Laughing gas and rubber: A recipe for suborbital flight?

Boston Uni's hybrid Starscraper rocket aims for 100km

This summer, the skies above Nevada will thunder to the sound of a mighty hybrid rocket motor, as the Boston University Rocket Propulsion Group (BURPG) sends its Starscraper vehicle past the symbolic 100km Kármán line.

BURPG group shot at motor test site

BURPG: Laughing gas and rubber

Having recently tin-rattled its way to a healthy $17k down at Kickstarter, BURPG is poised to hit the heavens burning rubber and laughing gas.

Starscraper diagram

The group describes hybrid motors as "relatively underdeveloped" rocket tech. We spoke to BURPG's Jeremy Pedro – a sophomore engineering student – who explained the group's choice.

Jeremy Pedro eyes a motor on a static test rig

Jeremy Pedro eyes a BURPG hybrid motor

He said: "Solid motors experience high forces and vibrations, but are fairly simple to fabricate. Liquid engines are complex and very expensive. Hybrid motors combine the two and give a best-of-both-worlds scenario. With a hybrid, we're able to carry sensitive payloads that sounding rockets otherwise wouldn't be able to carry."

"Hybrids also operate at a much lower cost than liquid engines and are easier to design and manufacture. At the end of the day, our motors give students the opportunity to work with liquid propulsion systems while gaining experience with solid motors as well."

Pedro did, however, admit that hybrids "are pretty tricky and no-one has really been able to master them". He asserted: "We have a very high confidence in our hybrid technology, and the reliability of the motor will only increase after testing and tweaking."

The Starscraper packs hydroxyl-terminated polybutadiene (HTPB*) fuel and nitrous oxide (N20), a traditional combination originally chosen for Virgin Galactic's SpaceShipTwo engine.

Virgin has now switched to polyamide plastic (nylon), which is one of a range of fuel options available to hybrid motor developers. Copenhagen Suborbitals has in the past deployed paraffin wax, while hobbyist supplier RATTworks favours polypropylene.

Tim Pickens, who led development of the HTPB/N20 powerplant for Scaled Composites' SpaceShipOne, demonstrated a few years back that asphalt works pretty well, too:

Generally, N20 is the oxidiser of choice, although the Copenhagen Suborbitals HEAT-1X used liquid oxygen, and the Bloodhound Supersonic Car's mighty Falcon hybrid rocket packs HTPB and high-test peroxide (HTP).

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