Aerospike Rocket Engine
By: Bor Kavcic
Tags: Space Exploration, Rockets, Rocket Engine, Space

A perfect propulsion system?

Photo by SpaceX on Unsplash

We consider rocket science to be challenging — and for a good reason. Crafting a precisely engineered vehicle that propels the spacecraft demands a deep understanding of physics, control theory, fuel chemistry, and engineering. Likewise, understanding the series of failures that led to a specific design is crucial: every blown-up rocket is a lesson in itself. The capability of the rocket comes from the efficiency and adaptability of its engine. It is no surprise that engineers dedicated a lot of effort to improve the technology and design of the rocket engines.

The main principle of rocket engine

The third Newton’s law (action-reaction) underlies the operation of a rocket engine: discharging the mass at high velocity propels the remaining mass in the opposite direction. You can observe this effect by holding a handheld showerhead and turning on the flow of water: you will feel a jolt when water comes through. This principle allows propulsion even in the absence of an external medium, as it is the case in the vacuum of space. Here, the rocket engine generates thrust by ejecting gases at high velocity.

Accelerating the ejected mass to high velocity requires energy. In the case of chemical rockets, this energy comes from combustion. A sudden release of thermal energy increases the pressure and the temperature inside the combustion chamber. Propellants are injected under high pressure, countering the enormous pressures of 10⁶-10⁸ Pa inside the combustion chamber.

The nozzle of the monstrous F1 engines that propelled the Saturn V. Circular plate on the right is the injector plate through which fuel and oxidizer entered into the combustion chamber. Photo by Hafidh Satyanto on Unsplash.

Nozzle design

Pressurized gases exit the combustion chamber through the nozzle and propel the vehicle. However, the way gasses exit the combustion chamber crucially determines the efficiency of the rocket engine —the nozzle can increase the thrust by ~67% in the ideal case. Most frequently, rocket designs use a bell-shaped “de Laval nozzle.” In this design, the rocket engine is roughly divided into two sections: high-pressure convergent and low-pressure divergent section connected by a narrow throat.

Gases in the combustion chamber converge towards the throat at high pressure and temperature but typically move with a subsonic speed — high-pressure convergent section. After passing through the throat, gasses start to expand, which increases the velocity but decreases the pressure and the temperature. In this divergent section, thermal energy is converted into the kinetic energy of rapidly exiting gases.

The geometry of the nozzle determines the expansion process of the exiting gasses. Ideally, if the nozzle shape allows gasses to reach a pressure equal to the ambient one, all gasses will leave the nozzle in a parallel manner. In this way, the thrust will be maximal. However, if the pressure of exiting gasses is higher than the ambient one, gasses will expand sideways, thus losing thrust. This nozzle is underexpanded. Likewise, if the nozzle geometry leads the exiting gasses to reach lower than ambient pressure, the outer gasses will constrict the flow of the exhaust. This nozzle is overexpanded.

Bell nozzles are most efficient at a specific altitude

The optimal shape of the nozzle thus depends on the ambient pressure. When a rocket launches from Earth, it starts at the ambient pressure of ~10⁵ Pa (1 bar) and makes its way towards the vacuum of space. Thus, the nozzle of a fixed shape will perform optimally only at a particular pressure. To some extent, rocket staging alleviates this issue. In this way, rocket engines most suited for higher ambient pressures at launch are discarded together with their stage; the rocket continues its ascent using engines optimized for lower ambient pressures at higher altitudes.

Taken together, the bell nozzle engine will perform optimally only at a specific pressure. However, if we would like to use a vehicle that requires no staging, we need a rocket engine with an optimal performance throughout the ascent. Engineers have often considered such a single-stage-to-orbit (SSTO) vehicle when designing concepts of reusable vehicles.

Aerospike engine

The aerospike engine drops the outer bell of the nozzle. Instead, it directs the outflow from the combustion chamber towards a “plug” in the middle and lets the ambient gasses surrounding the exhaust regulate its expansion. Thus, when the ambient pressure decreases, the exhaust shape is altered accordingly. In turn, this altitude compensation maximizes the thrust achievable at a particular altitude.

