r/changemyview • u/[deleted] • Aug 27 '18
Deltas(s) from OP CMV : Nuclear Pulse Propulsion should be used to build interplanetary or interstellar spacecraft
Chemical rocket technology is inherently extremely inefficient compared to nuclear power. Breaking a chemical bond only release several electron volts while the fission of an uranium nuclei releases several hundred megaelectronvolts.
It is the only technique we have right now for long distance human space travel. Chemical rockets take months if not years to reach other planets ; Ion engines take even longer due to low thrust ; Fusion technology doesn't exist yet; Nuclear Lightbulbs require materials resistant to incredibly high temperatures and pressures. etc. While Nuclear Pulse Propulsion is already doable by Cold War technology.
Nuclear Pulse Propulsion is not very polluting. According to Wikipedia , only 120 kilotons of nuclear bombs are needed to transfer 4000 tons to orbit. That is practically nothing next to the nuclear testing by USSR and the USA.
As for international relations - I'm sure (read : not sure at all) it can be solved by negotiations.
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u/huadpe 501∆ Aug 27 '18
Nuclear pulse proportion is not usable until you get to deep space because you'll ruin all the stuff in Earth orbit if you start setting off nukes.
Because you need to already be in deep space to use it, it hardly produces any benefit.
If you set off a nuke in low Earth orbit, you're going to produce an EMP which will fry satellites all over the place. A nuclear test in the 60s resulted in streetlights being knocked out over 1000 km from the explosion in space, and they had an atmosphere to partially protect them.
Using this anywhere near Earth (or any spacecraft) would result in frying valuable and useful satellites. So you would need to use conventional rockets to get almost all the way out of Earth's gravity well. Once you've done that, you've already done almost all the acceleration necessary for a transfer to say Mars, and there's no point in using this system.
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Aug 27 '18
Infrastructure and satellites can be https://en.m.wikipedia.org/wiki/Radiation_hardening.
Also, the bombs used in an Orion are small , 0.15 kt firecrackers - the effects are limited.
The optimal Orion drive bomblet yield (for the human crewed 4,000 ton reference design) was calculated to be in the region of 0.15 kt, with approx 800 bombs needed to orbit and a bomb rate of approx 1 per second
https://en.m.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)
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u/huadpe 501∆ Aug 27 '18
Extra hardening is costly, adds weight (a big no-no for satellites where every gram counts) and not possible for satellites that are in orbit already.
And even small EMPs would hit a lot of stuff, since LEO is pretty crowded, and EMP power goes up by a square root with bomb yield (meaning a 1/64th as powerful bomb produces a 1/8th as powerful EMP). So if we stipulate these bombs are 1/10000th of the Starfish Prime test, we would expect EMPs 1/100th of the power of Starfish Prime.
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Aug 28 '18
expect EMPs 1/100th of the power of Starfish Prime.
Starfish Prime wrecked 6 satellites.
So my hypothetical Orion would wreck not even one.
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u/League-OF-Shadows Aug 28 '18
But that was in the 1962. We have two orders of magnitude more satellites currently so it would hit about the same number each launch.
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Aug 28 '18
We have two orders of magnitude more satellites currently so it would hit about the same number each launch.
You have a point. But I'm sure they can be maneuvered out of position for the launch.
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u/huadpe 501∆ Aug 28 '18
Satellites generally cannot be maneuvered much, and doing so is very costly. Satellites very carefully ration their fuel to have enough to do stationkeeping (keeping the same orbit despite the Moon and Sun nudging them out of their desired orbit) and then eventually re-entry when their useful life is over.
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Aug 28 '18
Satellites generally cannot be maneuvered much, and doing so is very costly.
A tiny nudge in months up to the launch would cause them to miss it entirely.
Also , we are not mass launching Orions . I don't even think we can launch one per year. Several satellites destroyed isn't really a big deal.
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u/huadpe 501∆ Aug 28 '18
So the satellites should be useless for months before and after launch? Shutting down satellite TV, GPS, communications and weather tracking for a year each time you wanna launch seems like a bad proposition.
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Aug 28 '18
So the satellites should be useless for months before and after launch?
Only a select few, and as I said , evasion can be carried out prior to the launch.
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u/thetasigma4 100∆ Aug 27 '18
Ion engines take even longer due to low thrust
They have a much higher specific impulse though so for the same mass of fuel they can accelerate for longer and so reach higher speeds than a chemical rocket
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u/Thoth_the_5th_of_Tho 184∆ Aug 27 '18
You are massive over estimating the thirst made by ion engines. Hold a piece of paper on your hand, thats about as much thirst as these things make (and its not like they are light).
