The team that hopes to break the world land speed record in a rocket-assisted jet car could create an important spin-off for the nascent space tourism market: a safer rocket engine.
So says Daniel Jubb, who is the founder of military rocket motor company Falcon Project Limited (FPL), based in Mojave, California, and chief rocket engineer for the Bloodhound SuperSonic Car, which aims to reach a speed of 1600 kilometres per hour (1000 miles per hour) on the salt flats in Hakskeen Pan, South Africa, sometime in 2013. The current record is 1227 km/h, and was set in 1997 by the Bloodhound team’s driver Andy Green, in a car called Thrust SSC.
Bloodhound SSC looks like a wingless aircraft and will be powered by an engine from a Eurofighter Typhoon fighter jet as well as a hybrid liquid/solid fuel rocket motor designed by FPL. To keep the rocket fed with fuel, the vehicle uses a Cosworth Formula 1 racing car engine as a fuel pump.
Outsize fireworks
FPL considered many rocket engine options before settling for a hybrid. Solid fuel motors containing a powdered metal fuel were ruled out because they cannot be turned off in an emergency: they have to burn out fully, like outsized fireworks. They then investigated bipropellant liquid-fuelled engines burning a variety of fuels, such as kerosene, hydrogen and alcohol, with a liquid oxidiser such as nitric acid, nitrogen tetroxide, hydrogen peroxide or liquid oxygen.
“We looked at every combination of liquid fuels,” says Jubb. “But we needed a very compact system to provide Bloodhound with the impulse it needs to get to 1000 mph and none could provide that.” Basically, the all-liquid options were all too heavy. “The cryogenic temperatures that liquid oxygen needed would have placed an extreme risk to our driver Andy Green, too,” Jubb adds.
A key performance indicator with a rocket engine is its “specific impulse” – the change in
momentum it can achieve per unit of propellant mass burned, a quantity measured in seconds. “We need a specific impulse of 200 seconds for Bloodhound to reach 1000mph. That brought us into the range that can be delivered by a hybrid rocket motor,” says Jubb.
SpaceShipOne
FPL originally planned to give Bloodhound a hybrid rocket similar to that used by commercial spaceflight company Scaled Composites, whose SpaceShipOne was the first commercial craft to reach the edge of space in 2004, famously winning the $10 million Ansari X-Prize. Its motor used a mixture of solid fuel – an aviation rubber called polybutadiene – and a liquid oxidiser: nitrous oxide. The oxidiser is the source of oxygen for fuel combustion.
However, in 2007, Scaled Composites was testing how liquid nitrous oxide flows, at its headquarters in Mojave, when a tank exploded, killing three of its rocket engineers.
Members of Jubb’s team nearby were about to begin work on their own nitrous oxide-based motor when they heard the explosion. “We were very concerned,” Jubb says. “We immediately went to look for other instances of nitrous oxide accidents.”
Poring over reams of records dating back to the 1930s, they found that the compound has a history of explosive decomposition in the presence of contaminants, heat and/or high pressure. This had been a surprise to the Scaled team, too.
Dangerous oxidiser
“The body of knowledge about nitrous oxide (N2O) used as a rocket motor oxidiser did not indicate to us even the possibility of such an event,” Scaled Composites said in a statement after the conclusion of the investigation into the accident a year later.
(No Ratings Yet)