In January of this year NASA and the Defense Advanced Research Projects Agency (
DARPA) announced a collaboration to demonstrate a Nuclear Thermal Rocket (
NTR) engine in space. The demonstration is scheduled for 2026.
There has been interest in recent years in NTR by the United States Space Force and DARPA for orbital and cis-lunar uses. NASA is interested in the agreement for future missions to the moon and Mars. The project partnership is called the Demonstration Rocket for Agile Cislunar Operations, or DRACO.
NTR provides greater thrust than chemical rockets. Heat from a fission reaction replaces the chemical energy of the propellants in the engine. Liquid hydrogen is heated to a high temperature in a reactor and then expands through a nozzle, creating thrust. The nuclear heat source has a higher exhaust velocity and is expected to double or triple payload capacity compared to chemicals that store energy internally.
The kinetic energy per molecule of propellant is determined by its temperature, whether the heat source is a nuclear reactor or a chemical reaction. At any given temperature, lightweight propellant molecules carry just as much kinetic energy as heavier ones and therefore have more energy per unit mass. This makes low
molecular mass propellants more effective than high molecular mass compounds.
Chemical rockets use the waste products from their reactions to produce thrust. Most liquid-fueled engines employ either hydrogen or hydrocarbon combustion, and the exhaust is mainly water (
molecular mass 18) and/or carbon dioxide (
molecular mass 44). Nuclear thermal rockets using gaseous hydrogen (
molecular mass 2) have a theoretical maximum specific impulse that is three to four-and-a half times greater than those of chemical engines.
At temperatures above 1,500 degrees C, molecular hydrogen (
H2) dissociates to atomic hydrogen (
H), making the engine even more efficient.
NTRs use low enriched uranium fuel, so there is a risk of radioactive contamination. A rupture of the reactor vessel, whether caused by a launch failure, runaway reaction, flaws in design or material fatigue, could scatter material into the environment.
Before criticality occurs, solid core NTR fuel is not particularly hazardous. Once the reactor has been started for the first time, highly radioactive short-life fission products are produced, as well as less radioactive but extremely long-lived radio isotopes. Additionally, all engine structures exposed to direct neutron bombardment become radioactive.
Under DRACO, Idaho National Laboratory is advancing and testing fuel composites at its Transient Reactor Test (TREAT) facility. The lab is examining how they perform in the harsh thermal and radiation environments needed for nuclear thermal propulsion.
Current solid-core NTR designs are intended to greatly limit the dispersion and break-up of radioactive elements in the event of a catastrophic failure.
Launch from Earth may be impractical given the rocket’s higher exhaust temperatures and speeds. Consider the damage SpaceX’s April Starship lift-off caused to its launch facilities. Regardless, a cold launched Mars Transfer Vehicle would be assembled in-orbit utilizing a number of NASA SLS or SpaceX Starship payload lifts.
NASA is looking at the technology to facilitate its preferred shorter-stay class of missions to keep the roundtrip crewed mission duration to about two years. This takes advantage of optimal planetary alignment for a low-
energy transit for the first leg of the trip and making the higher-
energy transit for the return leg thanks to the new technology.
The more powerful NTRs offer a shorter flight time to Mars, estimated at three to four months, compared to six to nine months using chemical engines. Shorter transit time reduces crew exposure to cosmic rays for stays of up to fifty days in the vicinity of the Red Planet (thirty days on the surface). It also diminishes supply requirements and the need for more robust mission systems. Other benefits include increased science payload capacity and higher power for instrumentation and communication.
Nuclear thermal propulsion could allow for more flexible abort scenarios, allowing astronauts to return to Earth at multiple times if needed, including immediately upon arrival at Mars.
NASA is looking at two types of nuclear propulsion for human missions. Thermal, like DRACO, and electric as discussed in the
June 2021edition of JOTH and used in the fictitious DeepStar in my EPSILON Sci-Fi Thriller series. But to meet NASA’s two-year mission duration, a nuclear electric system would need a large chemical stage for the overall thrust required for a human mission. In the absence of a radical technological advancement, the space agency is committed to NTR for the first decade of its Mars missions.
NASA’s press releases discuss DRACO in terms of scientific missions to Mars. To get there, the space agency has fully embraced a lease for services model to use cheaper commercial launch vehicles and mission infrastructure. The Department of Energy enforces strict requirements surrounding the possession of fissile materials by private companies. The risks of testing and accidental loss of containment are high. In an about-face to their lease for services, I expect NASA to develop and lease nuclear thermal rocket boosters to qualifying corporations.
I foresee a bidding war developing between companies like EPSILON, and perhaps countries with Mars ambitions. SpaceX’s Starship and NASA’s NTR will compete to shuttle personnel and equipment to and from the Red Planet during the early frontier days of the 2030s.
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Want a deeper dive? Check out these sources, listed in the order of discussion.
https://www.nasa.gov/press-release/nasa-announces-nuclear-thermal-propulsion-reactor-concept-awardshttps://www.nasa.gov/mission_pages/tdm/nuclear-thermal-propulsion/index.htmlhttps://www.nasa.gov/press-release/nasa-darpa-will-test-nuclear-engine-for-future-mars-missionshttps://en.wikipedia.org/wiki/Nuclear_thermal_rocket
