10

The biggest challenge of our Mars journey is the journey time (9 months). Now I have seen news coming stating that atomic or nuclear rockets would cut the journey time in half. Is that right? When I googled for nuclear rocket, I found that this technology has been known since the 1960s. What took it so long to be considered a better technology than the chemical rockets?

David Richerby
  • 862
  • 10
  • 16
CodeIt
  • 208
  • 3
  • 7
  • 6
    news [...] stating that atomic or nuclear rockets would cut the journey time in half. Can you add references? –  Feb 16 '18 at 10:17
  • 10
    this technology has been known since the 1960s On paper yes, but not in practice –  Feb 16 '18 at 10:17
  • 2
    The Apollo programme had active plans to use a nuclear rocket on the S-IVB stage, but that got shelved because of all sorts of concerns. – GdD Feb 16 '18 at 10:38
  • Related: https://space.stackexchange.com/questions/26/is-there-any-ongoing-research-in-nuclear-pulse-propulsion – berry120 Feb 16 '18 at 11:24
  • @JanDoggen Here are the news articles 1) Daily Star 2) Bloomberg 3) Futurism 4) Daily Mail – CodeIt Feb 16 '18 at 12:53
  • 6
    @CodeIt, simply, (1) "nuclear rockets" have been fully explored, since back in the 60s. (In many ways they are just not very good.) Note that (2) these articles are "non-news" items. NASA happened to spend a tiny amount of $ on a study, involving the word "nuclear". By sheer coincidence, a newspaper noticed this and presented it as a "clickbait" headline. (Other newspapers then copy.) It is nothing. Zero. NASA continually does tiny studies like this on all sorts of things. You will see clickbait headlines like "NASA antimatter! NASA sex in space!" etc. Means, precisely, nothing! – Fattie Feb 16 '18 at 14:38
  • 1
    A nuclear bomb rocket would permit a flight time to Jupiter on the order of weeks, let alone halving the time to Mars. Can you be more specific? – Yakk Feb 16 '18 at 15:32
  • 1
    Note that SpaceX ITS plan aspires for trip times of Mars between 80 and 30 days - using chemical propulsion and aerocapture. It's worth noting that this is faster than many of the proposed nuclear schemes (often quoted 100 days or 3 months). Nuclear engines due to much higher specific impulse make better use of propellant mass, but if propellant can be launched cheaply into earth orbit then chemical engines can achieve very respectable trip times and Oberth effect is quite potent. – Blake Walsh Feb 16 '18 at 15:43
  • 2
    @CodeIt: The articles are all a good six months after the actual contract, which is for a ridiculously tiny $19 million. And they all repeat the same mistaken talking points about "last used in the 70s" (wrong: NASA nuclear tests occurred also in the 90s — and, yes, BWXT's ancestor company was involved then too). – Nathan Tuggy Feb 16 '18 at 17:14
  • @JanDoggen "...on paper yes but not in practice" Both the US and Russia extensivley built and tested successful nuclear engines. In fact, the US nerva engine ran for around 2.5 hours continuously at Max thrust and was going to be used for a Mars mission till the program was cancelled by congess. https://en.m.wikipedia.org/wiki/NERVA – Dragongeek Feb 18 '18 at 22:03

3 Answers3

31

The biggest challenge of our Mars journey is the journey time (9 months).

Well, maybe. More likely, the biggest challenge is making it economically worthwhile to support a viable colony. But if you're just talking about the problems of transport, assuming the will to do it, sure, that's plausible enough.

Now I have seen news coming stating that atomic or nuclear rockets would cut the journey time in half. Is that right?

It's right, or even understating the advantage, but this is not news at all. This has been known since the designs were drawn up in the first place, and the advantages fairly well understood.

When I googled for nuclear rocket, i found that this technology has been known since the 1960s. What took it so long to be considered a better technology than the chemical rockets?

Very simply, it didn't take so long. It's always been considered technologically superior, and several concepts are comparatively straightforward to develop or have already been mostly worked out (Orion, in particular, got quite far before being shut down hard, and Timberwind/NERVA got even farther). The problem has been that no one wants to deal with the risks, public perception, or definite hazards associated with putting high-powered nuclear reactors in space. Or even putting nuclear reactors in space at all. (Let's not even mention nuclear bombs every second, Orion/Medusa style.)

Until you can say "1 MW nuclear-thermal engine" to a US senator without them reaching for their smelling salts or their shotgun, this is a non-starter.

See also Are nuclear-powered engines the way to go for space exploration? for a slightly different perspective on the issues.

