The aerospike engine has been a topic of interest in the rocketry sector for the past decades. This article will answer some key questions regarding the attributes of the engines, why it isn’t serviceable, as well as a brief overview of the advantages and disadvantages of these engines.
But before we begin, we must first quickly cover a key topic in fluid dynamics called ‘Flow Separation’. As fluid flows from one point to another it will compress or expand depending on the pressure of its environment. An example would be the water from a garden hose expanding once it leaves the hose due to the air pressure outside being lower than inside the thin rubber hose. The expansion of the fluid due to lower environmental pressure causes the flow to become ‘over-expanded’ whereas the compression of fluid would cause it to be ‘under-expanded’. Therefore, a rocket’s ideal state for its fluid propellant is to be ‘optimally-expanded’ or ‘ambient’ for optimal efficiency.
We must cover another key concept, which is how first-stage modern rocket engines function. A high pressure gas from the combustion chamber is released through the nozzle, gradually becoming less pressurised and increasing in velocity until it leaves the nozzle at near atmospheric pressure. In summary, rocket fuel is burned to create an extremely high pressure gas which then ejects out the nozzle at a hypersonic velocity with far lower pressure, thus propelling the rocket thanks to Newton’s law (every reaction has an equal and opposite reaction. This is essential to understanding the advantages and disadvantages of aerospike nozzle engines.
Current traditional bell nozzle engines can operate at ambient conditions throughout certain stage(s) of the rocket launch. They cannot, however, operate at ambient conditions in a vacuum environment or throughout all stages of the launch as the ambient condition is determined by the expansion ratio (the difference in size between the two ends of the rocket nozzle). Therefore, bell nozzle engines are inherently inefficient in this regard. So to combat this inefficiency, the rockets are disposed of at the end of each stage to make way for a larger rocket that is more efficient at higher altitudes. Ambient flow is unachievable in a vacuum environment as it would require an infinite expansion ratio. This attempt at a solution to the bell nozzle inefficiency adds further complication and cost as more engines are used, making the rocket larger, heavier, and more complicated. The alternative though, is the aerospike engine.
In comparison, the aerospike nozzle engine adjusts itself to the ambient state of any environment. There are two main aerospike nozzle engine designs; linear and spike, both sharing similar advantages and disadvantages. Aerospike nozzle engines allow the outside/environmental pressure to determine the flow separation of the rocket fuel. The spike design is far harder to cool than its linear counterpart, however, the linear design is big and bulky, making it difficult to steer and maneuver the rocket.
In the above diagram, we can see the differences between the aerospike and traditional bell nozzle engines. While bell nozzle engines encompass the flow for most of its journey until it has gone from high-pressure-low-velocity to roughly atmospheric pressure and supersonic velocity; aerospike engines expose the flow to the environment much quicker. However, the angle at which aerospike nozzle engines expose the fuel to the environment is why they are more efficient. The engine itself uses the environmental pressure to compress or expand the flow, meaning at sea level the flow will be more narrow and will gradually expand until it reaches space as the atmosphere’s pressure will push the flow inwards towards that middle spike,‘squeezing’ it less until the engine reaches the vacuous space. This means that the aerospike engine will have an ambient fuel flow in any environment without the need to adjust its size or the fuel’s velocity/pressure.
This inherent adaptability of the engine improves the payload capacity by an estimated 15 to 23 percent for NASA’s X33 VentureStar SSTO rocket (a Single Stage To Orbit rocket is a vehicle that does not need to detach boosters, it can fly to space and back in one piece). Overall, the shape and layout of an aerospike gives it a valuable advantage over the standard bell nozzle engines we use today.
However, the aerospike engine is not without its issues. Due to all the high-temperature fuel flowing over a sizeable amount of the engine itself, the aerospike has issues with cooling and consequently with weight. To visualise, the bell nozzle has the hot fuel exhaust scraping along its sides whereas the aerospike has the hot fuel literally being squeezed onto it and this makes it harder to cool which adds weight. For example, Rocketdyne compared their bell nozzle J-2 engine to an aerospike configuration of the same engine called the J-2T which had a spiked design; the J2-T was around 15x harder to cool due to the surface area that is being subjected to the high pressure and temperature gas being far smaller on the aerospike. To comprehend, imagine the J-2 as having to cool the sun’s rays and maintain a temperature such that it doesn’t melt; now imagine that the J-2T has to cool itself from the sun’s rays but the rays are focused with a magnifying glass thus making it far more difficult. The issues of cooling require more R&D in the matter to come up with different designs as well as metal alloys to perhaps withstand such high temperatures for prolonged durations without adding too much weight. Overall, aerospike engines have their range of significant issues and complications which is a major contributing factor to why they are not currently being widely used in the rocketry industry.
In addition, taking the example of the X33 SSTO that NASA had big firms such as Lockheed Martin and Rocketdyne cooperating from the late 90s to early 2001; the project has been criticised in hindsight due to mistakes by the engineers of both NASA and the individual firms making some questionable choices. The adaptability of the aerospike to both sea and vacuum environments was the initial reason to have it on the SSTO, or RLV (Reusable Launch Vehicle) as it was called at the time. However, the 1.5 billion dollar project was terminated in 2001. Sadly, quite a lot of the issues ranging from the composite fuel tanks, new and lighter metal alloys/cooling mechanisms for the aerospike engine, and many more advancements over the past two decades have all shown that perhaps the VentureStar and the aerospike engine are not ludicrous ideas anymore. They were simply ahead of their time in 2001; so perhaps we will see a wider acceptance of aerospike engines in the years to come if NASA, Rocketdyne, and other rocketry firms have not been put off by the tough task of making a rocket with an aerospike engine.
On a final note, four of the XRS-2200 Rocketdyne aerospike engines were made and you can read more about their test results for the engines that function and were fired during the test runs but were never flight-tested.