Race you to space

IT TAKES a lot of oomph to launch a satellite into space. Typically, the payload represents only about 5% of the mass of a rocket as it leaves the launch pad. The rocket’s motors account for some of the rest, but the bulk of it consists of the propellants (the fuel and oxidant that react to produce the thrust required to reach orbit) and the gubbins needed to handle these propellants (tanks, pumps, valves, piping and the bodywork that contains them). The gubbins are not only expensive in themselves, but their mass also requires extra fuel to lift. Things would be more efficient if the gubbins could be dispensed with and a rocket designed that consists of only payload, motor and propellants.

This is exactly what those behind what they call the “autophage” rocket hope to achieve. This team, a group of researchers led by Patrick Harkness of Glasgow University, in Britain, and Vitaly Yemets of Oles Honchar Dnipro National University, in Ukraine, is designing a rocket that has a body made of a rigid cylinder of fuel and oxidant. At launch, the engine will sit at the base of this cylinder, but by the time the craft reaches orbit, it will have gobbled its way up towards the top, consuming the rocket’s structure on the way. That will save on launch weight, and thus on fuel. And, as they report in the Journal of Spacecraft and Rockets, Dr Harkness and Dr Yemets have now carried out the first static test-firing of such a rocket’s motor.

Self-evidently, the design they propose requires both fuel and oxidant to be solid. Solid-fuelled rockets are common in military applications, such as intercontinental ballistic missiles, but are less frequently employed for launching satellites because their thrust is hard to regulate. Like firework rockets (themselves solid-fuelled), once the metaphorical blue touchpaper has been lit, the fuel burns as it will. Liquid-fuelled rockets are preferred as satellite launchers because their thrust can be tuned by changing the flow of propellant to the motor. That makes it easier to position a payload into orbit correctly. But Dr Harkness and Dr Yemets think that their self-consuming design can overcome this difficulty, too.

The fuel for the motor is a hollow cylinder made from polypropylene, a plastic hard and strong enough to form a rocket’s outer casing. The middle of the cylinder is filled with a powdered mixture of ammonium perchlorate and ammonium nitrate, the oxidants. For their test firing, the researchers used a hydraulic ram to drive the cylinder into a preheated engine. Here, it made contact with a specially designed vaporisation surface, heated in order to turn both fuel and oxidants into gases and pierced by holes designed to collect the gases separately and channel them into a combustion chamber, where they mixed and burnt. To start the process, the vaporisation surface had to be warmed to its operating temperature by a gas burner (this would be done electrically in an operational model), but once the system was up and running, vaporisation and combustion became self sustaining. And, by varying the rate at which the propellant tube entered the engine, it was possible to control the amount of thrust developed.

A real rocket would, of course, have no ram to feed in the fuel. But Dr Harkness hopes Newton’s laws of motion will deal with that. Though the prototype under test is not yet powerful enough to make this work properly, the idea is that the acceleration of the motor will push constantly against the inertia of the propellant cylinder, forcing the cylinder against the vaporisation surface and causing it to be consumed. That process, moreover, is capable of regulation by using some sort of throttle to slow the cylinder’s feed-in speed, permitting control of the amount of thrust developed in a way not possible for a normal solid-fuelled rocket, in which the fuel burns in situ.

The autophage design Dr Harkness and Dr Yemets have come up with is not, in truth, likely to worry those who use large liquid-fuelled rockets to launch heavy satellites. The way rockets scale up means that freedom from gubbins is more valuable for small craft than big ones. But a small solid-fuel rocket fitted with an autophage engine might prove an ideal launcher for the growing number of small satellites being sent into space. Dr Harkness thinks such a vehicle could even be designed to launch an individual CubeSat, a type of satellite that has a volume of a litre and a maximum weight of 1.33kg.

At present, most CubeSats are taken up in batches alongside other payloads on big, liquid-fuelled rockets, and even Rocket Lab, a firm that has recently started offering dedicated CubeSat launches, uses liquid propulsion. A solid-fuel rocket would, though, be easier to handle than one full of liquid so, though a working autophage rocket is still several years from production, a launch vehicle that eats its way into space looks an attractive idea.