Nathan Tuggy
  • 4,566
  • 5
  • 34
  • 44
  • 5
    "high-powered nuclear reactors in space" Nuclear reactors are already in space! radioisotope thermoelectric generators are on many satellites and probes, and the TOPAZ nuclear reactor for example: https://en.wikipedia.org/wiki/Nuclear_power_in_space Projects like Orion are hardly reactors though. More like giant nuclear bomb bays detonating their contents in sequence... – Baldrickk Feb 16 '18 at 11:59
  • 4
    Or in getting the reactor/fuel safely into space in the first place. – GalacticCowboy Feb 16 '18 at 12:10
  • 1
    Not sure what power will US senators have to stop China or Russia put them up if they want to. China seems particularly keen on getting to Mars. Besides, the news was that NASA has subcontracted BWXT to design a reactor so I wouldn't discard it so quickly. – Diego Sánchez Feb 16 '18 at 14:43
  • 10
    @Baldrickk: RTGs are not reactors. TOPAZ and SNAP-10 and company aren't flying in new missions. (And I don't think it needs to be said that if there's public resistance to nuclear reactors, the resistance is that much greater to bomb propulsion.) – Nathan Tuggy Feb 16 '18 at 15:10
  • 9
    GalacticCowboy has the right idea. I don't think many lawmakers care much about reactors in space. They care about a launch disaster spewing radioactive fuel over millions of square miles of ocean off the Florida coast that we rely on for food, or winds wafting that junk over populated areas such as Orlando. Move the launch site to Edwards AFB and now you risk one of the largest metropolitan areas in the nation if the launch fails during a critical stage. None of this helps reelection. –  Feb 16 '18 at 15:53
  • Quick explanation of why just delivering nuclear material to space is difficult: We mess up space launches all the time. A rocket exploding on the pad or in the air is destructive enough with nothing but rocket fuel and a dummy payload inside. If you added radioactive material and it exploded, you could easily contaminate the area for hundreds of miles around -- it might not be a nuclear bomb, but it would be a dirty one, and it would be disastrous. No one has been willing to take that risk yet. –  Feb 16 '18 at 17:47
  • 6
    I feel compelled to point out that nuclear reactors are not a significant radiological hazard UNTIL THEY ARE OPERATED. Launch it before it's operated and you reduce your risks near to zero. Before startup, your fuel is enriched uranium, chemically bound as an oxide or carbide (not water soluble). Pretty tame stuff. A rocket with a freshly-built reactor exploding on launch and somehow rupturing the reactor presents nearly zero risk to the population. AFTER it's been run it starts accumulating fission products, like water-soluble iodine and bone-loving cesium. – Kengineer Feb 16 '18 at 20:39
  • Important to draw distinctions between radio-decay, active radio-thermal, direct nuclear... not all nuclear is the same. Active is a problem in space because because shielding is heavy. Direct is bad full stop, cause that much radiation is bad. – Kaithar Feb 16 '18 at 22:14
  • 1
    @Kengineer well yes, uranium fuel is /mostly/ safe before a reactor is activated, I still wouldn't want to risk aerosolize a signficant amount of it. I also wouldn't want to be responsible for designing a reactor that an both produce a decent amount of power without the possibility of a criticality incident if an explosion happened. Then there's the matter of the non-radioactive components, like neutron moderators and coolants, some of those aren't what you'd call conducive to well-being. – Kaithar Feb 16 '18 at 22:27
  • 1
    @Kengineer: So, a reactor is basically mostly safe until run. You know that, and I know that. Does a US senator know that? (Probably, but…) Does the US public know that? (No.) – Nathan Tuggy Feb 17 '18 at 02:50
0

Cut the journey in half compared to what? As far as I’m aware there is no physical limit for chemical rocket velocity or acceleration (except for the speed of light of course). With a huge enough rocket, thrust and enough stages you could go as fast as you want.

Nuclear rockets have a much higher efficiency of about ~800s of specific impulse compared to the ~400s of chemical rockets. So it’s true that it would take about half as much fuel for the same change in velocity. However, the most limiting factor in rocket flight is cost [citation needed] and I doubt that a nuclear rockets would be very competitive in that regard.

Michael
  • 472
  • 4
  • 10
  • 3
    Specific impulse has a distinctly non-linear (exponential, in fact) effect on total fuel mass. If your fuel fraction is, say, 90% (common enough for practical orbital rockets), halving specific impulse from 800 to 400 gets you a rocket that can be as much as ten times the mass, and that's before you increase the dry mass to account for the larger engines you need to maintain the thrust to weight. See e.g. http://quantumg.net/rocketeq.html to run the numbers yourself. (m0 is the wet mass, m1 the dry. For orbit, 9k or 10k m/s delta-V is necessary; for Mars trip, closer to 16k at minimum.) – Nathan Tuggy Feb 16 '18 at 17:06
0

Current flights from the Earth to Mars are routes that take the least amount of fuel (energy) to get there. If you have plenty of fuel (money) you could fly a route that goes directly to Mars, no need to wait for a launch window or anything.

Jes007
  • 11