Hydrogen in nuclear rocket engines and climate change fight?

NASA Hydrogen Afterburner

Mission to Mars

Some of the latest ideas with human space flight are associated with flight to Mars. The journey from Earth to Mars with traditional chemical rockets may take on the order of 500 days at a minimum, which is burdensome from human endurance perspective. A viable possibility to cut down on travel time by at least 3X is to use nuclear rocket engines, or nuclear thermal propulsion (NTP). In these engines, a liquid hydrogen is heated by an onboard nuclear reactor to hot state and exhausted at high speed to create thrust. These engines, like any others, need to be developed and tested before operational usage.

Similar to traditional chemical rocket engines, they can be tested on Earth as an option. During ground testing, all of the gaseous hydrogen (GH2) coming out the engine would ideally need to be fully contained without release into surrounding air due to concerns with potential contamination with radioactive materials introduced by the nuclear reactor into the heated hydrogen. One of the ways to contain the exhaust hydrogen is to burn it with liquid oxygen (LOX), thereby converting it to steam, and then condensing the steam to water that can be stored in a special container.

Nuclear rocket Hydrogen containment

A critical part of such containment concept is the LOX/GH2 combustor or hydrogen afterburner, where a stable combustion must be maintained at high speed of incoming hydrogen – close to 1700 ft/sec or 1200 miles/hour speeds are developed inside! And it’s happening at pressures of 20-25 psia, which are significantly lower than typical aero or industrial combustors. The challenge is similar to trying to keep a candle lit in a hurricane wind.

Hydrogen afterburner

This full exhaust capture concept has been of interest to NASA Stennis Space Center. High speed combustion is of interest to RPS as well, as we consider that as one of the fundamental technologies that we will need to develop the space planes in the long term future. This strong overlap in mutual interest gave rise to the hydrogen afterburner project led by RPS for NASA over the years 2019-2022 under NASA SBIR Phase I and Phase II awards.

The hydrogen afterburner developed by RPS has successfully operated at high combustion stability and efficiency within a wide range of conditions spanning:

Hypersonic research dual use

This afterburner concept can be rapidly manufactured due to 3D-printing and is scalable to any size of the nuclear rocket engine that is planned to be ground tested. It can also serve as combustion test bed for hypersonic combustion research at similar or higher speeds within DoD community.

Hydrogen to Fight Climate Change

Industrial decarbonization

In an effort to decarbonize their operations to meet greenhouse gas goals and mandates, US-based companies are seeking ways to move away from burning fossil fuels – see Department of Energy 2020 data below. However, many applications in power generation, industrial processes and manufacturing require continuous, dispatchable power and/or high temperature heat. These hard to decarbonize segments cannot meet their greenhouse gas reduction targets with current technologies.

industrial hydrogen burners

Hydrogen combustion can fight climate change by offering the potential to meet the needs of these hard to decarbonize segments because hydrogen has many of the qualities of fossil fuels like coal or natural gas, yet only emits water vapor when combusted.  Hydrogen is controllable, storable, abundant and generates high temperature heat when combusted.  However, current equipment used in power generation, industrial processes and manufacturing operations, for example combustion turbines, are not equipped to cost effectively and efficiently combust 100% hydrogen without some sacrifice of cost, efficiency or both and therefore have not been widely adopted by industry.   Decarbonization using hydrogen as a fuel drives the need for next generation combustion systems that can combust hydrogen safely, cost effectively and efficiently.  

In addition to the need for combustion systems to be able to combust hydrogen, another less obvious need exists that is preventing more widespread adoption of hydrogen as a fuel: the relative immaturity of hydrogen as a commodity results in supply and/or cost barriers to utilization of 100% hydrogen by a system.  As such, the ideal combustion system can effectively combust not only 100% hydrogen but any combination of hydrogen, natural gas (methane) and/or liquid fuels to best meet the needs of customers in different locations with different needs around the combination of fuels they have available and the speed at which decarbonization efforts are moving within local communities.  

NASA hydrogen afterburner use for decarbonization

Traditional industrial combustion systems consist of numerous machined/cast parts that are expensive to make and assemble, and are designed with large single-element injectors and secondary air dilution zone, creating non-uniform localized mixture ratio zones with off-nominal temperatures, and leading to incomplete combustion, with resulting increased nitrogen oxide emissions in high temperature regions.

RPS believes that the hydrogen combustion technology developed for NASA can be successfully adopted for use in new generation of industrial combustors to fight climate change due to its distinct advantages:

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