Nebula - n-Prize competition entrant No. 1

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Cerberus Engine

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Aerospikes explained

Aerospike Engine

An early CAD package capture of the original engine design.

The type of engine we're using is not a new design, but its not particularly old either. It does however, offer significant advantages over more 'traditional' designs that I believe make a SSTO (Single Stage To Orbit) more achievable.

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History

Rocketdyne (US) were the first to propose this type of engine design in the mid to late sixties. Even though a lot of static test data was collected, this engine has never been used as the primary thrust unit in an orbital craft, although it was initially proposed as the main engine type for the Space Shuttle.

The design has also been considered for numerous other projects in the last 30 years (including the X-33), but as far as I know it has only actually flown in a test configuration on the back of an SR-71 Blackbird.

Advantages

There are three main advantages in the type of engine which really made it the ideal choice:

1. It remains consistantly efficient at just about all altitudes including the near-vacuum of near-earth orbit. The conventional bell-type engine would have to be much longer than is currently practical in order to contain the expanding gases at higher altitudes.

2. Because the gas pressures at the bottom of the engine effectively form the remainder of the "inverted bell" (hence the term aero-spike), a significant weight saving can be made by truncating the bottom of the cone.

3. By modulating the oxidiser/fuel mixture/rate to individual injectors around the ring, a differential thrust can be set-up between opposite sides of the engine. This engine is therefore stearable without using moving parts.

Design

In practice our final engine design (codenamed 'Cerberus') has been reduced to a series of 8, inwardly canted small engines (codenamed 'Fido') arranged in a circular fashion around the spike cone. The 100% design thrust for each unit is 320N at sea level.

Each engine is direct-fed fuel at a constant rate, while the oxidiser to each is modulated using standard automotive fuel injectors so that the F/O ratio can be adjusted from rich down to fuel only (off)

The engines are all independently electrically started and they carry this ability all of the way into the flight. The start is made using a very high F/O ratio to minimise the risk of a hard-start, and the proper ratio is only applied after combustion is detected.

Cooling is extremely important in a reusable design (unless ablative engines are used), and this is partly accomplished by running the mixture fuel rich. However to augment this and provide fuel pre-heat, the fuel path for each engine takes it through a jacket around the exit nozzle, throat and combustion chamber before being injected via a ring a the top of the combustion chamber (which in turn provides some internal curtain cooling). The design allows the fuel velocity through this jacket to increase as it passes by the throat to avoid thickening of the thermal boundary layer which could lead to material failure.