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THE AEROSPIKE NOZZLE
FREQUENTLY ASKED QUESTIONS LIST

BY KEN DAVIDIAN


  1. WHAT IS AN AEROSPIKE NOZZLE?

    The following information comes from an article "Nozzle Design" by R. A. O'Leary and James E. Beck in the Spring 1992 (No. 8) issue of Rocketdyne's *Threshold* magazine (call (818) 586-2380/2771 or write to Rockwell Aerospace/ Rocketdyne Division, 6633 Canoga Avenue, Mail Code AB57, Canoga Park, CA, USA, 91304-7922 to get a copy).

    Briefly, a spike (or "plug") engine uses an exhaust nozzle that can be thought of as a conventional bell shaped nozzle turned inside-out. The aerospike nozzle is a truncated version of an ideal spike.

    For a diagram, seeGeorge P. Sutton, Rocket Propulsion Elements, 6th edition, J. Wiley & Sons, 1992, p. 70 orDieter K. Huzel and David H. Huang, Design of Liquid Propellant Rocket Engines, NASA-SP-125, 1971, p.92.

    In a bell nozzle combustion gases flow through a constriction (throat) and then the expansion away from the centerline is contained by the diverging walls of the nozzle up to the exit plane. Bells nozzles are a point design with optimum performance at one specific ambient pressure (i.e., altitude). Careful design is needed to achieve desired high altitude performance while avoiding flow separation at the walls of the nozzle near the exit when operating at low altitudes (launch), which can lead to loss of performance and possible structural failure of the nozzle due to dynamic loads [flow separation is responsible for the large nozzle motion on the SSMEs during startup transient - watch closely during next launch]. Therefore a compromise altitude must be used for the design point of a bell nozzle.

    In an spike nozzle the opposite takes place - the gas flow is directed radially inward from an annulus at some diameter away from the centerline. This flow is directly exposed to ambient pressure and its expansion is thus directly coupled to the external environment (continuous altitude compensation with no moving parts). Thus, a very high area ratio nozzle (high vacuum performance) can also operate efficiently and safely at sea-level.

    Truncating the ideal spike to save weight results in a wake at the base which has some performance loss. This can be offset by pumping secondary flow (about 1% of primary flow) into the base region to elongate the wake which then forms an aerodynamic countour similar to the truncated structure (hence the name "aerospike").


  2. WHY IS THE ENGINE CALLED AN AEROSPIKE?

    The aerospike engine is so named because the nozzle resembles a spike. I've always thought that it looked like a knight's lance. A plug nozzle is a truncated aerospike nozzle.

    I think the spike nozzle, the full length spike, came first. This was followed by the idea of chopping off a portion of the spike. I have seen the term plug nozzle refer to both a full spike and truncated spike nozzle. My impression is that a contoured, full-length spike nozzle is normally refered to as a spike, while a conical, full-length spike nozzle is called a plug nozzle. Aerospike comes from the idea of introducing an additional flow into the base region of the trucated spike, forming an "aerodynamic spike" with the base flow. Basically the base flow helps fill in the area underneath the base and helps make up for the performance loss from trucating the nozzle. The word Aerospike is actually a Rocketdyne trade name (trademark?) that refers to, usually, a trucated spike nozzle with a base flow. Over time it has become part of aerospace jargon, like "skunk works." That is probably more information than anyone really wants on the subject of the name "Aerospike". (Response by David Garza, dgarza@ccvf.cc.utexas.edu , December 3, 1997)


  3. WHY IS THERE A SPIKE?

    This question arose from the fact that the plume from a spike nozzle will compensate for changes in ambient pressure. In that case, why would you need to have a solid wall on either side of the plume?

    The answer is that the solid wall is the surface area upon which the axial component of the wall pressure pushes against to produce the thrust. If you didn't have a nozzle wall against which to push, there would be no thrust produced.

    The beauty of the aerospike nozzle is that you can truncate it and the thrust which is developed by the base pressure build up almost entirely makes up for the thrust loss due to the decrease in nozzle surface area.


  4. HOW IS IT POSSIBLE FOR AN AEROSPIKE NOZZLE TO PROVIDE OPTIMAL OPERATION AT DIFFERENT FLIGHT ALTITUDES?

    The phrase 'optimal operation' refers here to optimal expansion of the combustion gases which results in maximum thrust. This means that the pressure of the nozzle exhaust is the same as the ambient pressure thereby eliminating the (P-Pa) term in the thrust equation.

    In a bell nozzle, the exit pressure of the working gases are fixed by the nozzle geometry. With an aerospike nozzle, since there is no hard outer surface in which the gases expand, the gas expansion varies to match the current atmospheric pressure at all altitudes.

    Therefore, the performance is optimal for all altitudes of the nozzle flight.

