It’s not every day that you get to see rocket science in action. But in September, the team running the Bloodhound supersonic car project allowed viewers around the world to do just that. They tested their hybrid rocket engine in a shelter at Newquay airport, and broadcast proceedings live on the internet to 100,000 people.
Before this trial, parts of the hybrid rocket system had been tested only in isolation, so bringing everything together for the first time in front of the world was a gamble. Although the test comes after years of work to develop the rocket, it is the first in a series of development firings that will enable the Bloodhound team to hone the design ahead of the land speed record attempts that will begin next year at South Africa’s Hakskeen Pan.
Luckily for them, the day went relatively smoothly. The only drama came when the team had difficulty extinguishing the rocket. It continued to burn after the test had ended, filling the shelter with thick black smoke. This smoke knocked out a thermodynamic camera and obscured the live feeds from the test shelter to the engineers, who sat in a separate observation shelter.
But Daniel Jubb, creator of the hybrid rocket and Bloodhound’s rocket expert, is not perturbed. “As first firings go, it all worked pretty well,” he says. Now that the smoke has cleared, the Bloodhound team have been working hard to make sense of the test and take a closer look at the finer details of the firing. Analysis of the results will flag up any design changes that are needed for the rocket and car, so the team can get it up to speed for 2013.
Although the team quenched the flames on the day, the hiccup has affected the post-test analysis. During the fire, the rocket’s fuel supply continued to deplete, leaving the team unsure how much fuel was burned during the test.
Engineers need to know the exact mass of fuel burned to calculate the specific impulse – a measure of propellant efficiency. Without the data, the team are having to make do with an estimate of the fuel consumed pegged at 170.6 lb. According to Jubb, this indicates that the specific impulse of the rocket could be shallow of the 213 seconds target needed, but work is still ongoing to calculate the exact figure. He adds that the specific impulse target for Bloodhound is “modest”. A space shuttle burning liquid oxygen and liquid hydrogen, for example, reaches 430 seconds.
One thing the team know for sure is the amount of high-test peroxide (HTP) that the rocket consumed – 984.72 lb, about 87% of the total amount loaded into the tank before the test and within target. The HTP was pumped from the tank to the chamber at a rate of 98 lb per second and the pressure averaged 700 lb/in2, peaking at 820. The target pressure for the run was 800psi. The firing lasted about 14 seconds, with seven of those running at full target feed pressure producing 14,300 lb of thrust.
Not all the data has yet been analysed. There are still details of the stability of combustion to come. Initial data from the high-speed cameras and the pressure transducer suggest that there were no major fluctuations and combustion was stable, but further analysis of potential high-frequency fluctuations is necessary.
More detailed analysis of some of the rocket components has flagged up creases in the design that need to be ironed out ahead of the next firing. On dismantling the combustion chamber, for example, engineers found issues with the thermal insulation at the back end of the chamber. The team are looking at making some revisions in the design.
As well as yielding specific data, the firing has answered a key question that is central to rocket design, says Jubb. One of the biggest challenges with developing a hybrid rocket is getting the right balance of fuel burn speed and oxidiser. “The problem with hybrids is that there are all these intercoupled factors, so you can never change one thing without changing the other variables,” he says. “In the past, the thing that has killed many of these programmes off is that you usually get to a point where you can’t get sufficient fuel grain regression rate to efficiently burn with the amount of oxidiser that you are throwing at it.”
This means that adding more oxidiser to the combustion chamber will not always boost thrust. “We assumed that was going to be one of the limiting factors, but we have gone a bit too far the other way. We have made a fuel-rich hybrid, which I don’t think anyone has ever seen before on a large scale,” he says.
Before the next test, the rocket team will tweak the mix, by slightly reducing the fuel grain regression rate and increasing the oxidiser flow rate to reach the rocket’s “sweet spot”.
They also hope to avert another fire, which will require changes to the method of extinguishing the fuel grain. On the day, the team managed to quench the flames by carrying out an additional purge of the HTP tank. Running excess HTP through the system has been successfully used to extinguish the flame in earlier tests of the monopropellant rocket, too, and was the plan for the Newquay firing. The monopropellant rocket uses just the breakdown of HTP in the chamber to provide propulsion.
Jubb says the purge did not last long enough to quench the flames, so it needs to be extended in duration for future firings. Mark Chapman, chief engineer on the project, says the design of the car may need to be changed to accommodate this. The car may need to carry more deionised water and nitrogen to help ensure the flames are quenched, for example. Although the exact volumes required remain unclear, any redesign is likely to incorporate a self-contained purging system. Such a system had featured in earlier designs of the car that were shelved 18 months ago.
