What Future Propulsion Technologies Should NASA Invest In?

Researchers consistently complain about how difficult it is to fund breakthrough research. Most funding agencies, especially governmental ones, think funding incremental, evolutionary technological steps is the way to go, as it has the most significant immediate payback. But longer-term, higher-risk research is necessary to provide those incremental steps 20-30 years in the future. And in some cases, they are required to underpin completely new things that other researchers want to do.

That is the case with space propulsion systems. Current mature technologies, mainly derived from chemical rockets, cannot provide the necessary force to allow for a gravitational lens telescope out in the Oort cloud, an interstellar probe, or a round trip to Mars that would take less than a year. But other technologies on the horizon could if only we spent more time and resources developing them. So a group of  NASA and DoE engineers recently released a paper detailing some of those and where they think America’s space agency should direct its funding when developing new propulsion systems.

At the beginning of the paper, the authors lament that there hasn’t been any large-scale NASA investment in breakthrough propulsion technologies since the 1970s. And they’re right; the last significant effort was the Nuclear Engine for Rocket Vehicle Application (NERVA) project, which wound down in the 1970s. Despite the lack of prototyping, plenty of ideas were put forward then. Just none have made them into hardware.

Those ideas can be broadly categorized into four different groups of systems: chemical propulsion, nuclear thermal propulsion, solar electric propulsion (SEP), and nuclear electric propulsion (NEP). The rest of the study aimed to determine what if any, significant advancements could be made in those four systems that could lead to them lowering the round-trip time to Mars to less than one year.

The authors discard chemical propulsion and nuclear thermal propulsion, stating that they are simply not cut out for the rapid technological changes that could enable their use for these game-changing propulsion systems. Chemical propulsion suffers from “the tyranny of the rocket equation,” as Isaac Arthur puts it. But nuclear thermal propulsion suffers from the same underlying problem – they must carry too much weight in propellants to be viable for truly ground-breaking speed increases.

That leaves solar electric propulsion and nuclear electric propulsion. The authors break down the current state-of-the-art technologies for each technology and calculate the weight per kilowatt of energy they produce. Neither looks particularly promising at the state of the art – with NEP coming in at 51 kg/kWe and various solar arrays that could drive a SEP system ranging from 5 kg/kWe up to 22.73 kWe. None of those weight/power tradeoffs would result in anything approaching a sub-one-year time to Mars.

But why stop there? The authors do a deep dive into potential technologies on the horizon, ranging from materials to photovoltaics, that could dramatically impact those calculated ratios. The paper concludes with “transformative” technologies that could decrease the kg/kWe to below one. In that case, an extensive enough power system can reasonably transport humans to Mars in between 50 and 100 days. 

The researchers also looked at some early-stage propulsion concepts from NASA’s Institute for Advanced Concepts – including the ever-popular “pulsed nuclear” propulsion system, where a nuclear explosion is intentionally initiated behind the spacecraft to push it forward. These technologies are too early to be included in a deep-dive analysis, but they could lead to some promising alternative technologies.

To invest in those alternative technologies, the authors suggest NASA commit 1% of its Space Nuclear Propulsion budget to developing breakthrough technologies. At $45 million for FY2023, the whole budget isn’t exactly breaking the bank, and a mere $450,000 probably wouldn’t make too big of waves in the industry. But, it is undoubtedly better than what is currently allotted toward maturing these transformative propulsion technologies.

Learn More:
Dankanich et al. – Transformational Propulsion for In-Space Fast Transits
UT – Solar Electric Propulsion Systems are Just What we Need for Efficient Trips to Mars
UT – A Novel Propulsion System Would Hurl Hypervelocity Pellets at a Spacecraft to Speed it up
UT – Magnetic Fusion Plasma Engines Could Carry us Across the Solar System and Into Interstellar Space

Lead Image:
Mars/Lunar Transfer Orbits
Credit – Dankanich et al.