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Aerindel

Hypothetical design question. Would an LV-N create a radiation hazard?

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Finally working on designing a new class of interplanetary ships for 1.02. I used to always stick my atomic engines far away from my hab modules but no with 1.02 I want to place heat shields up front for aerobraking (wether I need them or not, it seems more realistic that way and looks awesome) This also means I need my fuel tanks up front or all the mass in the back flips my ships in hard aero braking maneuvers.

But, since you now have to stick your atomic engines directly to a large heat sink like the fuel tanks this means I need the engines up front too. This is easy to do but I find myself asking if there is would be a radiation risk to the crew if they where in a hab module behind the atomics. (obviously not directly behind but they would be within a few meters of the exhaust stream.

I realize the exhaust itself is not radioactive but would there be a cone of radiation emitted behind the engine itself?

I could also stick the heat shield array at the rear of the ship but then I would have my engines exposed to aero-braking forces which seems like a bad idea.

Obviously from a gameplay perspective none of this matters but I play KSP more like a roleplaying game and like to build craft that make sense.

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Using LV-N's near any hab modules is a very bad idea, in terms of 'roleplay'. The amount of radiation that would be emitted from them would be absolutely insane, and having any crew near it would not end well (malformed Jeb, anyone?). Put them as far away as possible from the hab modules, or make a containment area for the hab module so it isn't in the field of any radiation :>

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The reactor of a NERVA emits high amounts of gamma and neutron radiation when in operation. The only direction this radiation does NOT go (nearly as intensely) is in the forward direction because the NERVA has a radiation shield built in right on top of the reactor core.

Yes, the exhaust of a properly engineered nuclear engine need not be radioactive because the fuel passes through the reactor core very quickly and isn't likely to absorb may neutrons. If your materials are poor, you could get particles of radioactive material eroded off of the walls of the channels the fuel passes through in the reactor core. This stuff won't harm your ship because it leaves quickly in the exhaust, but you might want to not spew it around in an atmosphere. Better to use a design that does not suffer from this problem.

Putting propellant tanks and other mass between your kerbals and their nukes will reduce the radiation flux, and distance is always your friend since the radiation falls off as 1/r2.

The radiation levels would drop rapidly after the NERVA is turned off as the short-lived isotopes in the core decay away. But the reactor will still be "hot"* because the core now contains fission fragments, so I wouldn't go hug it.

*"hot" radioactively speaking. The temperature in the core of a NERVA is supposed to drop below something like 500° C after shutdown.

Edited by Brotoro

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I'm pretty sure the radioactive material inside of an atomic engine is very well shielded and there'd be little to no risk placing them within a few meters of habitation. The real safety risk regarding atomic rockets is if somehow the radioactive material were to escape containment, like say, a during catastrophic launch failure. This is a major reason why atomic rockets haven't been launched into space yet; launch failure could mean dangerous radioactive material spread throughout the upper atmosphere. Of course, if something were to also happen to the engine in space that could also mean bad news for the crew.

Also I'd just like to point out that the exhaust of nuclear thermal rockets is not radioactive. The radioactive material is contained separately and is just used to super heat the propellant, and not mixed with it. A lot of people seem to get this wrong.

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Personally I do not see why a properly designed core could not transfer sufficient heat out of the core and into an area of the engine to heat the propellant without also exposing radiation anywhere but the core. I think you could theoretically have a material that is a good radiation shield and good conductor of heat that can also withstand very high temperatures. Maybe some kind of thermal ceramic that is doped with metals that have high radiation blocking or absorption properties.

Of course for safety reasons you would still want a secondary buffer just in case of containment failure or leakage :-D

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For the sake of realism, always place your atomic rockets as far away from your habitation modules as possible, preferably behind meter after meter of liquid fuel. This is on top of the built-in radiation shield, which you must at all times recognize protects only a narrow cone of space drawn from the radiation source to the edge of the shield and onward. Should you need to dock with anything, your docking port will be in the nose and your final approach will keep the nozzle pointed away from all entities you are not presently at war with. Your exhaust is a weapons system. The exhaust may not be radioactive in an ideal world, but after a few cycles you'll have erosion of the materials sufficient to sprinkle radioactive dust around the place. Not to mention it is very very hot and moving very very fast. Oh, and should you land with the NERVA, use some sort of crane arm to keep your Kerbals from needing to pass too close to the reactor during descent. Then sprint behind the nearest hill.

