How safe is fusion rocket exhaust in the air?

[Spacescifi]

 

Fusion is seen like the holy grail for rocket scientists. With it SSTO scram jets would be easier to propel (although we could probably do it with a fission reactor spewing lethal exhaust).

The question is, if we ever managed to make a fusion powered rocket with a sustained fusion reaction, would the exhaust be relatively safe in the air? Or would it spew radioactive cancer death like nuclear thermal rockets?

Bonus question: Antimatter propellant combustion rocket exhaust? Safe in the air or not? I think not, since gamma rays. Unless you know of a way to make it safe?

 

What do you know and say about all this?

[kerbiloid]

Which one?

Spoiler

Fusion Reactor Fuel Modes

Fuel Mode	Reactors Types	Tech Requirement	Reactor Power	Reaction Energy	Reaction Rate	Power Requirement Multiplier Neutral Plasma / Non Neutral	Fuel	Products	Charged Particles	Neutron Energy Ratio
D-T	MCF / MIF	Fusion Power	1	1	1	1x	Deteurium + Tritium	Helium4	20%	80%
Cold D-D	MCF	Fusion Power	0.3537	0.7074	0.5	0.9x	Deteurium	Helium4 + Helium3		66.5%
D-He3	MCF	Advanced Fusion	0,884	1.04	0.85	3x	Deteurium + Helium3	Helium4 +Hydrogen	95%	5%
T-T	MCF / MIF	Advanced Fusion	0.5457	0.642	0.85	3x	Tritium	Helium4	20%	80%
Full D-D	MCF / MIF	Advanced Fusion	0.6135	1.227	0.5	3x	Deteurium	Helium4	41.8%	58.2%
p-B11	CBF	Exotic Fusion	0.3952	0.494	0.8	3x	Hydrogen + Boron	Helium4	99.9%	0.01%
Hot D-D	MCF	Exotic Fusion	0.3635	0.727	0.5	6x	Deteurium	Helium4 + Tritium		10%
Spin polarized He3-D	MCF	Exotic Fusion	0.8424	0.936	0.9	6x	Deteurium + Helium3	Helium4 + Hydrogen	98.5%	1.5%
He3-He3	MCF/CBF	Ultra Dense Fusion	0.551	0.73	0.7	6x	LqdHe3	Helium4 + Hydrogen	100%	0%
D-Li6	MCF	Ultra Dense Fusion	0.889	1.27	0.7	8x	Deteurium + Lithium6	Helium4	97.5%	2.5%
He3-Li6	MCF/CBF	Ultra Dense Fusion	0.672	0.96	0.7	8x	Helium3 + Lithium6	Helium4 + Hydrogen	100%	0%
p-D	MCF	Unified Field Theory	0.54212	1.3553	0.4	8x	Hydrogen + Deteurium	Helium4 + Hydrogen	99%	1%
p-Li7	MCF/CBF	Unified Field Theory	0.6839	0.977	0.7	8x	Hydrogen + Lithium	Helium4	99.9%	0.1%
Li6 Fusion Cycle	MCF	Unified Field Theory	0.5344	1.1875	0.45	10x	Lithium6	Helium4	99.9%	0.1%
p-Li6	MCF/CBF	Unified Field Theory	0.154	0.22	0.6	12x	Hydrogen +  Lithium6	Helium4 + Helium3	99.9%	0.1%
p-N15	CBF	Ultra High Energy Physics	0.1704	0.284	0.5	12x	Hydrogen + Nitrogen15	Helium4  + Carbon	99.9%	0.1%
p-O18	CBF	Ultra High Energy Physics	0.1363	0.227	0.5	12x	Hydrogen + Oxygen18	Nitrogen15 + Helium4	99.9%	0.1%
p-p	CBF	Ultra High Energy Physics	0.0239	0.0588	2.461	10x	Hydrogen	Deteurium	99.9%	0.1%

* MCF = magnetic confinement Fusion, MIF = Magnetic Inertial Fusion  CBF  = Colliding beam Fusion reactor   

