The rocket that eats itself

Burning a rocket’s fuselage as extra fuel could boost efficiency and tackle the space debris problem

Rocket engineering is a brutal and unsympathetic field. The so-called ‘tyranny of the rocket equation’ means that, if you want to increase the payload of a rocket, you need to add significantly more fuel – which in turn adds more weight, which requires yet more fuel. Based on the Tsiolkovsky rocket equation, the problem puts strict limits on payloads heading to space, and requires huge amounts of expensive fuel to do so.

But what if the structure of the rocket itself could be used as fuel? That is the unlikely aim of a project at the University of Glasgow, where engineers have successfully built and fired an ‘autophage’ self-eating rocket.

Building on a research partnership with Dnipro National University in Ukraine, which tested a solid rocket motor in 2018, the Glasgow team – supported by Kingston University in London – has now demonstrated that more energetic liquid propellants can be used.

Autophage rockets could help the UK take a bigger bite of the space industry, and tackle the growing problem of space debris. Such a radical rethink will bring its own set of problems, however.

Infinite staging

Rocket engineers have long dreamed of reaching orbit using a single ‘stage,’ with just one engine and no additional fuel tanks that need to be jettisoned, says postgraduate researcher Krzysztof Bzdyk, corresponding author of the new work. Advantages of single-stage vehicles could include lower overall mass and cost.

So far, such vehicles have not been “realistically feasible,” says Bzdyk, because of the need for extremely efficient engines and rocket systems. Instead, the Glasgow design is more akin to ‘infinite staging,’ constantly consuming the usable structure as it is no longer needed.

“That allows you to actually scale your systems down to be smaller, because you now no longer need as large a fuselage, as large tanks, in order to get to orbit,” says Bzdyk. “That doesn't mean that you couldn't use it just as an upper stage… you could still stage it and make it more efficient in that way.”

Known as the Ouroborous-3, the engine works by using waste heat from combustion of the two main propellants – gaseous oxygen and liquid propane – to sequentially melt the high-density polyethylene plastic fuselage as it fires. The molten plastic is fed into the engine’s combustion chamber as additional fuel, burning alongside the other propellants.

Controlled ascent

Test fires at Machrihanish Airbase in western Scotland produced 100N of thrust, while also demonstrating that the plastic fuselage could withstand the forces required to feed it into the engine without buckling.

A conventional rocket’s structure makes up 5-12% of its total mass. The self-eating rocket burned a similar amount of its own structural mass as propellant, with the plastic fuselage supplying up to 20% of the total propellant used.

The experiments also showed that the burn could be successfully controlled, with the team demonstrating its ability to be throttled, restarted and pulsed. These abilities could help future autophage rockets control their ascent from the launchpad into orbit.

The team is exploring several different options for the fuselage structure, one of which could find its way into a flight demonstrator planned to launch in five to eight years. Options include flexible or collapsible propellant tanks that are compressed as the fuselage is “eaten,” says Bzdyk, or even concentric tanks that are themselves used up.

Burning issue

The main challenge that conventional small launch vehicles face is reducing the cost of kilogram to orbit, says Bzdyk, with the proportion of structural to propellant mass growing as the vehicle becomes smaller.

“With autophage we're literally burning these scaling problems,” says project leader Professor Patrick Harkness.

A self-eating rocket should require less propellant in onboard tanks. This in turn should give operators bigger payloads compared to conventional rockets of the same mass, and more flexible launch options for ‘nanosatellites’ or CubeSats than launching on larger rockets.

A SpaceX launch, for example, might provide a lower cost per kilogram, says Harkness. “But where we can add value is that we can make that CubeSat the prime payload, so they will be able to say that they want to launch next week, to orbital parameters of such and such.”

The autophage system is unlikely to work on a larger scale owing to the huge diameter of rockets, which brings challenges for both manufacturing and combusting the fuselage. Small vehicles would be well-suited for UK launches, from new spaceports such as SaxaVord in the Shetland Islands.

Suborbital start

Development of the self-eating rocket has been supported by the Ministry of Defence and the Science and Technology Facilities Council, which, alongside the UK Space Agency, awarded the team an additional £290,000 for further pilot testing of the prototype engine. The current project will continue until March 2025, at which point the team hopes to scale the engine up another order of magnitude, to thrust of about 10kN.

The research will eventually investigate cryogenic propellants, says Bzdyk. The flight test in five to eight years is likely to be suborbital, after which the team hopes to spin off to an industry partner for orbital launches.

Some problems, like the tyranny of the rocket equation, seem unavoidable. Then one day – thanks to some innovative engineering – they no longer are.