Chapman says that this measure may add weight. “But we have to put that rocket out post-burn, and if that means putting more nitrogen through it then we will take the hit.”
Huge chunks of the car are now arriving at Bloodhound’s technical centre in Bristol’s docklands. The lower chassis arrived last month for final assembly, while the lower and upper monocoque are set to arrive in early 2013. With all these components arriving, the project has outgrown the premises and is on the hunt for a new home by Christmas.
“The technical centre is a fantastic facility, but it probably has about 3,500ft2 of workshop space and when you are trying to build a 30m-long car, it fills up very quickly,” says Chapman. He adds that the move is exciting as the new premises will have the capacity to work on the support trailer and equipment, and allow the car to progress into the build phase.
The team are also optimistic about funding. Chapman says that enough funding has come in over the past six months to secure the project right through to car build. The rocket firing has triggered more enquiries about sponsorship. “Funding-wise, we are probably the most secure we have ever been. Projects such as Felix Baumgartner and the Red Bull Stratos allow people to see how much publicity you can achieve,” says Chapman.
Chapman says that the biggest unknown now on the car is the air brake – one of the last elements of the vehicle that is completely new. The brakes will be deployed at around 800mph, and will experience huge forces. The air brakes of jet aircraft are not usually operated at supersonic speeds, so it is not clear how Bloodhound’s will behave. The team have done plenty of analysis to help them understand how the brakes are likely to perform.
They also face the complication of the Hakskeen Pan being at a low altitude. “Although we have come up with what we think is a robust system, how that actually works in the airflow is unknown. We need to be sure how the vortices and eddies off the air brakes affect the rear suspension,” says Chapman.
Once in South Africa, further data on the brakes will be collected during tests, which will start at low speeds and ramp up to an attempt in 2013 to break the land speed record of 763mph. Discussions have begun about the the camera data and telemetry that will need to be collected from the car, and about the equipment required to collect the best data from these test runs.
Another aspect that will be scrutinised during the initial tests will be the behaviour of the wheels. Digital modelling of how the V-shaped wheels react to the desert suggests that as the car gathers speed they will plane and start rising out of the dirt. This could mean that at very high speeds only a small amount of wheel is in contact with the desert floor – a feature that will have a massive impact on drag. “It’s hard to deduce. It’s only once we start running the car in South Africa that we will get some proper data,” says Chapman.
The test runs will require a rocket to power the car. With the success of Newquay under their belts, the team are working to ensure the system is up to speed. The next phase will see two more development firings for the monopropellant rocket, beginning in January. Initial test runs in South Africa will be driven by a monopropellant rocket, which has been tested up to 600psi pressure. Further work is needed to get this up to 1,100psi for a speed of 800mph and the record attempt.
Only in 2014, when the team attempt to push the car to more than 1,000mph, will the hybrid rocket and Eurojet 200 jet engine come into play. For these speeds, a total peak thrust of 47,500 lb is needed – 20,000 lb from the jet engine and 27,500 lb from the hybrid rocket.
Getting the hybrid ready for 2014 will be tough, says Jubb. He and his team will carry out a further four or five development firings of the hybrid between now and 2014. “There is a long way to get from here to 25,000 lb of average thrust and 27,500 lb peak thrust. And we are going to take the burn time up to 20 seconds,” he says.
Simply getting the hybrid rocket up to power is not enough – the team then have to prove the system is safe with 12 safety and acceptance tests. Five of these will be at full thrust duration. “That is quite a challenge,” says Jubb. “But we are pretty confident.”
How the hybrid rocket works
The rocket is called a hybrid because it uses solid rocket fuel in combination with a liquid oxidiser to generate power. The set-up weighs 450kg and is 4m long. It comprises a Cosworth F1 engine, a rocket pump, concentrated hydrogen peroxide (HTP) oxidiser, a catalyst pack, and combustion chamber with solid rocket fuel.
The F1 engine powers the rocket pump, which moves HTP at high pressure from a tank through the catalyst pack and into the combustion chamber. The catalyst pack contains layers of silver oxide mesh that break down the HTP into oxygen, water and heat. At this stage, the temperature inside the rocket chamber soars and ignites the rocket fuel, which sends out a long, thin, orange flame and produces thrust.
The initial runs of the Bloodhound supersonic car in South Africa will be driven by HTP acting as a monopropellant in the rocket chamber. Once the current land speed record is broken, the team will switch to the hybrid rocket to try to push the record over 1,000mph.