    The combustion occurs in an enclosed chamber, like in a conventional bell nozzle engine. The hot gasses are accelerated to low supersonic speed in an internal expansion section. In this part of the engine, the flow is still totally enclosed. A way to visuallize it is to imagine the gasses moving between two opposite walls which are coming closer to each other and then beginning to separate. At the point where the gasses have reached some low supersonic speed, one of the walls ends. This is the point where the external expansion ends. The other wall continues down and forms the spike contour. At the point where the other wall ends, the gasses expand around the egde of the wall. The amount of expansion is determined by the ambient pressure on the other side of the wall, the one which has ended. For details on the fluid process involved, look in any compressible flow textbook under the section on expansion fans and Prandtl-Meyer flow.(1) This is the source of the altitude compensation. As the ambient pressure drops, the gasses make a sharper turn around the edge. As the gasses turn the pressure drops. The amount of turning is determined by the pressure difference between the combustion gasses just arriving at the edge, and the ambient air on the other side of the wall. This really needs a picture, something better than ASCII art.

    A good reference for this material is Modern Compressible Flow with Historical Perspective by Anderson. (Response and reference given by David Garza, dgarza@ccvf.cc.utexas.edu , December 3, 1997)


  5. I CAN SEE HOW AN AEROPIKE TRAVELLING THROUGH THE AIR COULD CONSTRAIN THE EXPANDING COMBUSTION GASES WITH THE AIRFLOW COMING AROUND IT. BUT HOW DOES IT WORK AT A STAND-STILL? ALSO, HOW DOES IT WORK IN A VACUUM, WHEN THERE IS NO AIR PRESSURE TO CONSTRAIN THE EXHAUST?

    The air flow around the aerospike nozzle exhaust is not what constrains the plume but rather the ambient pressure. Therefore, whether or not the nozzle is moving, the exhaust plume will be more or less the same (shear layer effects between the nozzle exhaust and the quiescent air being neglected, of course). In a vacuum there is no ambient pressure to constrain the exhaust plume and the turning angle of the plume will be (approximately) determined by Prantl-Meyer expansion wave theory.

    A good reference for this material is Modern Compressible Flow with Historical Perspective by Anderson. (Reference given by David Garza, dgarza@ccvf.cc.utexas.edu , December 3, 1997)


  6. WHAT ARE THE ADVANTAGES AND DISADVANTAGES OF AN AEROSPIKE NOZZLE?

    Below are some of the very important advantages of the plug (truncated aerospike) nozzle concept as given in Huzel and Huang's Design of Liquid Propellant Rocket Engines:

    ADVANTAGES:

    DISADVANTAGES:


  7. WHAT ARE THE DIMENSIONS OF THE AEROSPIKE NOZZLE?

    The aerospike nozzle can be large or small. There is no fixed size of an aerospike nozzle.


  8. HOW DO YOU DEFINE AREA RATIO OF AN AEROSPIKE NOZZLE?

    There are a couple of ways to define the area ratio of an aerospike nozzle that I've heard of.

    1) For a toroidal plug, the total throat area divided by the area of the circle whose radius is the outer edge of the toroid would be the geometric area ratio.

    2) By running a one dimensional equilibrium (ODE) model of the expansion and comparing it to the performance of the plug, you can find what ODE area ratio gives that performance and that would be the equivalent performance based area ratio for that plug.

    I've had discussions with other people about more ways to define area ratio, but these are the two more common ways I've heard of.


  9. HAVE AEROSPIKE NOZZLES EVER BEEN TESTED?

    The following information comes from an article "Nozzle Design" by R. A. O'Leary and James E. Bech in the Spring 1992 (No. 8) issue of Rocketdyne's *Threshold* magazine (call (818) 586-2380/2771 or write to Rockwell Aerospace/ Rocketdyne Division, 6633 Canoga Avenue, Mail Code AB57, Canoga Park, CA, USA, 91304-7922 to get a copy).

    The article "Nozzle Design" states "During the 1960's, Rocketdyne tested numerous aerospike engines, ranging in size from subscale, cold-flow models to this 250,000-pound-thrust oxygen/hydrogen shown at a test stand in Nevada (picture of engine firing). The low altitude performance advantage of the aerospike over conventional bell nozzle is clearly seen".
    Various propellants and both conical (1-D) and axial (2-D) models have been tested. I have heard from several sources that Rocketdyne's *original* proposal for the Space Shuttle Main Engines used an aerospike design based on these tests. At the California Space Development Council's "Making Spaceflight Affordable" conference held in San Diego in February 1992, Vern Larson from Rocketdyne gave a presentation on the aerospike test program. Also, I've heard reports that the Germans experimented with them during WWII, but I have not seen documentation to confirm this.


