Next year, the Bloodhound supersonic car will attempt to make it through the sound barrier, aiming for the 1,000mph (1,609km/h) mark the year after. The hybrid rocket engines for the project are being developed by a team at Nammo, headed by Onno Verberne, vice-president for space and offshore.
Verberne is a Dutch aerospace engineer from Delft who started at Nammo in 1985, working on structural and thermal analysis and then space projects. One would expect the unique challenge of developing a rocket that travels horizontally, attached to a vehicle, to send a rocket engineer with more than 30 years of experience into a bit of a spin. But he is surprisingly unfazed by the unusual requirements of the Bloodhound project.
“This is a design of rocket that we haven’t seen since the 1960s,” he says. “We are doing a lot of old things new, at a larger size. Otherwise, it is a regular project. It is not that different from a rocket for space applications. It involves thrust levels and burn times, and uses the same parts. In a way, it’s a little easier because it’s not as weight-sensitive. The vehicle is not supposed to fly.”
Nammo, a technology-driven company in Norway, makes ammunition and rocket motors for the aerospace and defence sectors. Typical work for the company includes the rocket motors for the missiles used on fighter jets. It also supplies the separation rockets for the European Space Agency’s Ariane 5 main boosters separation. Each Ariane 5 launch uses a total of 20 rocket motors from Nammo to push the stages of the rocket to the side and to separate the central core.
The company has been making hybrid rockets, which use a solid propellant and a liquid oxidiser, since 2003. Bloodhound’s hybrid rockets are designed to use a rubber-like solid propellant called hydroxyl-terminated polybutadiene in the combustion chamber and high-test peroxide (HTP) as the liquid oxidiser. The solid fuel reacts with the liquid oxidiser when injected into it. Crucially, there are no explosives involved.
“It’s safer, and you can throttle the rocket engine by regulating the amount of liquid oxidiser to the solid propellant. It gives more versatility and it’s a green technology that enables, for example, smaller launch vehicles,” says Verberne.
“The way we feed our rockets with the oxidiser using a vortex injection means it is self-regulating. We only have to control the oxidiser mass flow and then the engine does its thing itself. You don’t need control logic – it is self-regulating.”
Nammo was approached to work on Bloodhound in 2013. Up until that point, the team had been developing its own rocket engine with a firm in Manchester called Falcon Project. However, the task of scaling up to a large enough engine while achieving the required performance proved tough and expensive. Nammo was working on similar technology, and, because the rockets were both fuelled by HTP, the fit for the project was good.
Development so far is going as planned. Nammo has invested in large-scale test facilities and, after development firings in autumn 2014, the first full-scale test firing of its Flight-Weight Unitary Motor was successfully conducted in May 2016 in Raufoss, Norway. “The test firing demonstrated, through a first firing and then a second firing of the same motor, the restart capability of Nammo’s hybrid technology at large scale,” says Verberne.
During the first firing, the engine burnt for five seconds, until it was terminated in a controlled manner by closing the main oxidiser valve. Two-and-a-half hours after the first pulse, the engine was restarted simply by opening the main oxidiser valve, and it burnt flawlessly for 10 more seconds. Ignition was performed by the catalytic reaction of the oxidiser, which enables an unlimited number of restarts. During both pulses, the engine delivered stable high-performance combustion and a thrust level of 30kN.
Last year, a significant change was made to the configuration of Bloodhound’s rocket. New drag calculations were causing engineers to worry that they needed more power to boost Bloodhound to 800mph. With just one hybrid engine, there could be no margins for error in the drag calculations. “A year ago it was a hybrid rocket, but now for the first record attempt we will use only the mono-propellant part of the rocket. HTP decomposes to produce gases at around 700°C and is a significant source of energy,” says Verberne.
The plan for the second, 1,000mph attempt remains to use three mono-propellant engines of 15kN each.
The single rocket for the first run will be the same design, just without the solid fuel and the related components. Hybrid rockets are inherently more complex than mono-propellant ones. They run at much higher peak temperatures, up to 2,700°C, and require more insulation. “It’s easier to implement one 30kN rocket than three 15kN rockets,” says Verberne. “But we need the car to run progressively faster and faster. A hybrid rocket motor gives a lot of power quickly. But this way, using just the mono-propellant, it is more controllable and there can be a quicker turnaround on the desert floor. You can just reload the HTP tank and go again. You don’t have to replace the insulation and you have a safer way of reaching the maximum speed.”
Nammo has achieved 11kN of thrust during tests so far, with just HTP fuel, but that was without a nozzle. The change to a mono-propellant is easy for the rocket motor side, because the previous design was already set up to supply a controllable amount of oxidiser to adjust the thrust, says Verberne. Indeed, most of the difficult parts of the car design, such as the aerodynamics and packaging the rocket into Bloodhound’s tightly packed chassis, are being handled by Bloodhound’s engineers.
“We have the easier side,” he says. “Using a rocket in a car is not that much different from what we normally do. Every rocket company tests its rockets in a different way, but we do it horizontally – identical with what we use in the car. But if it is in a manned vehicle, we have to make it so it can be stopped, which is determined by the pump and the valve.”
Under test conditions, Nammo’s engineers have a pressurisation system that uses a pump. In a way, this is recreated in the Bloodhound car. A pump, fuel tank and petrol engine will be used to get the HTP to the rocket engine.
Despite the simple approach, Verberne admits it will be a busy time for the engineers over the next year, as the team builds up to the record attempt in Hakskeen Pan, South Africa, next November.
“We haven’t put the car and rocket engine together yet – the next steps are to work on the feed system and the starts and stops,” he says. “The pump is in the UK, and the rocket engine is here in Norway. Some testing will need to be done here before we can go to the UK for testing the rockets with the system in the car.”
2018 challenge: a streamlined core swap
Bloodhound’s second record attempt, aiming for 1,000mph, is planned for 2018. The extra power needed will be provided by a combination of three hybrid rockets, says Onno Verberne. This run will be a tougher challenge for Nammo’s engineers, principally because of the turnaround during the record attempt. The team will need to change a significant number of insulated parts, within 45 minutes on three rockets, which wouldn’t normally be reused. This will require a pit-stop style approach to change a central core designed to be swappable. The technique has been done during tests, but not in less than 45 minutes.