The SCORPION Aerospike Engine
Aerospike rocket engines offer an excellent opportunity to improve the altitude-depending losses known from conventional bell nozzles throughout the whole flight. At the SCORPION engine the supersonic exhaust gases flow around a central body, while they self-adjust to ambient air pressure. This leads to a propulsion efficiency considerably closer to ideal rocket propulsion also offering a substantially wider operating range from ground to vacuum with only one engine.
As the aerospike engines' most characteristic feature is a spike, we named our engine "SCORPION" after the star constellation Scorpius which can be found in our RISE logo.
The annular combustion chamber and nozzle provide unique thrust vector control (TVC) methods that are only possible due to the exhaust flow being separated by the central body. It enables the manifestation of different thrust forces acting on opposite regions of the spike. The resulting thrust vector hereby gains a lateral component enabling TVC.
Outside the design point the efficiency of traditional bell shaped rocket engine nozzles deviates significantly from the optimum while moving through the atmosphere’s altitude-depending ambient pressure conditions until outside the atmosphere vacuum-optimized-nozzles are necessary. Aerospike rocket engines seek to increase the efficiency throughout theascent or descent through changing ambient pressures by the application of free supersonic expansion guided by a central body, thereby self-adjusting the supersonic exhaust flow to the ambient conditions.
AQUILLA rocket with cut-open SCORPION engine
SCORPION aerospike engine concept in a cut-open view
Test rig with force measurement platform and exemplary test engine
AQUILLA rocket with cut-open SCORPION engine
SCORPION's Working Principle
The supersonic expansion contour of traditional bell-shaped rocket engine nozzles leads to overexpansion or underexpansion of the exhaust stream under all conditions except for the moment while passing through the design point, where the exhaust pressure pe equals the ambient pressure p∞ yielding maximum thrust. This special case is shown in (c).
At p∞>pe conditions, the high ambient pressure leads to constriction of the exhaust stream as shown in (a) and (b). This overexpansion can cause the flow to detach from the nozzle wall, forming a shockwave inside the nozzle, usually leading to the destructionof the latter. With the ambient pressure below the exhaust pressure (p∞<pe), the exhaust stream expands at the nozzle exit as illustrated in (d). With the impulse vectors of the exhaust molecules gaining lateral components, the longitudinal component decreases, reducing net engine thrust, thus leading to a decrease in efficiency of more than 20 % in some cases.
Aerospike nozzles in contrast do not physically bound the supersonic expansion but let the ambient pressure and the expansion waves constrain the exhaust as free stream. Consequently, the supersonic exhaust adjusts itself to this ambient pressure while flowing around the spike. This leads to parallel boundaries of the exhaust stream under different ambient pressure conditions with minimal altitude-related losses as illustrated in (e)-(h). The current RISE spike contour is analytically calculated for the ambient pressure of (g) based on the Prandtl-Meyer-expansion with exhaust boundaries parallel and symmetric to the nozzle axis as constraint.
Efficiency and Operating Range
Outside the design point the efficiency of traditional bell shaped rocket engine nozzles deviates significantly from the optimum while moving through the atmosphere’s altitude-depending ambient pressure conditions until outside the atmosphere vacuum-optimized-nozzles are necessary. Aerospike rocket engines seek to increase the efficiency throughout the ascent or descent through changing ambient pressures by the application of free supersonic expansion guided by a central body, thereby self-adjusting the supersonic exhaust flow to the ambient conditions.
The SCORPION aerospike nozzle is much closer to ideal, lossless nozzles as compared to bell nozzles also providing a wider operating range. Single Stage To Orbit (SSTO) applications would therefore be possible with one nozzle from sea-levelto vacuum. With respect to the Ziolkowsky Equation SSTO applications should utilize expendable tanks (comparable to Space Shuttle external tanks) instead of staging to reduce superfluous structural mass during ascent.
Thrust Vector Control - TVC
In order to steer a rocket - until today - the whole engine is moved to control the direction of the exhaust stream. The annular combustion chamber and nozzle of the SCORPION aerospike engine provide unique TVC methods that are only possible due to the exhaust flow being separated by the central body. It enables the manifestation of different thrust forces acting on opposite regions of the spike. The resulting thrust vector hereby gains a lateral component enabling TVC without the need to gimbal the whole engine.
The RISE team evaluated several aerodynamic approaches such as locally...
injecting oxidizer in the fuel rich flow at one side of the spike
injecting fuel in the oxidizer rich flow at one side of the spike
changing the O/F ratio upon injection
changing the mass flow upon injection
changing the throat gap for example by displacing the central body or parts of the shroud
unequal base- or combustion chamber bleed
Right now we are focussing on the approach to locally change the mass flow upon injection by using throttleable pintle injectors. The concept incorporates a control ring ("TVC ring") comparable to a helilcopter swash plate. Tilting the TVC ring with actuators throttles the injection elements with the help of control rods. Hereby the center of mass flow (center of mass differentiated with respect to time) is moved providing a highly responsive unsymmetric combustion leading to a lateral force depending on the magnitude and the center of mass flow.
The image shows the result of a CFD computation locally throtteling pintle injectors. The exhaust stream's offset from the nozzle axis is clearly visible. The following images show the pintle concept and the TVC ring concept as well as a 3D printed pintle prototype in our test rig.
Bipropellants Ethanol - Nitrous Oxide
The propellants are pressurized separately in the rocket, while nitrous oxide is stored as a liquid in the tank. As further innovation concepts, the feasibility of an electric pump for ethanol is examined as well as the utilization of the self-pressurization of nitrous oxide. The propellants are fed into the combustion chamber through throttleable fuel-centered pintle injectors, used for aerodynamic thrust vector control by a jointly continuously variable actuation as shown in the section above. At the moment, we are working intensely on the design and construction of our first hot gas test stand. In 2022, the commissioning of the test stand and the first combustion tests of individual injectors are planned. The figure shows our current hot gas test stand tubing and valve concept.
As a student team, we obviously want to indulge our cravings for intoxicating substances. To offer something for every taste, we have chosen the combination of ethanol and nitrous oxide. Coincidentally, both substances can be used as rocket propellants - What a lucky chance!
Individually, these propellants allow fairly safe handling appropriate for a student project. However, the fun fully kicks into high gear as soon as the propellants are atomized, mixed and ignited in the combustion chamber.