Lunar lift

At an anonymous looking facility in Redondo Beach, California, engineers are applying the finishing touches to a rocket engine that could be at the heart of NASA plans to take us back to the Moon, and ultimately Mars.

Developed by Northrop Grumman, the company behind the lunar module descent engine used for the Apollo landings, the so-called TR408 is a reaction control engine (RCE) designed to provide the subtle bursts of power required for delicate manoeuvres such as docking.

Engineered specifically for the ascent phase of a lunar lander, the engine runs on cryogenic liquid oxygen and methane which, the company claims, could be manufactured from materials in the lunar or Martian soil and atmosphere.

Outlining the design challenges, Gordon Dressler, chief engineer at Northrop’s Propulsion and Fluid Products Centre, said for lengthy missions it is particularly important that the propellants are non-toxic, as the alternative air supply for astronauts is limited, and that they will not spontaneously ignite. It is also critical that exhaust gases do not contaminate the soil or atmosphere.

Fortunately, said Dressler, there is plenty of scope for innovation. ‘Where I work we pride ourselves on being able to rapidly develop cutting-edge — some would call it bleeding-edge — technology to take on the challenges of advancing spacecraft propulsion,’ he said.

The search for candidate propellant was triggered by the announcement of NASA’s vision for space exploration, the US space policy announced by US president George Bush in 2004.

Liquid hydrogen and oxygen remain the main candidates for tasks such as boosting spacecraft into orbit, but liquid oxygen is difficult to handle and store, so NASA turned to other propellants. One that surfaced quickly was the combination of liquid oxygen and liquid methane. ‘Curiously, the US hadn’t done a lot of work on this, though the former Soviet Union and Korea developed engines using them,’ said Dressler.

One issue that previously put NASA off was the problem of liquid cryogenic propellants boiling as they travelled over long links of plumbing. In a large spacecraft, propellant could travel through up to 10m of pipes. The heat leaks are such that cryogenic propellant will vaporise unless it is actively refrigerated or continuously recirculated using the tank thermal mask to offset the heat leaks.

NASA approached Northrop and a competitor, Aerojet, to develop an RCE where a thermal leak into the propellant line would not matter as the engine could start using vapour. Thereafter, the propellants would have enough thermal inertia to chill as they came down the line and become liquid at the inlet to the thrusters after about tens of seconds of operation.

Northrop already had an engine in its portfolio that used the capability to boil a storable oxidiser, in this case nitrogen tetroxide, and adapted it to use with the fuel too. ‘We determined that since we’re boiling it anyway, the engines would have a bit less thrust, but otherwise be thermally similar, and the engine became gas starting,’ said Dressler.

The company also developed a reliable igniter, an efficient, thermally designed thrust chamber assembly and cryogenic valve assembly.

Dressler said the configuration and integration of these components was tested on a modular test rocket. ‘We used a very modular test assembly so we could change things easily if we had to tweak an individual component, and it was loaded with instrumentation, so engineering could check we were headed down the right path,’ said Dressler. ‘The latest configuration we have is very near to flight weight and configuration — it has just a minor amount of state-of-health instrumentation located on it.’

Due to demanding mass restrictions for the journey to the surface of the Moon and back, NASA decided to go to the Moon with hydrogen and oxygen for the main journey and restrict use of the liquid methane and oxygen fuel to the ascent stage of the lunar lander vehicle.

As liquid methane and oxygen will not automatically ignite upon contact, Northrop developed an integrated igniter especially for the TR408, which could reliably fire hundreds of thousands of pulses over a mission. ‘It has to be reliable, as if you miss an ignition and you’re squirting those propellants out into space, space vacuum goes back into the chamber and flash-freezes the propellants,’ said Dressler. ‘If you do that repeatedly, then get an erratic ignition, the build-up of those propellants can cause the rocket to explode.’

For the regeneratively cooled chamber, Northrop used technology from a previous engine with a regeneratively cooled oxidiser and extended it to actively cool both the fuel and the oxidiser.

Northrop has carried out more than 65 test runs and more than 380 hot fire cycles and at the end of testing will be ready to proceed to formal qualification, if NASA chooses to go ahead with it.

During testing, the engine will be exposed to extreme operating conditions. The inlet temperature range is -183ºC to -149ºC for the liquid oxygen and -179ºC to -151ºC for the liquid methane at an inlet pressure range of 1.9 million Pa to 2.8 million Pa. It needs to operate on a variable ratio of between 2.6:1 and 3.5:1 to one oxidiser to fuel, which offers the flexibility to operate over a range of propellant loads and propellant uses. It needs to operate over a duty cycle of 0 to 100 per cent, meaning all the way from one per cent or less on-time compared to off-time — short pulse mode — all the way up to steady-state firings. The pulses themselves will be as short as 80 milliseconds.

‘These tolerances are unique compared to other engines which require a specific temperature and pressure,’ said Dressler. ‘To our knowledge this is the first engine designed to operate in both pulsing and steady-state mode that takes both fuel and oxidiser from 100 per cent liquid to 100 per cent gas and is able to operate over that range of pressures temperatures.’

The engine’s technology also fits NASA’s plans to travel beyond the Moon to Mars. The agency is studying in-situ propellant creation for the Mars mission, so the spacecraft would not have to carry the propellant for the return mission all the way to Mars, which would be a huge programme performance hit. The vision is that NASA would send a robotic refining plant ahead of the mission to make liquid oxygen and liquid methane from the CO₂ and ice water discovered on Mars materials.

Methane and hydrogen have an additional advantage over many storable low-pressure propellants such as hydrazines or amines and like the oxidiser nitrogen tetroxide, which are caustic or toxic in very small doses. One of the main concerns about near-Earth orbit, lunar missions and, in particular, Mars missions is a leak invading the capsule. This would be toxic to the astronauts, who would not be able to get in their space suits and breathe the contained air long enough to survive the mission.

There is also the environmental issue, where a launch pad spill could be an environmental nightmare for mitigation or clean-up. Liquid oxygen and liquid methane are not hypergolic so even if there was a massive spill of both propellants simultaneously, which is unlikely, they would not automatically catch fire or explode. The oxygen would evaporate harmlessly into the air, and methane, though a greenhouse gas, is not toxic if inhaled.

The engine would also offer the advantage of not contaminating the Martian surface or atmosphere, which would be subject to scientific analysis. ‘The exhaust species that come out when you burn liquid oxygen and methane are water vapour, which we know is already present on Mars, CO₂, which is 95 per cent of the atmosphere of Mars, and a trace amount of carbon monoxide, which might be viewed as a minor contaminant, but there’s some in the Martian atmosphere due to the action of sunlight on CO₂,’ said Dressler.

While waiting to hear whether NASA is interested in specifying the engine for future missions, Dressler is hopeful that the engine will find applications elsewhere. The defence industry, he said, is particularly keen to investigate environmentally-friendly propellants.

He said the propellant technology used on the TR408 could also be used in reusable launch vehicles, which would avoid the use of solvents to flush out the engines to prepare for re-flight.