# Nuclear propulsion: viable long-term launch strategy?

So recently there was this interesting theory -the paper itself is quite readable, but here's an article link that expands on it. Basically, it states that aliens on those very habitable but massive super-earths might not be able to escape their planet's gravitational grasp with chemical rockets. Well in theory they could, but it would require a rocket design with preposterous weight requirements on the order of an ocean battleship just to launch a satellite.

The most obvious alternative -to our mind- would be nuclear propulsion, but this made me wonder: what are the long-term effects of repeatedly blasting regions into wastelands while sending such rockets into space? Is it possible to re-use a lap of land already annihilated by what is really a nuclear bomb, or would the aliens have to resort to using other fertile launch pads?

• Nuclear rockets design do not require the remass to be radioactive nor for it to contain nuclear waste. Just place it in a desert an you'll be fine. May 29 '18 at 8:02

You do not need to create a nuclear wasteland to have a nuclear rocket. All you need is heat and propellant. If you need more propulsion you need more heat, and you deploy more nuclear power. One idea about how to do this would be a nuclear ramjet. It turns out they exist. https://en.wikipedia.org/wiki/Project_Pluto

The principle behind the nuclear ramjet was relatively simple: motion of the vehicle pushed air in through the front of the vehicle (ram effect), a nuclear reactor heated the air, and then the hot air expanded at high speed out through a nozzle at the back, providing thrust.

The notion of using a nuclear reactor to heat the air was fundamentally new. Unlike commercial reactors, which are surrounded by concrete, the Pluto reactor had to be small and compact enough to fly, but durable enough to survive a 7,000-mile (11,000 km) trip to a potential target. The nuclear engine could, in principle, operate for months, so a Pluto cruise missile could be left airborne for a prolonged time before being directed to carry out its attack.

Nuclear ramjets do not shower the area they fly over with nuclear waste. Probably the remnants of the nuclear engine are deposited at the impact site, but their effect is lost in the larger nuclear explosion.

Back to your heavy worlders: for the last piece of the trip outside the gravity well, there is not enough atmopshere remaining to use as propellant so your nuclear ramjet will need onboard propellant to be heated and exhausted out the back. Something which expands greatly on a phase change would serve - like rocket fuel. Or water.

• Actually one reason given why the nuclear ramjet was abandoned was because they could not figure out how to test it safely. Most nuclear rockets are actually cleaner. May 28 '18 at 18:39

There was a detailed exploration of launching from heavy planets in the Space Exploration Stack Overflow; I think it's a better explanation of the problems with conventional rockets than the cited paper.

Nuclear engines get around the problem because they can give more energy, hence a higher velocity, to the exhaust stream than a chemical reaction can. Unless they have an accident, they don't make the exhaust stream radioactive in the process. So the keys to reducing the radioactive contamination of the launch site are (1) fewer accidents and (2) less radioactivity from accidents.

Fewer accidents is hard. Starting a space program without accidents is almost unbelievably hard. There are likely to be some.

But reducing the radioactivity has some promise. What makes a reactor dangerously radioactive is, well, the reaction in the reactor. The original fuel load isn't so bad, particularly if the fuel is encapsulated in rods or pellets. The radioactivity builds up as the fuel "burns" because it's (mostly) the fission products. So the nuclear engine starts as not so dangerous, but becomes more so as it runs. You'll want to get it launched and on its way.

If you can get your experimental rockets headed out over an ocean, you'll also minimize the problems from eventual explosions. Big chunks will sink, and smaller dust will be dispersed. Oceans are big.

We ended up blowing up a lot of equipment, infrastructure and people getting to space. If there was an easier way, we would have taken it, but there wasn't. So long as a culture on a heavy planet thought getting there was worthwhile, using nuclear rockets isn't that much harder than our progression from throwing things, to firework rockets, to modern chemical rocket engines. It's just one more step.

• This is a bit too simplistic. Yes, nuclear rockets can in principle give you much higher specific impluse and not exhaust the radioactive material. However for a launch, you need not only $I_\mathrm{sp}$ but also lots of thrust, and that's another thing – e.g. the various types of ion thrusters, hall thrusters etc. give you plenty of $I_\mathrm{sp}$ but only little thrust. For a nuclear rocket, you could probably get the combination of both high $I_\mathrm{sp}$ and thrust, but it may be only possible by reacting the nuclear fuel in the gas phase, and that would cause radioactive exhaust. May 29 '18 at 9:06
• @leftaroundabout The “Nuclear Thermal Rocket” Wikipedia article discusses how this was actually done with reactors. None of those used gaseous radioactives. en.m.wikipedia.org/wiki/Nuclear_thermal_rocket May 29 '18 at 13:16

You could repeatedly use one piece of land for take-off/landing operations provided you had good shielding and/or remote systems for permanent structures and transportation equipment bringing rockets and supplies in. You have a much bigger problem though; the atmospheric radiation pollution from launching volleys of nuclear powered spaceships would start to kill the planetary ecology in short order.

Project Excalibur was a "Star Wars" plan to use satellites that would explode atomic bombs and use lasers to focus the X rays to destroy incoming satellites.

https://en.wikipedia.org/wiki/Project_Excalibur

Thus it seems possible for a somewhat similar system to use mostly underground nuclear explosions to power powerful ground based lasers used to power rocket launches.

Nuclear pumped lasers can also use the energy from nuclear reactors to power ground based lasers, which could be used to power rocket launches.

https://en.wikipedia.org/wiki/Nuclear_pumped_laser

Other methods have been suggested for transmitting energy from ground based generating systems to launching rockets.

If and when fusion power generators become practical they can be used to generate power that could be transmitted to launching rockets.

Assuming that the biology of the aliens is not very different from ours when it comes to dealing with radiation, it is straightforward that the place where a nuclear explosion took place is barren for access.

The only solution for reusing a launch spot a la Cape Canaveral or Baikonur would be to have remotely operated machinery to execute most of the operation needed for site and launch preparation.

Astronauts in the ship would be protected by the shielding of the ship itself. I mean, if you can survive a nuke exploding below your buttocks thanks to the shielding, you are not going to worry about some radioactive nuclei scattered on the ground.

To further limit fallout it would be good to use a sort of potato cannon, with the explosion taking place underground. Few hundreds meter more to rise won't be a challenge for a nuke, but the subsequent collapse of the ground around the detonation site would prevent most of the fallout of a surface explosion. Not all, as some would exit following the ship.