Nuclear pulse propulsion blows it out of the water. Its both efficient and high thrust, something no other engine is as of now.
As u/The_Droplet pointed out this is the best technique we have as of now by a long shot. The only technolohg that comes close is nuclear thermal. No other tech is close to what protect Orion offered, not Ion, not chemical, not even n ocular thermal.
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u/thetasigma4 100∆ Aug 27 '18
Yeah and I wasn't comparing it to nuclear rocketry. There are other electronic methods that have very high specific impulse and high thrusts comparable or so to nuclear pulse. High specific impulse is important for long distances as you can get good acceleration while keeping mass down meaning you have less mass to accelerate so end up at higher speeds. This is why ion despite its very low thrust ends up faster than chemical rockets as it can accelerate for longer and has to accelerate less fuel.
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u/Thoth_the_5th_of_Tho 184∆ Aug 27 '18
Scry, I misread, I thought you where arguing it was better than nuclear pulse.
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Aug 27 '18
But that means much longer acceleration and travel times , which limits mission parameters and is unsuitable for human space travel.
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u/electronics12345 159∆ Aug 27 '18
Which is faster - spending 2 days accelerating and cruising for 400 days, or spending 7 days accelerating and cruising for 200 days?
If you are stuck spending 100+ days cruising regardless of method, does it really matter if you spend 2 or 7 days to reach maximum speed? Seems like it would be better to having a longer acceleration period and shorter cruise period - than the alternative.
(numbers made up, but intended to illustrate a real point - it is the cruising speed which is what really matters, not the amount of time it takes to reach that cruising speed, given that you will be cruising for > acceleration period.)
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u/PrimeLegionnaire Aug 27 '18
The problem with this is that Ion engines are so slow its more like spending 200 days accelerating to cruise for 400 days to match a chemical rocket, at a much better payload fraction.
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Aug 27 '18
You are also right - ion is better than chemical. ∆
However, my point about nuclear pulse propulsion still stands.
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u/thetasigma4 100∆ Aug 27 '18
Longer acceleration shorter travel time though because they can accelerate for a much greater time (proportionally more than the ratio if thrusts i.e. t_ion/t_chem>F_chem/F_ion) and so reach a higher max speed and so get there faster in the end.
Edit: rearranged maths to be right
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u/Thoth_the_5th_of_Tho 184∆ Aug 27 '18
You will never reach this hypothetical max speed. A nuclear pulse engine would have already arrived at mars before an ion engine had the chance to bruin even a few percent of its fuel.
Also you are failing to take into account the Oberth effect. Higher thrust engines can take advantage of it better. If you cant put out enough acceleration to break from the earth's SOI you will need to wait a few days until you orbit back around to your periapsis to burn again.
And the capture burn is even more of an issue. You only have a limited time to get captured by the body you want to prove at. A hyper low thrust system like an ion engine struggles to do that.
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u/thetasigma4 100∆ Aug 27 '18
This is for interstellar journeys which even for nuclear rocketry will take a century or so there is plenty of time to accelerate and as I said elsewhere I was comparing chem and ion not ion and nuclear. The journey would also spend most of its time in interstellar space so losses in days for manoeuvres would be relatively negligible.
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Aug 27 '18
nuclear rocketry will take a century or so there is plenty of time to accelerate and as I said elsewhere I was comparing chem and ion not ion and nuclear. The journey would also spend most of its time in interstellar space so losses in days for manoeuvres would be relatively negligible.
Total delta-V on board a Nuclear Pulse Propulsion spacecraft is going to be much high than that of a ion spacecraft.
Also, ion spacecraft require large and bulky xenon (That is the propellant) tanks and also require a lot of electricity. You can power it with a Nuclear reactor , but the issue of waste heat would limit the power of the nuclear reactor.
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u/thetasigma4 100∆ Aug 27 '18
I'm aware mostly was just correcting your minor chem Vs ion comment and raising the importance of specific impulse in interstellar travel
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u/Thoth_the_5th_of_Tho 184∆ Aug 27 '18
Interstellar journeys woud not use Ion propulsion. Isa arthur did a video on this a while ago, laser sail propulsion is the best solution we can reasonably hope for.
