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Most enviromentally friendly Propellant choices


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Say a nuclear space faring civilization does not want to pollute and damage the environment if propellant leaks.

 

This does matter because:

https://www.google.com/amp/s/www.theverge.com/platform/amp/2019/10/14/20913959/rocket-launch-environment-cleanup-soil-water-pollution

 

So what is the most enviromentally friendly propellant in case of a propellant leak? That will get you to orbit with nuclear thermal rockets.... with airbreathing turbojets if necessary.

Water? Hmmm... don't think so.

LH? Maybe? How bad would liquid.hydrogen pollute the soil?

 

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Why would hydrogen pollute the soil? It will quickly evaporate and unless it burns, rise to the higher altitudes and eventually leave the atmosphere altogether.

It would freeze whatever it splashes on, but that's not really pollution.

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Spoiler

Once a pool of liquid hydrogen has frozen and turned into metallic hydrogen, it becomes a metal hydrogen scrap to be removed before somebody slides.

Also imagine a whole crop field covered by metal hydrogen pieces if a liquid hydrogen tank had splashed in midair. Any farmer's tech will crash its wheels and cutters.

 

Edited by kerbiloid
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If you just want to get to orbit, water will do fine.

Assume we're using  DUMBO nuclear thermal engines.

Using water as a propellant, and assuming the Project Rho figures are correct, a 5 ton engine will give you a thrust-to-weight ratio of a little over 71 and an ISP of 410. That T/W appears to be an average - three different DUMBO versions are listed with T/Ws of 20, 55 and 130. But lets roll with 71 for the sake of argument.

Delta-V to LEO is about 9.4 km/s, so lets go with 10 km/s to give ourselves a margin and make the maths easier.

Using the Tsiolkovsky equation and plugging in 410 as the ISP, I get a mass fraction of 11.9. Again, lets give ourselves a bit of a margin and go with 12 which, for example, would give us a 100 ton spacecraft (dry mass) using 1100 tons of water as propellant for a total wet mass of 1200.

[For what it's worth, Wikipedia gives a mass fraction of 10 for SpaceX's Starship and that's made of stainless steel. So a mass fraction of 12 doesn't seem completely unreasonable.]

We equip our spacecraft with four DUMBO engines, for a combined mass of 20 tons which is well within that 100 ton dry mass budget. Each of those engines can push 355 tons, so four can push 1420 tons, giving us a lift-off thrust to weight ratio of 1.4. In other words, enough to get it off the ground at a reasonable pace. :)  Bear in mind that's the 'average' DUMBO too - the numbers get much better if we can assume a T/W of 130.

1100 tons of water needs a 1.1 million litre tank to hold it all which, from a quick bit of Googling, is about half a Space Shuttle external tank or about 80% of a SpaceX Starship. So big but not ridiculously big. 

Assuming that all my napkin maths checks out, this certainly isn't a particularly convenient shuttlecraft for getting an away team down to an uninhabited planet and back but it's a somewhat plausible SSTO for getting that crew from a suitably equipped launchpad to orbit in the first place, if we can assume it's capable of glide returns from orbit. And, to answer the original question, water is a pretty environmentally friendly propellant compared to the alternatives. Not perfect as @RCgothic pointed out but not bad. And its a lot easier to handle than liquid hydrogen!

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And also what exactly is "pollute" and "environmental friendly" in this context ?
"Non-toxic for people" or "requiring to be produced nothing except what is already widely present in wild nature" ?

If the latter, then water and alcohol. Actually, whiskey (vodka won't burn) or moonshine (if DIY).
Some ammonia, too, especially if launch it above the ocean.

Edited by kerbiloid
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3 hours ago, KSK said:

If you just want to get to orbit, water will do fine.

Assume we're using  DUMBO nuclear thermal engines.

Using water as a propellant, and assuming the Project Rho figures are correct, a 5 ton engine will give you a thrust-to-weight ratio of a little over 71 and an ISP of 410. That T/W appears to be an average - three different DUMBO versions are listed with T/Ws of 20, 55 and 130. But lets roll with 71 for the sake of argument.

