DLR Creates the Rocket Fuel of the Future

• Two advanced “green” fuels have been successfully tested to replace hydrazine.
• State-of-the-art laboratory and analysis techniques in the physical-chemical laboratory form the cornerstone of future fuel technologies.
• 3D-printed combustion chamber compatible with “green” fuels.

LAMPOLDSHAUSEN, Germany (DLR PR) — Sustainability and environmental compatibility are also increasingly important standards in space travel. To achieve this, scientists at the German Aerospace Center (DLR) in Lampoldshausen are developing fuels for next-generation space applications. The focus is on application-relevant properties such as improving environmental compatibility, safety, behavior at different temperatures and reducing fuel costs. 

“In order to meet the high requirements of new types of fuels, the selection of the right“ chemical components ”is crucial right from the start,” says Dr. Dominic Freudenmann, Head of the Chemical Fuel Technology Department at the DLR Institute for Space Propulsion. 

The DLR scientists therefore research innovative “chemical concepts” in their own laboratory and systematically compare them with one another in order to develop ideal new fuels for space travel. A diverse synthesis repertoire of functional materials and novel substances such as ionic liquids is available as a basis. Two new fuels have now been successfully tested. A new, particularly heat-resistant combustion chamber is crucial for this.

“Green” fuels replace conventional fuel combinations with hydrazine

In demand are fuels that contain hydrazine (N₂H₄) can replace. Whether satellites, probes or rocket stages – hydrazine has been used in space propulsion systems as monergol, which means as a one-component fuel or as diergol, i.e. in a fuel combination of fuel with an oxidizer. Hydrazine has the advantages, among other things, that it is efficient and it can be stored for a long time. 

As monergol, it can be cold-started, which means that as soon as the hydrazine hits a catalyst, it decomposes and chemical energy is released to drive the engine. The disadvantage of hydrazine: It is toxic and carcinogenic for humans and the environment. The handling of these substances therefore requires a high level of security, which in turn is very expensive.

In two projects, DLR researchers are now showing alternatives that are less polluting but just as efficient. Felix Lauck and Dr. Michele Negri successfully tested the green fuel “Hypergolic Ionic Propellant” (HIP_11) for the first time in a test combustion chamber on the M11 research test rig in Lampoldshausen. The fuel consists of hydrogen peroxide (H₂O₂) as an oxidizer and an ionic liquid (liquid salt) as fuel. 

Due to its hypergolic properties, which means that the substances self-ignite on contact, HIP_11 has the potential to replace the fuel combination monomethylhydrazine and nitrous oxide that has proven itself in upper stage engines. This ability saves complex ignition devices and enables multiple ignitions of an engine. 

“We therefore see the application of HIP_11 in orbital propulsion systems such as large satellites or spacecraft,” says Negri. “Such a green fuel combination is particularly interesting for manned space missions, since the hazard potential of HIP_11 is significantly lower than with previous fuel combinations.”

3D printing compatible with green fuels

In addition, scientists have for the first time successfully tested a test combustion chamber produced using a 3D printing process with a green rocket fuel in a unique way. 

“The holistic view of innovative combustion chamber design and advanced fuels is the focus of our work, which we do as part of the DLR project Future Fuels”, says Dr. Lukas Werling, head of the M11 test bench complex. The green rocket fuel “Hydrocarbons mixed with Nitrous Oxide” (HyNOx) used in the tests is a mixture of nitrous oxide (N₂O) as an oxidizer and ethane (C₂H₆ ) or ethene (C₂H₄) developed by DLR) as fuel. With this type of fuel, the oxidizer and the fuel are either premixed and stored in a tank like a monergol or used as a conventional diergol.

This combines several advantages: HyNOx fuel combinations have a higher specific impulse compared to conventional monergols and are therefore more efficient. But they are not toxic and the components are very cheap because they are manufactured on a large industrial scale and are available worldwide. Laughing gas and ethane have a high vapor pressure that is used to pump the fuel into the combustion chamber. This eliminates the need for external pressure on the fuel tank and reduces the complexity of the drive system. 

Diverse possibilities of 3D printed research combustion chambers further explored

No combustion chamber material previously used can withstand these high combustion temperatures on its own. To this end, DLR engineers have further developed a conventionally manufactured combustion chamber: A 3D-printed test combustion chamber made of the low-alloy copper material (CuCr1Zr) was created. This material has both high strength and very good thermal conductivity. These properties are required to manufacture the combustion chamber for the HyNOx fuel, because the combustion chamber is also regeneratively cooled. 

Regenerative cooling is used to protect against overheating. Some of the fuel flows through channels in the wall of the combustion chamber before it is injected into the engine. The high thermal conductivity of CuCr1Zr is used in order to dissipate the high combustion temperatures via the fuel fed through in the combustion chamber walls. In the case of combustion chambers made of conventional materials without high conductivity, the combustion chamber would melt. 

“Based on the tests, the occurring heat loads and the combustion efficiencies were analyzed and compared with conventionally manufactured combustion chambers. The knowledge gained from this is used to design and construct future engines, ”Werling sums up. “In general, combustion chambers manufactured using the 3D printing process are suitable for use with the green HyNOx fuel.”

Future-oriented manufacturing processes are therefore also used with advanced fuels in order to increase the power density of space propulsion and significantly reduce production costs.