Comparison of bell-shaped (left) and aerospike (right) rocket engine. A bell-shaped nozzle is optimal only at a particular pressure (altitude). At very low pressure (or in a vacuum; top) or high pressure (bottom) the nozzle is underexpanded or overexpanded, respectively. Altitude compensation of the aerospike engine maximizes the thrust achievable at a particular altitude. (Schematic by the author, public domain)

This principle allows other geometries of the engine. Besides the toroidal aerospike in which the name-giving spike replaces the bell skirt, the rocket engine can take a more futuristic-looking linear shape (see image below).

There are several additional features of the aerospike engine that make its use very attractive. In the case of a linear aerospike engine, its multiple nozzles (see image below) allow thrust vectoring through differential throttling. Classical bell-nozzle or toroidal aerospike engines achieve vector control by gimballing the engine.

Linear aerospike engine “Rocketdyne XRS-2200” developed by Lockheed-Martin. Note a series of small combustion chambers ordered in a row. The rocket engine used hydrogen and oxygen. (CC BY-SA, source)

Linear aerospike engines are stackable: the figure above shows an engine made from two blocks of with ten combustion chambers each. Additionally, this arrangement allows the spreading of the thrust loads along a wider area of the vehicle, which reduces the structural reinforcements and consequently reduces weight.

Issues

The main caveat of the aerospike engines comes from heat management and associated engineering complexities. While cooling the nozzle bell is already challenging as it is, there are effective ways of doing it. Most efficient is regenerative cooling in which the propellant flows through piping running around the nozzle and thus cooling it. Some nozzles — especially those not meant for reuse — use ablative cooling in which material deposited on the wall of the nozzle vaporizes or melts away and thus reduces the temperature. Other methods, such as film- and radiation cooling are used as well.

The geometry of the aerospike engine makes cooling challenging: its spike is put directly amid the infernally hot exhaust gases. Even more so, as the spike narrows, it gets more difficult to cool. Truncating the spike partially alleviates this issue but harms performance. To counter this the exhaust of the turbopumps can be used to create a pressure cone that mimics the reminder of the spike. Additionally, the area next to the combustion chamber(s) is under particularly heavy heat load and is difficult to cool due to large area. Aerospike engines are also heavier due to the large combustion chamber(s).

Proposed projects

Engineers have considered aerospike engines for a variety of projects. Space Shuttle orbiter, whose engines operate throughout the range of ambient pressures (from sea-level pressure to the vacuum of space), was considered to have aerospike engines but went with tried-and-tested bell-shaped engines.

Lockheed Martin started developing “VentureStar” as the Space Shuttle replacement. VentureStar was an SSTO and would — if successful — dramatically simplify the pre-launch preparations. As an SSTO vehicle, its design incorporated aerospike engines. VentureStar would have a stack of seven linear aerospike engines and could launch up to twenty tonnes payload to Low Earth Orbit. After launching vertically, it would land like an airplane — similar to a Space Shuttle orbiter, but with less stringent airport requirements. Its design was to be proven by a sub-scale demonstrator X-33. The project was canceled after difficulties with composite fuel tanks.

VentureStar releasing a satellite. Note the stack of seven linear aerospike engines. (public domain).

Today, very few companies are developing rockets using aerospike engines. ARCAspace, for example, is developing a Haas 2CA rocket, which would utilize the linear aerospike engine. Its rocket engine would sport sixteen combustion chambers, allowing for thrust vectoring without nozzle gimbaling. It is, however, still cheaper to improve and use flight-tested bell-shaped rocket engines in combination with staging rather than develop the technology required to make aerospike engines flight-ready.

The aerospike engine is — with all its issues — a thought-provoking concept. As space technologies advance, we might see the resurrection of the aerospike engine and its use in a flown rocket. Yet, for the time being, it is likely that more classical, bell-shaped rocket engines will continue to dominate the rocket design.