** = not implemented yet.

This is an overview off all fuel modes and there effects on performance

Non Fusion Reactor Fuel Modes

This is an overview off all fuel modes and there effects on performance

Reactor Fuel Modes

Fuel Mode	Type	Reactors	Tech Requirement	Core Temp Modifier	Reaction Energy	Fuel Efficiency	Fuel	Products	Charged Particles	Brems-strahlung	Neutron Energy Ratio
Uranium Oxide	Fission	NERVA / JUMBO	Nuclear Propulsion	100%	1	85%	EnrichedUranium	DepletedUranium **	0	n.a	2%
Uranium Hexafloride	Fission	Molten Salt / Gas Core	Nuclear Power	100%	1	15%	UF6	94% DepletedFuel + 6% Xenon	0	n.a	2%
Uranium Fuel Cycle **	Fission	Molten Salt	Nuclear Fuel Systems	80%	0.8	80%	UF6	80%DepletedFuel + 10%Plutonium 10%DepletedUranium	0	n.a	2%
MOX Plutonium Burnup **	Fission	Molten Salt	Nuclear Fuel Systems	115%	0.9%	30%	7%Plutonium+ 93%Anticides	DepletedFuel	0	n.a	1%
Thorium	Fission	Molten Salt	Nuclear Power	138%	1.38	15%	ThoriumTetraflouride	Anticides	0	n.a	2%
Thorium Fuel Cycle **	Fission	Molten Salt	Nuclear Fuel Systems	69%	0.69	99%	ThoriumTetraflouride + Anticides	96%DepletedFuel + 2%Anticides + 2%Plutonium	0	n.a	2%
Uranium Nitride Pellet	Fission	Pebble Bed	Nuclear Fuel Systems	100%	n.a.	5%	UraniumNitride	DepletedFuel	0	n.a	2%
Uranium Nitride Nanoparticle	Fission	Dusty Plasma	High Energy Nuclear Power	100%	n.a.	97%	UraniumNitride	DepletedFuel	83.5% * 0.46	n.a	2%
Microfusion	Fussion-Fision Hybid	AIM	Exotic Fusion Reactions	100%	1	94%	LqdDeteurium + LqdHe3 & UraniumNitride +AntiMatter	Helium4 + Hydrogen + DepletedFuel	95%	n.a.	5%
AntiMatter	AntiMatter	Antimatter	Antimatter Power	100%	1	22%	AntiMatter	none	80%	20%	n.a

* MCF = magnetic confinement Fusion, MIF = Magnetic Inertial Fusion  CBF  = Coliding beam Fusion reactor   

** = not implemented yet.

 

[Spacescifi]

22 minutes ago, kerbiloid said:

Which one?