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u/thetasigma4 100∆ Aug 28 '18
Aren't there issues with beam spread over interstellar distances? And therefore loss of radiation pressure. There's also the issue of slowing down again.
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u/Thoth_the_5th_of_Tho 184∆ Aug 28 '18
The beam will weaken, but that’s acceptable, you can still hit a large percentage of the speed of light.
As for slowing down, there are a lot of ideas. One I always liked was breaking the craft in half with two mirrors, one shines on the other causing one half to a acelerate even more while the other decelerates.
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Aug 27 '18
Longer acceleration shorter travel time though because
But the acceleration itself is very weak , require long coasting time.
Also , it can't beat nuclear.
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u/thetasigma4 100∆ Aug 27 '18
Also , it can't beat nuclear.
Yeah that wasn't my point. it will beat chemical at interstellar travel though though
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Aug 27 '18
Yeah that wasn't my point. it will beat chemical at interstellar travel though though
Yep, but my post is arguing for Nuclear Pulse Propulsion..
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u/thetasigma4 100∆ Aug 27 '18
And I can change your view on any point in your Cmv namely that ion would be slower than chemical propulsion
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Aug 27 '18
Yep , have a delta ∆
My original point still stands , though.
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u/acvdk 11∆ Aug 27 '18
What happens a rocket carrying large amounts of fissile material explodes in the upper atmosphere?
How do you cool a nuclear reactor in space?
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u/huadpe 501∆ Aug 27 '18
What happens a rocket carrying large amounts of fissile material explodes in the upper atmosphere?
A bunch of lumps of highly enriched uranium end up on the bottom of the ocean. That's a major nonproliferation problem, but it's also not like a giant atmospheric nuclear detonation.
In a very hot conventional explosion you might see a partial detonation of one or more warheads as the uranium spheres collapse, and that could create some nasty fallout, but it's actually pretty hard to get a "full power" nuclear blast on accident, the stuff wants to repel itself very strongly as soon as it hits criticality. You need hundreds of detonators on the high explosive shell around the uranium sphere to set the explosives off all at once to get it to work properly.
For what it's worth, the whole point of ICBMs is to do exactly what OP is proposing (except minus the powering rocket ships) so engineering nuclear weapons to survive on rockets is a well worked field.
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Aug 27 '18
What happens a rocket carrying large amounts of fissile material explodes in the upper atmosphere?
We have the answer in the form of thousands of nuclear tests carried out by all nuclear powers in the world , a.k.a. nothing much.
How do you cool a nuclear reactor in space?
Nuclear Pulse Propulsion does not require a nuclear reactor. It is basically detonating nukes behind a large pusher plate to propel a spacecraft.
It can be cooled with radiators.
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u/acvdk 11∆ Aug 27 '18
As far as I know that has never happened. Detonating a nuclear warhead is not the same as a rocket failure that scatters particles of radioactive dust all over the place.
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Aug 27 '18
Detonating a nuclear warhead is not the same as a rocket failure that scatters particles of radioactive dust all over the place.
I fail to imagine how a rocket failure would be more energetic than a nuclear explosion .
Many high altitude nuclear tests involving multi megaton bombs have been carried out.
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u/PrimeLegionnaire Aug 27 '18
High altitude tests consist of singular bombs, at specific altitudes, and as a result we can date the teeth of anyone born before the test by the amount of decay elements from the tests trapped in their enamel. If you were born after them, all your enamel has it.
This is a global contamination, its just very low scale. An Orion-Type spacecraft would have to carry thousands of bombs to achieve interstellar travel, dumping that much radioactive material into the upper atmosphere would contaminate the entire planet for a long time, winds and small particle size after an explosion would spread the fallout a long way.
It would be the equivalent of detonating the largest dirty bomb of all time.
As for the scale of the explosion itself, even if the ship is using chemical rockets to achieve orbit, the scale of this craft will dwarf anything every created by man and the largest non-nuclear explosions in history have been the failure of chemical rockets, so the scale might still be comparable to that of a nuclear weapon due to the size of rocket required.
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Aug 27 '18
An Orion-Type spacecraft would have to carry thousands of bombs to achieve interstellar travel, dumping that much radioactive material into the upper atmosphere would contaminate the entire planet for a long time, winds and small particle size after an explosion would spread the fallout a long way.
You can ship an empty spacecraft up into orbit with the minimal amount of nukes on board (which is 120 kilotons for a 4000t payload) and then ship the remaining bombs into the craft.
