The signs of a healthy industry isn't just growth or innovation, but a tendency to reach out and fill niche markets. A case in point is the small satellite launch company Firefly Space Systems, which recently unveiled its planned Alpha launcher. Aimed at the small satellite launch market, it's designed to launch satellites into low-Earth orbit (LEO) at very low cost with an unconventional aerospike engine. It is also the first orbital launcher to use methane as fuel.

The Firefly Alpha is a specialized design to launch light satellites at low cost into low Earth and Sun-synchronous orbits for broadband communication and Earth observation missions. Designed to carry payloads of up to 400 kg (880 lb), the Alpha features carbon composite construction and uses the same basic design for both of its two stages to keep down costs and simplify assembly.

The launcher uses methane and liquid oxygen as propellants, which Firefly says is a first for orbital launchers, though methane has been proposed for landers. Methane was chosen because it’s cheap, plentiful, clean-burning and, unlike more conventional fuels, self-pressurizing, so it doesn't require a second pressurization system.

However, the interesting thing is that when you look at the first stage, you see a curious bulge at the base ringed with rocket burners rather than the usual cluster of rocket engines. That’s because, while the second stage uses conventional rocket engines, the first stage uses a more exotic plug-cluster aerospike engine putting out 90,000 lb (400.3 kN) of thrust.

Aerospike engines have been under development since the 1960s, though, until now they've never got much past the design phase. The idea behind them is that rockets with conventional bell-shaped nozzles are extremely efficient, but only at a particular altitude. Since rockets are generally used to make things go up, this means that an engine that works best at sea level will become less and less efficient as it rises.

The plug aerospike is basically a bell-shaped rocket nozzle that’s been cut in half, then stretched to form a ring with the half-nozzle now forming the profile of a plug. The clever bit is that the open side of the rocket engine is now replaced with the air around it. As the rocket fires, the air pressure keeps the hot gases confined on that side like the wall of the bell nozzle did. As the craft rises, the change in air pressure alters the shape of the “nozzle;” keeping the engine working efficiently.

One of Alpha's rocket burners

The result of this arrangement is a lighter rocket engine that works well across a range of altitudes. Because the second stage operates in a near vacuum, it uses conventional rocket nozzles.

"What used to cost hundreds of millions of dollars is rapidly becoming available in the single digit millions," says Firefly CEO Thomas Markusic. "We are offering small satellite customers the launch they need for a fraction of that, around US$8 or 9 million – the lowest cost in the world. It’s far cheaper than the alternatives, without the headaches of a multi manifest launch."

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