once ignited, the grain will bum away until it is all gone. If something goes wrong along the way, you cannot slow down or stop. In fact, if there is a problem with the motor itself, like a crack in the propellant grain or a piece of material blocking the nozzle, the propellant will keep on burning and the increasing pressure will result in a violent explosion. Solid rockets either work well or they blow up; there is no ‘benign failure’ which merely results in a loss of thmst, as is possible when using liquid propellant rocket engines. Nevertheless, by cleverly designing the shape of the propellant grain it is possible to vary the amount of thrust desired at certain times after ignition. Many solid propellant rockets used for launch vehicles have star-shaped cross sec­tions. At first the burning surface will be large and the rocket will provide a maximum of thmst in order to get the vehicle off the ground quickly. As the pie-shaped sections burn away, the active surface is reduced and the thrust diminishes in order to limit the aerodynamic forces on the vehicle while it is flying at high speed through the atmosphere.

Another important disadvantage is that for any given mass of propellants, solids cannot provide as much thmst as liquids. A rocket engine that uses liquid hydrogen and liquid oxygen can have a specific impulse of 450 seconds but solid propellants can achieve no better than 290 seconds. On the other hand, because of their relative simphcity it is

Space Shuttle [NASA],

easier to develop and build big powerful solid propellant rockets than large liquid propellant rocket engines, albeit they are much less efficient in terms of the amount of energy to be derived per kilogram of propellant. Also, the storage of solid propellants (in the rocket motor) is generally easier than of liquid propellants, which often require cooling and can be corrosive or toxic.

Probably the most famous solid propellant rockets are the Solid Rocket Boosters of the Space Shuttle. Each of these huge rockets provides a maximum thrust of 13,800,000 Newton, which means one booster could lift some 1,380 cars. Together the two boosters generate about 83% of the lift-off thrust of the Space Shuttle, with the more efficient but far less powerful liquid propellant engines of the Orbiter vehicle only providing the remaining 17%. The Space Shuttle’s rocket boosters were actually recovered (by parachute, splashing into the ocean) and reused, but this required much cleaning and refurbishment, and was only economical because of the booster’s huge size and production cost.

In contrast to a rocket engine used in an expendable missile, one that is meant to propel an aircraft has to comply with more requirements: it needs to be restartable, reusable, maintainable, and reasonably safe for both the pilot and the ground crew. Ablative cooling, where the rocket’s throat and nozzle lose heat by slowly burning away, is not a viable solution for a reusable system; an aircraft rocket engine needs active cooling which pumps the propellant through cooling ducts in the combustion chamber and nozzle. The engine’s igniter, which was an external piece of ground support equipment for the series of missiles of which the A4/V2 was part, must be incorporated into the rocket motor itself. On the other hand, the required reliability and safety means that performance may require to be sacrificed to improve safety margins, reduce wear and tear and simplify maintenance.

Overall, their non-reusability, lack of a throttle, poor efficiency and explosion hazards make solid rocket motors generally a poor choice for propelling manned aircraft, other than briefly for an assisted take-off. Liquid propellant rocket engines are more controllable, more efficient and can be made reusable, so are generally a better choice for aircraft propulsion, even if they are more complex.

The main benefit of rocket engines over jet engines and propellers, and the reason that they are used in spaceflight, is that they can operate outside the atmosphere. Jet engines uphold Newton’s ‘action equals reaction’, just like rockets, but they depend on oxygen from the air to burn aircraft fuel. Propellers push air backwards to make a plane go forward, and are driven by engines that need atmospheric oxygen. Rockets carry both the necessary oxidizer as well as the fuel. Independence from the atmosphere is rather handy if you are flying at high altitudes or through the vacuum of space, where there is either insufficient or no oxygen available for your engines. Interestingly, rocket engines also offer an advantage in the thicker atmosphere because their operation is totally independent of velocity. Propellers lose efficiency because of aerodynamic shock waves which form when the rotation of their blades approaches the speed of sound. The same holds for the compressor fans in turbojet engines, although these can still be used at supersonic flight velocities because the airflow entering the engine can be slowed down to subsonic speed by the air intakes (but this does cost energy at the detriment of the plane’s velocity, and becomes increasingly problematic at higher supersonic flight speeds).

Rocket engines are also intrinsically less complex than jet engines. This is not to say that rocket motors cannot reach very high levels of complexity (just take a look at a schematic of the Space Shuttle Main Engine) but it is generally easier to build a simple rocket motor than it is to construct a simple jet engine. Basic solid propellant rocket motors are much simpler than any piston engine and propeller combination; an alternative history in which the Wright brothers power their aircraft using not a primitive piston engine but a few simple solid rockets is not completely unrealistic. After all, solid propellant rockets had already been in use for several centuries when the piston engine and – even later – the jet engine were invented.

Rocket engines running on liquid propellants are much more efficient in terms of the amount of energy that can be obtained out of a certain amount of propellant, but the requirement for pumps, valves and cooling makes them more complex than their solid propellant counterparts. Nevertheless, without large compressors and turbines, complex air intakes, supersonic shock problems and intake drag, they were, at least initially, a simpler solution for high-speed, high-altitude aircraft than turbojets. This is why so many of the high-speed, high-altitude aircraft developed during the 1940s and 1950s were propelled by rockets rather than jet engines. For the same thrust, a liquid propellant rocket engine is also much less heavy than a turbojet engine: a large modern rocket engine can produce a thrust that is 70 (the Space Shuttle Main Engine) or even 138 times (the Russian NK-33) as great as its own mass (although this number is much lower for smaller rocket engines), whereas for a turbojet the thrust to weight ratio approaches eight at best. For the same thrust, rocket engines are also considerably smaller than jet engines. However, a rocket engine quickly loses this advantage if account is taken of the weight and volume of the propellants that must be carried. Considering just the fuel flow, since the oxidizer comes from outside, the specific impulse of a jet engine is some 20 times that of a rocket engine, and is thus much more efficient. For a plane that needs to have a considerable range it soon becomes more economical to use an air-breathing jet engine rather than rocket propulsion. In aviation rocket engines have therefore been mostly used for early high-speed, short range aircraft such as experimental planes and high altitude military interceptors.