Delta-V to LEO is about 9.4 km/s, so lets go with 10 km/s to give ourselves a margin and make the maths easier.

Using the Tsiolkovsky equation and plugging in 410 as the ISP, I get a mass fraction of 11.9. Again, lets give ourselves a bit of a margin and go with 12 which, for example, would give us a 100 ton spacecraft (dry mass) using 1100 tons of water as propellant for a total wet mass of 1200.

[For what it's worth, Wikipedia gives a mass fraction of 10 for SpaceX's Starship and that's made of stainless steel. So a mass fraction of 12 doesn't seem completely unreasonable.]

We equip our spacecraft with four DUMBO engines, for a combined mass of 20 tons which is well within that 100 ton dry mass budget. Each of those engines can push 355 tons, so four can push 1420 tons, giving us a lift-off thrust to weight ratio of 1.4. In other words, enough to get it off the ground at a reasonable pace. :)  Bear in mind that's the 'average' DUMBO too - the numbers get much better if we can assume a T/W of 130.

1100 tons of water needs a 1.1 million litre tank to hold it all which, from a quick bit of Googling, is about half a Space Shuttle external tank or about 80% of a SpaceX Starship. So big but not ridiculously big. 

Assuming that all my napkin maths checks out, this certainly isn't a particularly convenient shuttlecraft for getting an away team down to an uninhabited planet and back but it's a somewhat plausible SSTO for getting that crew from a suitably equipped launchpad to orbit in the first place, if we can assume it's capable of glide returns from orbit. And, to answer the original question, water is a pretty environmentally friendly propellant compared to the alternatives. Not perfect as @RCgothic pointed out but not bad. And its a lot easier to handle than liquid hydrogen!

 

Interesting.

So this nicely explains how specifc impulse works.

The harder a rockets 'throws' exhaust the less propellant it takes to to travel a distance. The less hard a rocket throws propellant the more propellant is needed to cover the same distance.

Thrust is a measure of how much mass a rocket is throwing out.

So that is why an AM rocket can travel farther with less propellant than larger rocket with more propellant.

 

Still interesting though.

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Only if you're assuming a constant acceleration drive where you're thrusting for the entire journey. It would be more generally correct to say that the harder a rocket throws propellant, the less propellant it takes to give that rocket a given velocity change.

Think Newton's 3rd law. Any change in momentum of a rocket requires an equal and opposite momentum created by throwing propellant out the back of that rocket. Since momentum is mass x velocity, the faster you can throw that propellant the less mass of propellant you need.

Formally, specific impulse is the total impulse (or change of momentum - Newton's 2nd law*) generated per unit of propellant consumed, and it's proportional to exhaust velocity.

For high specific impulse you want the molar mass of your propellant to be as low as possible. I always picture this as a rocket engine being able to give a molecule of propellant a maximum amount of kinetic energy depending how hot that engine can run. Since kinetic energy = 1/2mv2, the lighter the molecule (or atom, depending on exhaust temperature), the faster it goes. That's probably not a very elegant way of thinking about it though.

Either way, hydrogen has the lowest possible molar mass, so it makes the best propellant considered purely in terms of specific impulse. It's not necessarily the most practical propellant for a given application though, since it also has a low density and high boil-off rate, so needs very big, extremely well insulated tanks, particularly for long-duration deep space missions.

 

*Edit

Impulse = force x time or Ft
change in momentum = m(v1-v0) where v1 and v0 are your final and initial velocities respectively.
Therefore:    Ft = m(v1-v0)
Therefore:    F = m(v1-v0)/t

Since (v1-v0)/t is simply acceleration (or a), we can rewrite the above as  F=ma, which is maybe a more familiar equation for Newton's 2nd law).

 

 

Edited by KSK
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6 hours ago, KSK said:

If you just want to get to orbit, water will do fine.

Assume we're using  DUMBO nuclear thermal engines.

Using water as a propellant, and assuming the Project Rho figures are correct, a 5 ton engine will give you a thrust-to-weight ratio of a little over 71 and an ISP of 410. That T/W appears to be an average - three different DUMBO versions are listed with T/Ws of 20, 55 and 130. But lets roll with 71 for the sake of argument.