If this sounds insane, hundreds of similar lowest-bidder military reactors cruise around in underwater cans full of nuclear missiles, and nuclear submariners have lower radiation exposure levels than the general population. Perfectly safe!

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I'm not sure how large your interplanetary ships are planned to be, but one thing you could do is create a mini rcs powered unmanned tug, and use it to move the heat shield from the front to back of the ship as needed. Stick it on the front when you need to use the engines, and stick it on the back for aerobraking, that way you don't need to have the fuel in the front of the ship, and your kerbals are safe from radiation

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Your exhaust is a weapons system.

Ah yes, the kzinti lesson.

Okay, well, I've played with my design and managed to come up with a aero-braking, reentry stable vessel with enough delta to get to eloo, LV-N's that can burn for 22 minutes before overheating parts and living space for kerbals that is on the opposite end of the ship from the engines. They aren't as protect as they could be but COG concerns mean they the fuel tanks need to be where they are, and its better than my other design that had the engines mounted up front with the kerbals living behind them only a few meters from the exhaust streams. It even has two spots to mount landers (yet to be designed)

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Screen%20Shot%202015-05-04%20at%2011.53.33%20PM_zpsnvkfrojv.png

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Actually, there are some puller configurations that would shield the crew and still keep the tanks on the front. Fore example, my Medusa transports, those were designed to look incredibly realistic and draw on real life concepts like tension structures working better:

i2ZCUZS.png

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The crew is behind the whole length of the ship, with no straight line between the engines and them that doesn't go through the propellant tanks (excellent radiation shields themselves). These engines, I roleplay, would have their radiation shields on the inner sides, facing the spine of the ship.

Rune. I mean, they are encased in docking ports that actually look like a cover.

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Personally I do not see why a properly designed core could not transfer sufficient heat out of the core and into an area of the engine to heat the propellant without also exposing radiation anywhere but the core...

The reason is mass. It takes a substantial amount if mass to shield an operating nuclear reactor, and excess mass is a killer in rocketry. That's why the NERVA engine only had radiation shielding in the front. To provide shielding all around the engine would increase its mass dramatically, and you'd end up with an ineffective propulsion system.

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The reactor of a NERVA emits high amounts of gamma and neutron radiation when in operation. The only direction this radiation does NOT go (nearly as intensely) is in the forward direction because the NERVA has a radiation shield built in right on top of the reactor core.

Yes, the exhaust of a properly engineered nuclear engine need not be radioactive because the fuel passes through the reactor core very quickly and isn't likely to absorb may neutrons. If your materials are poor, you could get particles of radioactive material eroded off of the walls of the channels the fuel passes through in the reactor core. This stuff won't harm your ship because it leaves quickly in the exhaust, but you might want to not spew it around in an atmosphere. Better to use a design that does not suffer from this problem.

Putting propellant tanks and other mass between your kerbals and their nukes will reduce the radiation flux, and distance is always your friend since the radiation falls off as 1/r2.

The radiation levels would drop rapidly after the NERVA is turned off as the short-lived isotopes in the core decay away. But the reactor will still be "hot"* because the core now contains fission fragments, so I wouldn't go hug it.

*"hot" radioactively speaking. The temperature in the core of a NERVA is supposed to drop below something like 500° C after shutdown.

OMG Brotoro, i saw your shielded Nerva designs in your Mission Pages and it seems like you put a lot of though into that :)

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The reason is mass. It takes a substantial amount if mass to shield an operating nuclear reactor, and excess mass is a killer in rocketry. That's why the NERVA engine only had radiation shielding in the front. To provide shielding all around the engine would increase its mass dramatically, and you'd end up with an ineffective propulsion system.

Actually, if you are willing to accept horribly low TWRs (by chemical propulsion standards, you would still leave ion engines on the dust), and sufficiently big starting masses so you get roughly similar mass ratios, the losses would only be those for non-Oberth spirals out of gravity wells, which while significant, are probably more than offset with the high Isp. Then again, nowadays aerospace engineers think so much "razor-thin margins" and "every gram counts" that that is, indeed, difficult to picture.

Rune. Perhaps in a future with routine orbit-to-orbit deep space transportation and space-fabricated radiation shields?

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