3 ms·

> In the 1970’s, an experimental nuclear thermal rocket engine gave an energy equivalent of 8.3 km/s. This engine used a nuclear reactor as the source of energy

> In the 1970’s, an experimental nuclear thermal rocket engine gave an energy equivalent of 8.3 km/s. This engine used a nuclear reactor as the source of energy and hydrogen as the propellant. That was intriguing, but didn't go into detail on why a nuclear thermal rocket hasn't been tried since. The obvious explanation is that there could be serious consequences if such a rocket exploded, spreading radioactive material, etc. And rockets tend to explode sometimes. However, it sounds like while that is certainly a concern, it is not as great of one as it might seem: http://en.wikipedia.org/wiki/Nuclear_thermal_rocket#Risks http://en.wikipedia.org/wiki/Nuclear_thermal_rocket#Risks And indeed there is still work ongoing on such designs.

daeken 12y ago
> And rockets tend to explode sometimes. Chemical rockets -- the only ones we've ever actually used -- explode because that's what they're intended to do. The only difference between a successful rocket firing and a catastrophic rocket failure is the speed at which the explosion happens. A nuclear rocket engine has basically no risk of explosion; tearing itself apart at speed maybe, if the aerodynamics aren't done properly or there's a structural weakness. But that's about it.
- pjc50 12y ago
  Normal rocket operation is a burn, not a detonation, and there's a clear difference between those that's not just about speed.
- tempestn 12y ago
  Normally at least the first stage of the rocket will still be chemical, so the risk of explosion is still there. Fortunately the risk of spreading radioactive material appears limited regardless.
- snowwrestler 12y ago
  Pretty much any large rocket today has an explosive self-destruct mechanism to cover failure modes where the rocket goes in an unintended direction. So that's one reason a nuclear rocket might explode at low altitude.
yoha 12y ago
Nuclear engines are not practical for lift-off. They can, however, be used for inter-planetary travel for a big enough payload.
tempestn 12y ago
There's a nice easy to read primer on nuclear rocketry here: http://www.lanl.gov/science/NSS/issue1_2011/story5full.shtml http://www.lanl.gov/science/NSS/issue1_2011/story5full.shtml
- deleted 12y ago
  [deleted]
RBerenguel 12y ago
I wonder if the treaty about nuclear weapons in higher atmosphere/space has something to do with it. A reactor is far from being a bomb, even if it explodes mid-air the explosion would essentially be hydrogen+oxygen (with maybe a high amount of radioactive plume, but not a proper nuclear detonation).
anovikov 12y ago
Nuclear thermal rockets also provide a very poor mass fraction of fuel because their propellant has very low density (liquid hydrogen) and the reactors are heavy. So their advantage is much small than it seems. And low thrust makes things even worse, they can be used only for upper stages/space tugs. In the end they were never used mainly because cost outweighed benefits. Imagine, nuclear fuel contains about millionx the energy per mass of chemical fuel, and yet exhaust velocity is only 2x higher, and effective combined ISP of the stage (velocity change vs mass fraction assuming dead weight of stage being zero, and with gravity/ballistic losses subtracted) is only about 50% higher than state of the art chemical systems, at vastly higher cost and risks involved. If i was Elon Musk i won't launch a nuclear rocket unless i had a really good liability insurance, and i was an insurer i'd say nah unless federal government backs me, and if i was uncle Sam i'd say nah, too. It simply doesn't worth it. Also, problems with space access are mostly market size-related problems. Cheap access to space is possible, only requires a vast market to pay back the investments, which is simply not there.
Symmetry 12y ago
More information than you require about different nuclear (and other) rocket designs. http://www.projectrho.com/public_html/rocket/enginelist.php http://www.projectrho.com/public_html/rocket/enginelist.php

daeken 12y ago

> And rockets tend to explode sometimes. Chemical rockets -- the only ones we've ever actually used -- explode because that's what they're intended to do. The only difference between a successful rocket firing and a catastrophic rocket failure is the speed at which the explosion happens. A nuclear rocket engine has basically no risk of explosion; tearing itself apart at speed maybe, if the aerodynamics aren't done properly or there's a structural weakness. But that's about it.

pjc50 12y ago
Normal rocket operation is a burn, not a detonation, and there's a clear difference between those that's not just about speed.
tempestn 12y ago
Normally at least the first stage of the rocket will still be chemical, so the risk of explosion is still there. Fortunately the risk of spreading radioactive material appears limited regardless.
snowwrestler 12y ago
Pretty much any large rocket today has an explosive self-destruct mechanism to cover failure modes where the rocket goes in an unintended direction. So that's one reason a nuclear rocket might explode at low altitude.

yoha 12y ago

Nuclear engines are not practical for lift-off. They can, however, be used for inter-planetary travel for a big enough payload.

tempestn 12y ago

There's a nice easy to read primer on nuclear rocketry here: http://www.lanl.gov/science/NSS/issue1_2011/story5full.shtml http://www.lanl.gov/science/NSS/issue1_2011/story5full.shtml

deleted 12y ago
[deleted]

RBerenguel 12y ago

I wonder if the treaty about nuclear weapons in higher atmosphere/space has something to do with it. A reactor is far from being a bomb, even if it explodes mid-air the explosion would essentially be hydrogen+oxygen (with maybe a high amount of radioactive plume, but not a proper nuclear detonation).

anovikov 12y ago

Nuclear thermal rockets also provide a very poor mass fraction of fuel because their propellant has very low density (liquid hydrogen) and the reactors are heavy. So their advantage is much small than it seems. And low thrust makes things even worse, they can be used only for upper stages/space tugs. In the end they were never used mainly because cost outweighed benefits. Imagine, nuclear fuel contains about millionx the energy per mass of chemical fuel, and yet exhaust velocity is only 2x higher, and effective combined ISP of the stage (velocity change vs mass fraction assuming dead weight of stage being zero, and with gravity/ballistic losses subtracted) is only about 50% higher than state of the art chemical systems, at vastly higher cost and risks involved. If i was Elon Musk i won't launch a nuclear rocket unless i had a really good liability insurance, and i was an insurer i'd say nah unless federal government backs me, and if i was uncle Sam i'd say nah, too. It simply doesn't worth it. Also, problems with space access are mostly market size-related problems. Cheap access to space is possible, only requires a vast market to pay back the investments, which is simply not there.

Symmetry 12y ago

More information than you require about different nuclear (and other) rocket designs. http://www.projectrho.com/public_html/rocket/enginelist.php http://www.projectrho.com/public_html/rocket/enginelist.php