From that link it appears that DUMBO uses liquid hydrogen - which would imply a higher T/W when using water. 130 may be much more realistic in that sense.

However the 410 ISP may be a bit high. Nuclear Thermal generally operates at lower temperatures than chemical rockets - so the ISP will be lower as well than a comparable rocket with water exhaust - that is the performance may be closer to kerolox. 

But perhaps a LANTR DUMBO system would work where the oxygen/hydrogen mix is changed as it gains altitude and velocity - losing thrust but gaining ISP. All of the LOx is used up like a first stage, and then it continues with just hydrogen past some point. So really good engine T/W for launch and the early ascent where fighting gravity is the most important aspect and then good T/W but really high ISP for when getting up to orbital speed is the priority.

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1 hour ago, Bill Phil said:

From that link it appears that DUMBO uses liquid hydrogen - which would imply a higher T/W when using water. 130 may be much more realistic in that sense.

However the 410 ISP may be a bit high. Nuclear Thermal generally operates at lower temperatures than chemical rockets - so the ISP will be lower as well than a comparable rocket with water exhaust - that is the performance may be closer to kerolox. 

But perhaps a LANTR DUMBO system would work where the oxygen/hydrogen mix is changed as it gains altitude and velocity - losing thrust but gaining ISP. All of the LOx is used up like a first stage, and then it continues with just hydrogen past some point. So really good engine T/W for launch and the early ascent where fighting gravity is the most important aspect and then good T/W but really high ISP for when getting up to orbital speed is the priority.

Not sure about the details but there's a table further up the page with various engines and (where applicable) different propellants used in the same engine type. Fairly sure that the 410 was for DUMBO with water propellant but I may have misread. 

Edit.  A LANTR-DUMBO sounds very interesting, although I do like the simplicity of running your nuclear rocket on plain tap water!

Edited by KSK
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2 minutes ago, KSK said:

Not sure about the details but there's a table further up the page with various engines and (where applicable) different propellants used in the same engine type. Fairly sure that the 410 was for DUMBO with water propellant but I may have misread. 

It's a table with ISPs for different propellants assuming 3200 Kelvin.

410 was for water.

I'm not sure how hot DUMBO's core is but nonetheless the thrust would be higher if you were to use water.

Though methane actually has good ISP and would also have higher thrust to weight than LH2. Approximately 600 seconds at 3200 K and higher molecular weight. So higher specific impulse without much sacrifice for thrust. 

Though it's probably less environmentally friendly. A specific ratio of LOx to LH2 can probably achieve the same performance.

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1 hour ago, RoninFrog said:

Most environmentally friendly?  Nuclear pulse, duh!

Can't polute an environment that isn't there anymore Forehead :D

On a more serious note, you need to consider not just the propellant itself, but what the exhaust products will be, how they can react to the air, and how the air responds to the intense localized heating of an engine.

Regardless of the propellant, if it's hot enough, some of the surrounding air will be converted into the nitrogen oxides NO2, NO, and N2O, which are all powerful greenhouse gasses (N2O is ~15x worse than methane and 300 times worse than CO2) and are toxic. As these are also created by natural biological and geophysical processes, they aren't too big of an issue, and there really isn't anything that can be down about this while still using a rocket powered launch vehicle anyways.

  • Kerlox, Methalox, and Hydrolox all produce water as an exhaust product, with kerolox and methalox also producing some CO and CO2. Carbon monoxide is only ~0.6 times as potent of a GHG than carbon dioxide, but is toxic. Water is only ~0.4 times as potent of a GHG as CO2 (but contributes the majority of Earth's greenhouse heating because of its atmospheric abundance) and its clouds are reflective to create a small cooling effect, so is pretty mundane. Some unburnt RP1, or methane could be problematic though.
  • Alumilox (Aluminium + LOX) doesn't produce water or CO2, but does produce a fine Alumina dust that could potentially be harmful.
  • Solids, Hybrids, and Hypergolics are all less efficient than the big three cryo fuels, and will produce more toxic fumes to various extents.
  • Fluorine or Beryllium based fuels can get higher isps than hydrolox, and even rivaling some nuclear rockets, but both are pretty nasty substances to work with and would be quite bad for the environment.

