With SpaceX garnering so much attention with the recent launch of their new flag ship Falcon Heavy, the first operational rocket powerful enough to take passengers to the moon since the retirement of the Saturn V, it would seem that the world is once again looking toward the possibility of manned space flight with destinations beyond low earth orbit in mind. Although NASA’s much touted SLS (Space Launch System) is expected to dethrone the Falcon Heavy as the most powerful rocket on the planet whenever it’s completed, repeated delays have sucked much of the wind from America’s deep space sails… but then, maybe NASA doesn’t need sails at all.
A manned mission to Mars would be an expensive endeavor, in large part because of how long the trip would take and all the variables to manage in the name of the crew’s safety. For instance, NASA’s two primarily spacecraft propulsion methodologies are effectively the same as everyone else’s: chemical and electrical (solar) propulsion. Each have their drawbacks. Chemical propulsion (think rocket engines) require a massive amount of fuel, creating a complex algebraic equation for each launch: the further you hope to go, the more fuel you need to carry, the more fuel you need to carry, the heavier the spacecraft, the heavier the spacecraft, the more fuel you need to carry… and so forth. Because of this, it’s likely that a manned mission to Mars would involve launching fuel storage at the planet in advance of the mission’s arrival, giving astronauts a chance to refuel before heading back home.
The alternative, often used on satellites and unmanned spacecraft, is solar electrical power. The problem is, these spacecraft can travel far distances on extremely low levels of fuel, but they travel far too slowly to be a reasonable approach to manned space travel. The options, then, seem to be carrying tons of fuel for an inefficient power plant that can get humans to Mars fairly quickly, or carry tons of food and water to sustain a crew on an extremely long duration solar fueled trip.
A chemical system has tremendous thrust force, but low miles per gallon, and you can only carry enough propellant to thrust for minutes,” Jeff Sheehy, chief engineer of NASA’s Space Technology Mission Directorate, explained. “A solar electric system has very low thrust force, but very high miles per gallon, and you can carry enough fuel to thrust for years.”
There is, however, one more option… one that would require very little fuel compared to chemical rockets, but still reach the Red planet significantly faster than current projections. Where can one find this miraculous technology? Well, sitting on NASA’s shelf for the past forty or so years…
Back in the 50’s and 60’s, NASA devoted considerable resources to the development of a nuclear propulsion system that could be used in spacecraft. This nuclear thermal propulsion (NTP) proved possible, but at the time, there were a number of technological limits to what we were capable of managing in a safe way, and further, there didn’t seem to be sufficient interest in deep space exploration to warrant the continued effort. By the 1970s, when the NTP was being shelved, the American public had already lost interest in the Apollo missions. The space race was over, and America was just beginning to settle in for another fifty (plus) years of never venturing further than a few miles from our planet’s surface.
Now, though, many of those technological hurdles are no longer as daunting, and perhaps more importantly, public interest in manned space travel is once again beginning to grow.
I mean the desire has always been there,” Sheehy said, “but the push or the emphasis that NASA has had for the last few years about developing that capability—that has renewed the interest in NTP as an option.”
In August of last year, NASA awarded a nearly $19 million contract to BWXT Nuclear Energy, who has a long and storied history of developing nuclear fuel solutions for the United States Navy. Their goal is to develop a nuclear reactor that runs on low-enriched uranium (rather than the more strictly controlled highly-enriched uranium often used in weapons applications. If predictions are correct, this style of propulsion could mean huge changes to the current Mars mission models.
A functional NTP system on a spacecraft could mean reaching Mars as much as 20-25 percent faster than we are currently capable of with chemical propellant crafts, but because of the long duration output capability allotted by an on board nuclear power plant would also mean changes in the way we think of these missions. Currently, missions to Mars would only be feasible during a 30 or so day window every 26 months, based on the locations of each planet within their respective orbits. An NTP wouldn’t have to worry about running out of fuel as quickly, meaning the launch window could grow dramatically.
More importantly, a Mars mission powered by an NTP system could choose to abort and return to Earth if an emergency were to arise.
You might have the opportunity to abort a month or two into the mission [with NTP],” Sheehy said. “Whereas with a chemical system, once you put yourself on a trajectory to Mars, you’re basically going to Mars. You maybe wish you could come home.”
A chemical fueled rocket, once set on its course to Mars, would almost certainly need to complete its journey no matter what, as it would need to rely on fuel in place in Mars orbit, or potentially a gravitational orbit assist from the planet itself to make the trip home. A nuclear-powered spacecraft, on the other hand, could simple turn around and head home if it needed to.
There are no guarantees that a nuclear propulsion system in NASA’s next generation spacecraft may come to fruition – as some of the same hurdles that forced this project onto the back burner decades ago do still persist. Concerns about the safety of nuclear power, regulations pertaining to the use of nuclear materials, and the cost of development dragging at an already sparse NASA budget may be enough to keep the NTP system from ever taking to the skies, but Sheehy intends to be ready if his department ever gets the call.
Whether NASA ultimately decides to build the nuclear thermal propulsion system for in-space transportation is still TBD, but what we are doing now is development that will allow the agency to take that step,” Sheehy said.
Image courtesy of NASA