  10. HAS ANYONE PURSUED THE IDEA OF USING AN AEROSPIKE ENGINE ON A HYPERSONIC AIRCRAFT?

    The combustion occurs in an enclosed chamber, like in a conventional bell nozzle engine. The hot gasses are accelerated to low supersonic speed in an internal expansion section. In this part of the engine, the flow is still totally enclosed. A way to visuallize it is to imagine the gasses moving between two opposite walls which are coming closer to each other and then beginning to separate. At the point where the gasses have reached some low supersonic speed, one of the walls ends. This is the point where the external expansion ends. The other wall continues down and forms the spike contour. At the point where the other wall ends, the gasses expand around the egde of the wall. The amount of expansion is determined by the ambient pressure on the other side of the wall, the one which has ended. For details on the fluid process involved, look in any compressible flow textbook under the section on expansion fans and Prandtl-Meyer flow.(1) This is the source of the altitude compensation. As the ambient pressure drops, the gasses make a sharper turn around the edge. As the gasses turn the pressure drops. The amount of turning is determined by the pressure difference between the combustion gasses just arriving at the edge, and the ambient air on the other side of the wall. This really needs a picture, something better than ASCII art. (Response by David Garza, dgarza@ccvf.cc.utexas.edu , December 3, 1997)


  11. WHY AREN'T AEROSPIKE NOZZLES USED?

    The following information comes from an article "Nozzle Design" by R. A. O'Leary and James E. Bech in the Spring 1992 (No. 8) issue of Rocketdyne's *Threshold* magazine (call (818) 586-2380/2771 or write to Rockwell Aerospace/ Rocketdyne Division, 6633 Canoga Avenue, Mail Code AB57, Canoga Park, CA, USA, 91304-7922 to get a copy).

    At that same conference, I asked Max Hunter ("father" of the Delta rocket and a major player in the SSTO field) why it seemed that an aerospike was not baselined for the DC-X or the proposed DC-Y and DC-1. He replied that there was concern regarding the lack of *flight-test data* (he acknowledged that there was plenty of ground test data), in particular for the transonic regime. However, the Rocketdyne article states ". . . from Mach 1 to about Mach 3, *wind tunnel tests* (emphasis mine) indicate a drop in nozzle efficiency due to the slipstream turning into the nozzle region .. . . Nevertheless, the interval of time that is spent in this adverse flight regime is short for typical flight trajectories, and overall performance of the aerospike nozzle remains well above that of a conventional bell-type nozzle". Note that wind tunnel test results combined with CFD simulations are usually sufficient for preliminary design of experimental aircraft flight vehicles.


  12. IS IT POSSIBLE TO USE A SPIKE NOZZLE WITH A SOLID FUEL MOTOR?

    Sure. The propellant would have to be made to conform to an annular combustion chamber instead of a central conical combustion chamber but if that doesn't present any incredible technological challenges, I don't see why an aerospike nozzle couldn't be used on a solid fuel motor.

    Since I'm a liquid rocket guy, this may not be the best answer in the world. Is there anyone out there who works in the solid rocket world who could give a better answer to this question and the following questions as well?


  13. IS THE LASRE SPIKE AND COMBUSTION CHAMBER COOLED BY THE LIQUID FUEL? IF SO, COULD AN EXPANSION CYCLE VERSION BE USED? WOULD THIS BE ANY MORE EFFICIENT OR LOWER COST TO BUILD?

    In answer to your FAQ regarding the cooling of the ramp (spike) and combustion chambers on the LASRE project: The lasre project uses gasseous hydrogen so there is no source of LH2 for cooling. The chambers and ramp walls are cooled by water that is pumped from special tanks in the canoe (structure below the reflection plane). The water is fed to the engine where it enters water jacket type channels in the ramp walls and walls surrounding the chambers. From there the water is mixed into the chambers to provide extra cooling before it is expelled with the plume.

    In addition to the engine cooling water, there is another water tank that resides in the a bay in the SR'. This water is fed to several water jacket type cold plates used to mount various electronic black boxes. It also provides mist cooling to the electronics boxes and propellant tanks in both the model and the canoe. This is to provide cooling at the high mach number test points where the skin temps will reach an estimated 650 deg F. The misting is accomplished by the very same misting nozzles that you may have seen at your local hardware store for cooling patios in the summer time!

    My name is Edward A. Nauman. I am a senior flight test instrumentation engineer for the skunkworks. I am currently the lead instrumentation engineer for the X33. Before that, I was the lead instrumentation engineer for the LASRE program from it's conception until we delivered it to Phillips Laboratory for testing. If I can, I would be happy to answer any non-proprietary LASRE questions. you can e-mail me at work: enauman@ladc.lockheed.com or at home: enauman@aol.com .


  14. WHAT DOES PC/PA IN THE EFFICIENCY CHART STAND FOR?

    Many of the graphs of aerospike performance are given as a function of the ratio of chamber pressure (Pc) and ambient pressure (Pa). For a constant chamber pressure, the Pc/Pa ratio increases as the altitude increases (as the ambient pressure decreases). Since aerospike nozzles are altitude compensating, this is an effective and general way to depict the performance for a wide range of chamber pressures and altitudes.


  15. WHERE CAN I GET MORE INFORMATION?

    Try the Aerospike Nozzle Homepage!