As for nuclear propellants: H2 is probably the most environmentally friendly; maximizing ISP while exhausting a gas with negligible GHG contributions (on par with O2). Some hydrogen may react with nitrogen and/or oxygen in the air to produce water and ammonia, which are greenhouse gasses, though in only small amounts. Water, as mentioned is one of the cleaner GHGs. Ammonia however, while very short lived (breaking down in a few hours or days), is nearly as potent a GHG as methane, and is pretty smelly. It is naturally produced by many organisms though, so can be tolerated by the environment to a degree.

Helium could also be a good nuclear propellant, though there are issues with supply. Methane is a non-starter for a nuclear propellant, dissociating into H2 and soot in the reactor, which could be problematic for the reactor, or be exhuasted out and enter the atmosphere, neither of which are very good. Ammonia is probably a better option. Although is has a slightly lower ISP than methane does, it breaks down into inert N2 and H2, which will also be handled by the reactor better. Solid core reactors aren't hot enough to cause water to dissociate, but higher power liquid, colloid, droplet, vapor, gas, and plasma cores would be. The O2, H2, and H2O exhausted by such a rocket are non-problematic.

Bipropellant mixtures like Hydrolox can also be used in nuclear rockets, but give lower ISPs in favor of higher TWRs. This may be useful in less developed NTRs, where TWR can be rather lacking. More advanced designs can get high enough TWRs and ISPs to be useful without bipropellants.

As for what type of nuclear thermal rocket, it depends. A closed cycle is best, as no nuclear fuel can escape, but has reduced performance over open cycles, making them more difficult. Hotter engines also get better performance, but are also harder to contain. Solid cores don't get hot enough to take a performance hit from a closed cycle (other than TWR), so these types kind of blend together, especially since solid fissile materials can't be exhausted out unless something Really Bad happens. Liquid, Colloid, and Droplet cores get hot enough for open cycles to be problematic, but not hot enough for closed cycles to be much better than a simpler solid core. Vapor and Gas cores do get hot enough where closed cycles start to significantly out performing solid cores, but there are issues designing close cycles with current material science (especially the hotter gas cores). Plasma core are simply too hot to contain in any meaningful way, and despite magnetic confinement being a plausible option, it could still allow for some fissile losses during operation.

All nuclear rockets have to worry about the potential environmental impact of a failure occurring, which would be significantly worse than the failure of any chemical rocket. When operating as intended would have a lower impact.

For comparison of closed cycle options (assuming H2 as the propellant. Keep in mind that any other propellant would give worse performance):

  • Solid core NTR: ISP 800-1200 s, TWR 0.8-40
  • Vapor core NTR: ISP 1100-2000 s, TWR 0.5-10
  • Gas core NTR: ISP 1300-2800 s, TWR 2-15
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Any propellant first needs to be produced.

Say, hydrogen is usually produced out of methane. This requires chemicals to purify it, and produces various by-products, especially carbon dioxide (because you have to remove carbon from the methane).
Also it requires fuel to heat this kitchen, so usually the methane burning (and producing additional dioxide).
So the chemically produced, such "environmentally friendly", hydrogen pollutes atmosphere much greater than if you just use that methane directly.

If produce the hydrogen electrolytically, you need a lot of electric energy, so either need the same methane to run your powerplant, or nuclear fuel, or a lot of toxic chemicals to process the solar panels (and later utilize them, turning into same carbon dioxide).

So, any fuel except the ones who are naturally pre-existing in wild nature, is just hiding crap from under the table to under the bed.

***

So. let me still insist on whiskey and oxygen balloon.

Edited by kerbiloid
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1 hour ago, kerbiloid said:

Say, hydrogen is usually produced out of methane. This requires chemicals to purify it, and produces various by-products, especially carbon dioxide (because you have to remove carbon from the methane).
Also it requires fuel to heat this kitchen, so usually the methane burning (and producing additional dioxide).
So the chemically produced, such "environmentally friendly", hydrogen pollutes atmosphere much greater than if you just use that methane directly.

