THE FINAL FRONTIER

Dave Wilson looks at the engine that will power the Venturestar into space at the turn of the century

Designers across the US are working to perfect the Venturestar, an all rocket-powered single stage to orbit reusable launch vehicle (RLV). But before its launch, they must demonstrate the feasibility of the concept. Key to achieving that goal will be the X-33 – a half scale version of the real thing that will make its first flight in 1999. Capable of vertical take off and glider like landings, it is a pilotless vehicle, that will fly up to 15 times the speed of sound at altitudes of almost 50 miles.

To propel the spacecraft into orbit, a new engine has been developed by Rocketdyne dubbed the Aerospike. The difference between the linear Aerospike and conventional rocket engines is the shape of the nozzle. Whereas the bell nozzle of a conventional engine expands the hot combustion gas on its inside surface, the Aerospike nozzle expands the gas on its outside surface. The linear Aerospike nozzle is not a bell shape at all, but the shape of a `V’ called a ramp. The shape enhances performance and allows a more optimum vehicle design. Aerospike nozzles can be circular or linear with the latter being ideal for the X-33/RLV application.

HOW THEY WORK

Rocket engines all turn energy stored in propellants into thrust. Pump-fed liquid rocket engines perform this by ingesting the liquid propellants stored in the vehicle, increasing their pressure and flow rate by the use of turbine-driven pumps, delivering them to the combustion chamber for ignition, then exhausting the hot combustion gases out of a nozzle to produce thrust.

The enhanced performance of the Aerospike is due to the external expansion of combustion gases. In a bell nozzle engine, a particular nozzle shape and length will expand its combustion gases outward only as far as the nozzle allows. They are designed to be the best compromise of shape and length for a particular vehicle and flight path.

At higher altitudes, gases could expand farther if there were more nozzle length, thereby improving performance. But since the nozzle cannot easily change shape – that is, growing longer – it essentially gives up some performance. The Aerospike offers a solution to this problem. Its plume is open to the atmosphere on one side and free to move, allowing the engine to operate at its optimum at all altitudes. It compensates for decreasing atmospheric pressure as the vehicle ascends, keeping the engine’s performance very high throughout the entire trajectory.

While the free side blooms outward to adjust the combustion gas pressure to the local atmospheric pressure, it is pushing against the nozzle ramp surface on the other side, producing thrust. This altitude compensating feature also allows a simple, low-risk gas generator cycle to be used. Instead of recirculating and burning the turbine exhaust gases, as in the Space Shuttle Main Engine, or dumping the exhaust, as with most other bell-nozzle engines, the Aerospike is designed to exhaust the gases through the truncated end of the spike, creating more thrust.

The linear Aerospike allows the smallest, lowest cost vehicle to be developed because the engine fills the base of the vehicle, reducing installed weight when compared to a bell-shaped engine. A rectangular-shaped `linear’ Aerospike engine fits nicely into a vehicle with a rectangular back end. The weight of the aerospike is less than a bell engine, thereby reducing the vehicle’s size and cost.

In addition, it produces a hot gas plume which nearly fills the base area of the vehicle, thereby reducing the `base drag’ caused by the open area at the base of the vehicle. The base drag reduction allows the vehicle to be smaller than a similar vehicle with a bell engine.

Like the X-33 vehicle itself, the linear aerospike engine designed for the X-33 is subscale. Each of the two engines on the X-33 will have a series of 20 combustion chambers – ten aligned along the forward end of each nozzle ramp – and will produce 206,400lb thrust at sea level.

The full-scale Aerospike for the RLV will produce 431,000lb thrust. Propellants for both the subscale and prototype engines are liquid oxygen and liquid hydrogen. The only combustion by-product from burning these chemicals together is steam.

The X-33 and RLV vehicles will be steered by using differential thrust – that is, varying the thrust of the Aerospike engine segments to produce pitch, roll and yaw – as opposed to moving or gimballing the entire engine to change direction.

This feature also reduces the number of mechanical systems and contributes to a lighter weight vehicle. The X-33 engine can throttle from 40 to 119% of its designed thrust level, while the RLV engine will throttle from 18 to 100% for the RLV prototype. The life of the RLV linear Aerospike is expected to be one hundred missions.

Design work on the full-scale aerospike prototype began in the middle of this year. The 15 month design phase on this project ends in mid-1998, and after fabrication and component testing, engine assembly will start in early 2000. Engine ground tests are due to take place in the second half of the year 2000.

Figure 1: When the Venturestar re-useable launch vehicle enters service, spaceflight will become a matter of gas up and go

Figure 2: A comparison of the bell nozzle with the aerodynamic spike nozzle

Figure 3: Inboard and outboard views of the Rocketdyne Aerospike engine and a schematic showing the operating principles of the engine

Figure 4: The Venturestar will be capable of vertical lift-off