  Reveal hidden contents

Fusion Reactor Fuel Modes

Fuel Mode	Reactors Types	Tech Requirement	Reactor Power	Reaction Energy	Reaction Rate	Power Requirement Multiplier Neutral Plasma / Non Neutral	Fuel	Products	Charged Particles	Neutron Energy Ratio
D-T	MCF / MIF	Fusion Power	1	1	1	1x	Deteurium + Tritium	Helium4	20%	80%
Cold D-D	MCF	Fusion Power	0.3537	0.7074	0.5	0.9x	Deteurium	Helium4 + Helium3		66.5%
D-He3	MCF	Advanced Fusion	0,884	1.04	0.85	3x	Deteurium + Helium3	Helium4 +Hydrogen	95%	5%
T-T	MCF / MIF	Advanced Fusion	0.5457	0.642	0.85	3x	Tritium	Helium4	20%	80%
Full D-D	MCF / MIF	Advanced Fusion	0.6135	1.227	0.5	3x	Deteurium	Helium4	41.8%	58.2%
p-B11	CBF	Exotic Fusion	0.3952	0.494	0.8	3x	Hydrogen + Boron	Helium4	99.9%	0.01%
Hot D-D	MCF	Exotic Fusion	0.3635	0.727	0.5	6x	Deteurium	Helium4 + Tritium		10%
Spin polarized He3-D	MCF	Exotic Fusion	0.8424	0.936	0.9	6x	Deteurium + Helium3	Helium4 + Hydrogen	98.5%	1.5%
He3-He3	MCF/CBF	Ultra Dense Fusion	0.551	0.73	0.7	6x	LqdHe3	Helium4 + Hydrogen	100%	0%
D-Li6	MCF	Ultra Dense Fusion	0.889	1.27	0.7	8x	Deteurium + Lithium6	Helium4	97.5%	2.5%
He3-Li6	MCF/CBF	Ultra Dense Fusion	0.672	0.96	0.7	8x	Helium3 + Lithium6	Helium4 + Hydrogen	100%	0%
p-D	MCF	Unified Field Theory	0.54212	1.3553	0.4	8x	Hydrogen + Deteurium	Helium4 + Hydrogen	99%	1%
p-Li7	MCF/CBF	Unified Field Theory	0.6839	0.977	0.7	8x	Hydrogen + Lithium	Helium4	99.9%	0.1%
Li6 Fusion Cycle	MCF	Unified Field Theory	0.5344	1.1875	0.45	10x	Lithium6	Helium4	99.9%	0.1%
p-Li6	MCF/CBF	Unified Field Theory	0.154	0.22	0.6	12x	Hydrogen +  Lithium6	Helium4 + Helium3	99.9%	0.1%
p-N15	CBF	Ultra High Energy Physics	0.1704	0.284	0.5	12x	Hydrogen + Nitrogen15	Helium4  + Carbon	99.9%	0.1%
p-O18	CBF	Ultra High Energy Physics	0.1363	0.227	0.5	12x	Hydrogen + Oxygen18	Nitrogen15 + Helium4	99.9%	0.1%
p-p	CBF	Ultra High Energy Physics	0.0239	0.0588	2.461	10x	Hydrogen	Deteurium	99.9%	0.1%

* MCF = magnetic confinement Fusion, MIF = Magnetic Inertial Fusion  CBF  = Colliding beam Fusion reactor   

** = not implemented yet.

This is an overview off all fuel modes and there effects on performance

Non Fusion Reactor Fuel Modes

This is an overview off all fuel modes and there effects on performance

Reactor Fuel Modes

Fuel Mode	Type	Reactors	Tech Requirement	Core Temp Modifier	Reaction Energy	Fuel Efficiency	Fuel	Products	Charged Particles	Brems-strahlung	Neutron Energy Ratio
Uranium Oxide	Fission	NERVA / JUMBO	Nuclear Propulsion	100%	1	85%	EnrichedUranium	DepletedUranium **	0	n.a	2%
Uranium Hexafloride	Fission	Molten Salt / Gas Core	Nuclear Power	100%	1	15%	UF6	94% DepletedFuel + 6% Xenon	0	n.a	2%
Uranium Fuel Cycle **	Fission	Molten Salt	Nuclear Fuel Systems	80%	0.8	80%	UF6	80%DepletedFuel + 10%Plutonium 10%DepletedUranium	0	n.a	2%
MOX Plutonium Burnup **	Fission	Molten Salt	Nuclear Fuel Systems	115%	0.9%	30%	7%Plutonium+ 93%Anticides	DepletedFuel	0	n.a	1%
Thorium	Fission	Molten Salt	Nuclear Power	138%	1.38	15%	ThoriumTetraflouride	Anticides	0	n.a	2%
Thorium Fuel Cycle **	Fission	Molten Salt	Nuclear Fuel Systems	69%	0.69	99%	ThoriumTetraflouride + Anticides	96%DepletedFuel + 2%Anticides + 2%Plutonium	0	n.a	2%
Uranium Nitride Pellet	Fission	Pebble Bed	Nuclear Fuel Systems	100%	n.a.	5%	UraniumNitride	DepletedFuel	0	n.a	2%
Uranium Nitride Nanoparticle	Fission	Dusty Plasma	High Energy Nuclear Power	100%	n.a.	97%	UraniumNitride	DepletedFuel	83.5% * 0.46	n.a	2%
Microfusion	Fussion-Fision Hybid	AIM	Exotic Fusion Reactions	100%	1	94%	LqdDeteurium + LqdHe3 & UraniumNitride +AntiMatter	Helium4 + Hydrogen + DepletedFuel	95%	n.a.	5%
AntiMatter	AntiMatter	Antimatter	Antimatter Power	100%	1	22%	AntiMatter	none	80%	20%	n.a

* MCF = magnetic confinement Fusion, MIF = Magnetic Inertial Fusion  CBF  = Coliding beam Fusion reactor   

** = not implemented yet.

 

 

A thermo-fusion plasma rocket would be the one

 

The idea is to use free air as propellant by shunting it over the fusion reaction, the use rocket fuel once the air thins out. Again shunting ot over the reactor for greater thrust.