If produce the hydrogen electrolytically, you need a lot of electric energy, so either need the same methane to run your powerplant, or nuclear fuel, or a lot of toxic chemicals to process the solar panels (and later utilize them, turning into same carbon dioxide).

Introducing the Kværner process, which converts natural gas or biogas directly into nearly pure carbon and hydrogen. A carbon neutral biogas powered and biogas fueled system could be implemented relatively easily by any eco-conscious future civilization if so inclined. As a bonus, the carbon could be used in ceramics, composites, or high strength carbon allotropes like carbon nanotubes and graphene. So technically it would be a carbon sink, rather than simply carbon neutral.

Much more broadly speaking, an eco-conscious space fairing future civilization could migrate all power, manufacturing, and living space off of Earth to preserve its environment. As I mentioned previously, though in a less serious way, you can't pollute an environment that isn't there. Because of this, any environmental impact caused by the manufacturing of any of the rocket's systems (fuel, structure, power, electronics) could be disregarded entirely.

 

Edited by wafflemoder
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17 minutes ago, wafflemoder said:

the Kværner process, which converts natural gas or biogas directly into nearly pure carbon and hydrogen.

Quote

The endothermic reaction separates (i.e. decomposes) hydrocarbons into carbon and hydrogen in a plasma burner at around 1600 °C.

So, you need a powerplant for the plasma burner, and it requires more energy than CH4 bounds contain,

So, this process just moves the pollution from the hydrogen plant to the power plant.

17 minutes ago, wafflemoder said:

Much more broadly speaking, an eco-conscious space fairing future civilization could migrate all power, manufacturing, and living space off of Earth to preserve its environment.

Then it will be delivering additional energy to the Earth, breaking its heat balance.
(Unless you plan to abandon the Earth, as the "living space" is probably where people live.)

And we should remember that the Earth is the only known place in the Universe with significant sedimental and hydrothermal ore deposits.
So, you should mine on the Earth, and produce goods also here.
Any agriculture is also Earth-based. At least for gravity.

Edited by kerbiloid
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3 minutes ago, kerbiloid said:

So, you need a powerplant for the plasma burner, and it requires more energy than CH4 bounds contain,

So, this process just moves the pollution from the hydrogen plant to the power plant.

Which is why I suggested running it off of a biogas powerplant rather than natural gas. Also its abilty to sequester carbon in elemental can reduce, or even give it a negative carbon footprint.

Interestingly, the Kværner process is very similar (and possibly identical) to what happens in a nuclear rocket when you use methane as a propellant, only the carbon soot builds up in the engine, which can cause blockages or affect the neutron moderation in the reactor, and isn't desirable in that situation.

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Just now, wafflemoder said:

hich is why I suggested running it off of a biogas powerplant rather than natural gas.

You anyway need to spend more energy by burning the biogas rather thatn you can store in the separated hydrogen.
And the "biogas" is just methane and methanol, just biologically produced.
It produces same amount of carbon dioxide, so it's just a way to save money on mining.

Tthe Kværner process is just another way to apply external energy to the molecules of methane rather than electrolysis.

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17 minutes ago, kerbiloid said:

You anyway need to spend more energy by burning the biogas rather thatn you can store in the separated hydrogen.
And the "biogas" is just methane and methanol, just biologically produced.
It produces same amount of carbon dioxide, so it's just a way to save money on mining.

Tthe Kværner process is just another way to apply external energy to the molecules of methane rather than electrolysis.

First off, if a civilization's goal is to preserve the environment, monetary cost will be a non-issue. And biogas isn't even cheaper than minning, thats why we still mine.

Secondly, Biogas is biologically produced methane and methanol, as are all fossil fuels. What makes biofuels "cleaner" than petroleum fuels isn't that they're a different chemical, its that biofuels are made from the carbon already in our atmosphere, rather than carbon which has been stored in the ground for hundreds of millions of years. Burning biofuels simply returns the CO2 that was used to make it back into the atmosphere, whereas burning natural gas adds more in.