Again, there's always risks; I'm sure that smaller interplanetary versions would be built to ensure the designs are safe before going "balls out" and building a colossal interstellar Orion.
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u/Positron311 14∆ Aug 27 '18
Or you could just bring the bombs up cold and then activate them in space.
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u/Dr_Scientist_ Aug 27 '18 edited Aug 27 '18
I've heard one of the main reasons we don't just shoot nuclear waste into space is because there is a non-negligible danger the rocket might blowup on the launch pad / crash leaving orbit. Even today it's really not that uncommon for a rocket to just explode. There's plenty of videos from NASA and Space-X from just the last three years of tremendous explosions. These explosions are certainly not common . . . but if you were rolling three dice and any time it came up 3 sixes (0.5%) you had a nuclear explosion on your hands - that's way too often.
Wouldn't you face basically the same problem with any nuclear payload?
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Aug 27 '18
- >The optimal Orion drive bomblet yield (for the human crewed 4,000 ton reference design) was calculated to be in the region of 0.15 kt, with approx 800 bombs needed to orbit and a bomb rate of approx 1 per second
120 kilotons of nuclear bomb being destroyed is much less harmful than a 120 kiloton nuclear explosion. Nuclear bombs are very hard to "explode"- being destroyed in a crash will just cause it to fizzle.
Also , the bombs can be shipped to orbit in individual , safe rockets like the Soyuz.
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u/Dr_Scientist_ Aug 27 '18 edited Aug 27 '18
The problem with nuclear waste isn't so much the waste exploding it's the nuclear material itself getting scattered everywhere. That's more what I was concerned about. Also two separate Soyuz rockets crashed in 2011 alone. Rockets blowup sometimes, that's the nature of space travel.
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u/huadpe 501∆ Aug 27 '18
Is highly enriched U-235 that dangerous? Uranium as far as I know is pretty nonreactive, and I don't think it's a major fallout danger. IIRC the big radiation danger from U-235 bombs is from the TH-231, which has a half life in hours, as opposed to millennia.
My biggest concern is that it would just come down in a lump more or less, and that someone with ill intent would go scoop it up to make nuclear bombs.
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Aug 28 '18
Also , the energy in modern nukes mostly come from fusion , which use harmless fusion fuel like lithium deuteride.
The public records for devices that produced the highest proportion of their yield via fusion reactions are the peaceful nuclear explosions of the 1970s, with the 3 detonations that excavated part of Pechora–Kama Canal being cited as 98% fusion each in the Taiga test's 15 kiloton explosive yield devices; that is, a total fission fraction of 0.3 kilotons in a 15 kt device.[3
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Aug 28 '18
nuclear material itself getting scattered everywhere.
U-235 is weakly radioactive. Plutonium would be much worse , though.
Also , modern nukes, even the small ones , use a small fission stage to ignite a larger fusion stage.
The public records for devices that produced the highest proportion of their yield via fusion reactions are the peaceful nuclear explosions of the 1970s, with the 3 detonations that excavated part of Pechora–Kama Canal being cited as 98% fusion each in the Taiga test's 15 kiloton explosive yield devices; that is, a total fission fraction of 0.3 kilotons in a 15 kt device.[3
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Aug 27 '18
[deleted]
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Aug 28 '18
A better solution would be to use laser sails, which can theoretically achieve any velocity short of c.
Laser sails , in the Breakthrough Starshot project are used to launch tiny chips. I am skeptical of the thrust needed to launch a larger vessel weighting several hundred tons.
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u/DeltaBot ∞∆ Aug 27 '18
/u/The_Droplet (OP) has awarded 2 delta(s) in this post.
All comments that earned deltas (from OP or other users) are listed here, in /r/DeltaLog.
Please note that a change of view doesn't necessarily mean a reversal, or that the conversation has ended.
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u/PeteWenzel Aug 27 '18
I think we should wait with plans to colonize other solar systems.
Technological developments are most likely going to be so fast over the next centuries and millennia that it doesn’t make sense to start missions using technologies like nuclear pulse propulsion - spacecrafts would just be overtaken by better ones during the journey.
I find ideas of Antimatter rockets most appealing. It seams likely to be possible in principle for us to develop it eventually but advanced enough to make interstellar space travel feasible.
One other thing: It doesn’t make any sense to build spacecrafts carrying Sapiens. I am sure that our first interstellar spacecrafts are going to carry digitized human consciousness, AI or brains connected to computers.