Key Takeaway

NASA and DARPA are collaborating on a nuclear rocket engine to accelerate space exploration, reducing transit time and providing higher payload capacity. This technology offers a competitive advantage in military defense, scientific inquiry, and commerce. The US is developing DRACO, a nuclear engine, spacecraft, and launch vehicle by 2027, using high-assay low-enriched uranium (HALEU) fuel for efficient, lightweight, and efficient nuclear operations. HALEU demonstrates potential for manned Mars travel, with a proposed nuclear thermal engine achieving 4,000 seconds impulse, but environmental concerns remain. DARPA confirms nuclear reactions will propel craft forward in outer space, a groundbreaking technology that could expand human space exploration beyond Earth’s borders.

In an attempt to facilitate mankind’s exploration of the Solar System, NASA has partnered with the Defense Advanced Research Projects Agency (DARPA) to finally bring to life one of our most ambitious and controversial engineering projects: the creation of a nuclear rocket engine. This thermal beast would propel us from Earth to Mars faster than ever before, creating new speed records in outer space.

A shortened transit time would reduce the number of supplies needed and would mitigate travel strain and danger for the onboard crew. When traveling through space, astronauts face the harmful effects of radiation and decreased bone and muscle density, as well as unique psychological stressors. A longer transit time also increases the number of provisions required to keep the passengers alive.

Nuclear thermal engines are able to reduce this transit time — and increase the payload capacity — because they are 3 to 5 times more efficient than standard chemical rockets and offer 10,000 times greater thrust-to-weight ratios than electric propulsion. Nuclear thermal engines have a very high specific impulse, a measurement of how efficiently fuel is converted to thrust, typically measured in seconds of acceleration. Traditional chemical rockets have a top specific impulse of about 450 seconds or 7.5 minutes whereas nuclear engines double these numbers to 900 seconds or 15 minutes of specific impulse. The higher the specific impulse, the faster you can move.

The highest specific impulse ever achieved by a chemical propellant was 542 seconds using a combination of lithium, fluorine, and hydrogen. This is, however, impractical because lithium and fluorine are both corrosive and easily ignited while hydrogen is explosive. This tripropellant is also toxic to humans and a danger to the environment, making it difficult to obtain the permissions needed to work with it.

The increased payload capacity of nuclear thermal engines also allows for more valuable scientific and communication instruments to make the interplanetary journey. Space itself is becoming of greater importance across the domains of military defense, scientific inquiry, and commerce. Having the ability to move larger payloads at greater speeds will give the US a notable advantage over other global space programs.

The exact project the US will use to achieve this advantage is called the Demonstration Rocket for Agile Cislunar Operations, or DRACO. It involves creating not only a working nuclear engine, but also an all new spacecraft and launch vehicle by the end of 2027, an ambitious timeline to bring this concept to life and have an in-space demonstration. The spacecraft has been named the experimental NTR vehicle (X-NTRV).

One of the biggest differences between DRACO and traditional chemical rockets is that the nuclear engine doesn’t aim to burn fuel. Chemical rockets carry fuel and oxidizer onboard in order to burn that fuel. Just like with campfires, oxygen is needed to sustain this chemical reaction. But the oxidizer comes with a heavy price — it takes up a huge amount of space onboard the rocket and contributes a great deal of weight, limiting the craft’s specific impulse.

Instead of burning the fuel, nuclear thermal reactors heat the propellant to ultrahigh temperatures (as high as 5,000 degrees Fahrenheit for DRACO) using a fission reaction like those found in nuclear power plants. The propellant expands before being exhausted through a nozzle, creating thrust. Because there is no need to carry oxidizer onboard the rocket is spared the extra weight and space-consuming tank.

The fuel used in this reaction is high-assay low-enriched uranium (HALEU), a uranium that’s enriched so that the fissile isotope concentration is between 5–20% of the fuel mass. The “assay” of HALEU refers to the concentration of the fissile isotope U-235. The nuclear engines would have an assay greater than light water reactors but lower than nuclear weapons, submarines and aircraft carriers. To give an idea of the potency of HALEU, just 3 tablespoons is enough to meet the electricity requirements for a person’s entire life.

HALEU is equally as impressive when we consider what it can do for manned travel to Mars. With current chemical rockets a one-way trip to the red planet will take about 7 months. This becomes just 3.3 months when using a nuclear rocket and 1.5 months when using the newly-proposed nuclear thermal engine from Professor Ryan Gosse of the University of Florida. If Gosse can provide evidence that his “Bimodal NTP/NEP with a Wave Rotor Topping Cycle” can perform as expected, the engine could achieve a specific impulse of 4,000 seconds. This engine is still in the beginning phases and is being elaborated on conceptually by Gosse, with funding from NASA.

Perhaps one of the biggest concerns regarding the engine is the environmental damage it could cause, though it would be far from the first time nuclear material would be launched into space. RTGs (radioisotope thermoelectric generators) are nuclear batteries that have been launching into space for over 60 years to provide power to satellites and other spacecraft that aren’t well-positioned enough to use solar power. While some aerospace failures have caused the release of plutonium, uranium and polonium in regions across the planet, no major catastrophe has ever served as a warning against launching nuclear material. DARPA has confirmed that nuclear reactions will not be used to launch the craft from Earth and into space, but rather they will be used once in outer space to propel the craft forward.

Part of the reason why the timeline has been so ambitious may lie in the fact that nuclear thermal engines are not necessarily new. The concept was studied in the 60’s by both NASA and the Department of Energy as part of the Nuclear Engine for Rocket Vehicle Application program. Scientists from the Los Alamos National Laboratory in New Mexico — the same laboratory that delivered the first atomic bombs — were able to build and test several successful nuclear thermal rockets that are still influencing designs to this day. The program ended a decade later but there has been ongoing research into the fuel, material, and designs for these systems.

Nuclear thermal engines have been called many things. Among those an “exciting investment in the future of human space exploration” and “a revolutionary technology that will allow the United States to expand the possibilities for future human spaceflight missions”. It is this potential which inspires the continued interest in nuclear spacecraft. Many go as far as to say that nuclear systems are a necessity for the exploration not only of our neighboring Mars, but of every planet after. These are the first steps of a technology that could take us far beyond our Solar System’s domestic borders, allowing us to someday venture into the oceans of interstellar space.


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