Third, the Kværner process is another way of turning methane into hydrogen. The difference is the waste product. Steam reforming creates ten times as much CO2 as it does hydrogen. The Kværner process produces no CO2 as a waste product, and so less greenhouse gasses. The kværner process instead creates carbon dust, which can be stored and removed from the carbon cycle.

When taken as a whole: CO2 is taken from the air by plants to produce biomass. Some of this biomass is burnt, generating power and turning it back into CO2. The rest is put through the kværner process (using the power generated from before) and converted into hydrogen and carbon. The carbon is not returned to the air, so there is an overall net decrease in atmospheric CO2.

Edited by wafflemoder
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20 minutes ago, wafflemoder said:

if a civilization's goal is to preserve the environment, monetary cost will be a non-issue.

The environment preservation will stop very soon without monetary supply.
The money is just a redistribution of stored energy (and information, a little).
Only balanced solutions can survive.

20 minutes ago, wafflemoder said:

Biogas is biologically produced methane and methanol, as are all fossil fuels. What makes biofuels "cleaner" than petroleum fuels

All fossil fuels are by definition stored biofuel produced millions years ago.
The nowadays biofuel hype is just an attempt to DIY this process, exhausting what has already been exhausted, absorbed, and buried under rocks.

So, no modern biofuel process can be more environmentally friendly than the fossil fuels are. Because those are already partially post-processed, while these are raw.
The only advantage of the biofuel besides  the absence of mining industry is that they release as much carbon dioxide as they store on production.
But this just means that you need to produce more by supporting infrastructure which allows the biofuel farms operate. Say, at least you need to clean the rot gas and utilize the unreacted remains. This means chemicals (usually ammonium-based, like diethanolamine) and fuel to run the purifiers.
Once the biofuel production reaches the productivity comparable to the fossil energetics, there will be much more environmental problems with it. Currently we are just parasiting on these process which were taking place long before the human appeared.

20 minutes ago, wafflemoder said:

Third, the Kværner process is another way of turning methane into hydrogen. The difference is the waste product. Steam reforming creates ten times as much CO2 as it does hydrogen.

I was comparing it to the electrolysis of water. In both cases you need a powerplant, an amount of energy greater than the methane or water inner bounds have, and a way to deliver this energy from the power plant to the unmethanizer/electrolizer.
Just in one case on the final phase you use electrolysis, in another case - plasma heating. In one case you spend water (with no carbon at all), in another case - methane.

Edited by kerbiloid
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for launchers i tend to make the case leave the reactor on the ground, make hydrolox. you can always build bigger rockets. especially large multi-stage reusable rockets. once in space all bets are off and you can use whatever you want from the nastiest of hypergolics to the cleanest fusion drives. though you might have a nuclear exclusion zone for really nasty drives like fission fragment and orion (for the latter we might be talking in a solar orbit no closer than mars). 

 

actually im curious how far actual exclusion zones would be for various drives. not just to keep fission products from entering the atmo, but also to protect satellites. i dont think there has ever been a study as no one has ever flown a dirty drive in space to the best of my knowledge.

Edited by Nuke
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1 hour ago, Nuke said:

for launchers i tend to make the case leave the reactor on the ground, make hydrolox. you can always build bigger rockets. especially large multi-stage reusable rockets. once in space all bets are off and you can use whatever you want from the nastiest of hypergolics to the cleanest fusion drives. though you might have a nuclear exclusion zone for really nasty drives like fission fragment and orion (for the latter we might be talking in a solar orbit no closer than mars). 

 

actually im curious how far actual exclusion zones would be for various drives. not just to keep fission products from entering the atmo, but also to protect satellites. i dont think there has ever been a study as no one has ever flown a dirty drive in space to the best of my knowledge.

Starfish Prime messed with a bunch of satellites but I think it wouldn’t be impossible to harden future satellites against such a thing - robust in-space infrastructure would make deploying higher mass satellites easier.

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5 hours ago, Bill Phil said:

Starfish Prime messed with a bunch of satellites but I think it wouldn’t be impossible to harden future satellites against such a thing - robust in-space infrastructure would make deploying higher mass satellites easier.

still it would suck to lose your slowboat ion probe because someone with a bigger budget than you fired up their torch drive nearby. 

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