World’s first SSTO Aerospike aircraft to fly later this month

If you’ve ever watched a sci-fi movie then you’ll notice that all the spaceships take off from the planet they’re on and head straight to orbit – unlike the multi-stage reusable rocket systems we see today like those from SpaceX and even the retired US NASA Shuttle program. This is only made possible by something called a Single Stage to Orbit (SSTO) system, and only a few companies are developing the technology to achieve our sci-fi fantasies.

While I’ve talked about SSTO Aerospike engines before, like the SABRE engine whose developers recently went into administration, they’ve generally been prototypes but a few months ago the first flight test for an aerospike rocket engine ended in disaster. But now Polaris Aerospace is back on track, preparing to fly two new prototypes for its MIRA Supersonic-Hypersonic aerospike spaceplane platform within weeks.

Immediately after the MIRA I crashed on take off Polaris Spaceplanes stated it would be going forward with the MIRA II and III. True to word, Polaris has unveiled two new, yet-to-be-fully completed airframes in a recent LinkedIn post.

The MIRA II and III are identical 16.4 ft (5 m) airframes with 30% more wing area than their predecessor, the MIRA I. Polaris opted to create two identical airframes in order to speed up flight testing as well as have a "reserve" aircraft if need be.

"In addition," writes the team, "the design has been greatly improved compared to MIRA with incorporating all the lessons learnt so far."

Both are fiberglass airframes, as they are demonstrators only. The company intends to use Carbon Fiber Reinforced Polymer (CFRP) for its supersonic and hypersonic aircraft.

While the MIRA I did make several successful test flights before crashing, it did so using its four conventional kerosene turbine engines. It was only once the AS-1 LOX (Liquid Oxygen)/Kerosene linear aerospike rocket engine was fitted that it was destroyed on take off at over 100 mph (160 kph) – preventing it from becoming the first aircraft ever to fly under aerospike power.

See more details on the test, updated.

Most rockets only work efficiently in a certain altitude range, largely defined by the shape and size of their bell-shaped exhaust nozzles, which tune the pressure and flow of the expanding gases to produce maximum thrust. As the ambient air pressure and speed of the vehicle change, so does the ideal shape of that exhaust nozzle – and this is one reason why most space launch vehicles use multiple stages.

The aerospike engine is different – and in theory, it should be effective from sea level all the way up into space. Essentially, the aerospike design doesn't use a traditional nozzle – it fires its rocket exhausts down the sides of a central surface shaped to mimic one side of the interior wall of a bell-shaped nozzle, and the ambient airflow around the rocket acts as the remaining walls of the nozzle.

So the aerospike's virtual nozzle characteristics change constantly in response to aerodynamic flow. The rocket naturally tunes itself as speed and altitude changes. It'll never exceed the efficiency of a bell-shaped nozzle running right in its sweet spot, but its average efficiency from take off to orbit should be good enough to do the job, in a way that cuts down on moving parts, and hopes are high that all goes well with the 2nd generation aircraft.

Earlier test flights were held at the Peenemünde Airfield in northern Germany, and over the waters of the neighbouring Baltic Sea. It's unknown if that's where testing will begin again with the latest iterations of the MIRA – however, Polaris has announced that it expects to have them airborne within this month.

So, we should soon see the very first aerospike engine in flight since Rocketdyne invented them over 70 years ago. I'll definitely keep you posted when it happens. In the meanwhile, you can see the first prototype giving its aerospike engine a blast at 60% power for a few seconds in a runway roll test, in the video above.

The aerospace company is already making plans to test NOVA, a 26.2 ft (8 m) supersonic prototype it intends to develop into a commercial product, starting next year in 2025.

A video from Zweites Deutsches Fernsehen Today, in German only, offers a few glimpses of footage from behind the scenes, as well as what appears to be the moments before the MIRA I crash when it diverted off the runway moments before take off.