Principles
The basic concept of any engine bell is to efficiently expand the flow of exhaust gases from the rocket engine into one direction. The exhaust, a high-temperature mix of gases, has an effectively random momentum distribution, and if it is allowed to escape in that form, only a small part of the flow will be moving in the correct direction to contribute to forward thrust.
Instead of firing the exhaust out of a small hole in the middle of a bell, an aerospike engine avoids this random distribution by firing along the outside edge of a wedge-shaped protrusion, the "spike". The spike forms one side of a virtual bell, with the other side being formed by the outside air—thus the "aerospike".
The idea behind the aerospike design is that at low altitude the ambient pressure compresses the wake against the nozzle. The recirculation in the base zone of the wedge can then raise the pressure there to near ambient. Since the pressure on top of the engine is ambient, this means that base gives no overall thrust (but it also means that this part of the nozzle doesn't lose thrust by forming a partial vacuum, thus the base part of the nozzle can be ignored at low altitude).
As the spacecraft climbs to higher altitudes, the air pressure holding the exhaust against the spike decreases, but the pressure on top of the engine decreases at the same time, so this is not detrimental. Further, although the base pressure drops, the recirculation zone keeps the pressure on the base up to a fraction of 1 bar, a pressure that is not balanced by the near vacuum on top of the engine; this difference in pressure gives extra thrust at altitude, contributing to the altitude compensating effect. This produces an effect like that of a bell that grows larger as air pressure falls, providing altitude compensation.
The disadvantages of aerospikes seem to be extra weight for the spike, and increased cooling requirements due to the extra heated area. Further, the larger cooled area can reduce performance below theoretical levels by reducing the pressure against the nozzle. Also, aerospikes work relatively poorly between Mach 1-3, where the airflow around the vehicle has reduced pressure, and this reduces the thrust.
Read more about this topic: Aerospike Engine
Famous quotes containing the word principles:
“With our principles we seek to rule our habits with an iron hand, or to justify, honor, scold, or conceal them:Mtwo men with identical principles are likely to be seeking fundamentally different things with them.”
—Friedrich Nietzsche (1844–1900)
“Ah, I fancy it is just the same with most of what you call your “emancipation.” You have read yourself into a number of new ideas and opinions. You have got a sort of smattering of recent discoveries in various fields—discoveries that seem to overthrow certain principles which have hitherto been held impregnable and unassailable. But all this has only been a matter of intellect, Miss West—superficial acquisition. It has not passed into your blood.”
—Henrik Ibsen (1828–1906)
“When great changes occur in history, when great principles are involved, as a rule the majority are wrong.”
—Eugene V. Debs (1855–1926)