[satnet]

You could probably design a system that was safe. For fusion the exhaust isn't really a problem being mostly helium, though some reactions do produce radioactive atoms like tritium. You probably don't want a fusion torch drive because of the electromagnetic radiation (particularly UV and X-rays), but one where the reaction is contained should be something that can be safe (assuming you can afford the weight of a reasonable amount of shielding). All of this assumes of course that net-positive fusion can be miniaturized once we work out how to achieve it. Tokamak's that might achieve net-positive fusion are building sized (ITER), so that design is probably out. There are reactor designs that are smaller like the Lockheed Martin Compact Fusion Reactor, though I'm a little dubious being such a far departure from existing efforts. You do have the problem of neutron activation of the reactor material itself, though most proponents consider that manageable with the right materials. Assuming you aren't using tritium or another naturally radioactive fuel you don't have the problem of raining radioactive fallout in the case of a RUD event. The challenges for a fusion based in atmosphere rocket are going to be public perception and getting the size/weight down to the point where it is practical.

Even nuclear fission can avoid a radioactive exhaust with the right design. The problem there is you if the rocket explodes you now have an uncontrolled nuclear material falling from the sky. You also can't really turn it off (just slow it down) which means you need to be cooling it all the time. This is another advantage of fusion since it can be shut down when not in use.

[kerbiloid]

Another question: whether there is a reason for a reactor on every vessel.
Or maybe a power transmission is much easier and much safer to implement.

(Microwave and laser transmitters/receivers in the same mod above)

Edited May 26, 2019 by kerbiloid

[Xd the great]

Probably the biggest concern would be radiation leakage, X-ray and stuff.

Or formation of NOx...

[Nuke]

if you can go aneutronic you wouldn't have to worry about neutrons and charged particles are very easy to accelerate or slow down for direct conversion to electricity. you would probably want to do one or the other. in space it would be beneficial to spew the high energy alpha particles out the tail pipe as you would likely get more bang for your buck that generating power to run an ion cluster to do the same job the reactor is already doing. having the option to switch modes or to partition the output between power and propulsion would be good to have. if you have an isru capability the ability to pick up arbitrary remass and using it in ion engines or arcjets then you need power for that should your high isp/low mass fusion products be insufficient thrust. you can also have a lot of power for launch systems like first stage electric ducted fans, electric turbo pumps for chem engines and arcjet enhanced engines. of course you could argue that the reactors place is on the ground making carbon neutral cryofuel depending on the size of your reactor anyway. 

[farmerben]

Supposing you did drop x-rays and neutrons into the atmosphere.  The radiation would be scattered by atoms in the atmosphere.  

Scattering of neutrons is basically moderating them, until eventually they are absorbed.  This could create some tritium, carbon14, and maybe some radioactive argon.  Most of the other elements in the atmosphere are either totally stable or totally unstable.   A question that can be solved by experiments, is how much radioactive air the craft makes compared to the normal solar wind.  There are also chemical reaction in the air, again basically the same ones that occur naturally due to solar wind.  It would take a lot to bring the carbon14 ratio back to where it used to be before we started burning depleted carbon.  

X-ray scattering would also happen in the air.  At first approximation, 1 km of sea level air has a similar shielding as 1 m of water.  Whatever shielding the spacecraft has, can be compared to a couple of kilometers of air behind.  

 

I wouldn't call it "safe".  But it is millions of times safer than project Orion.  Freeman Dyson esimated that Orion would cause between 1-10 cancers per launch in the most optimistic scenario.  If it were down to 1/100 of a delayed cancer he would have approved of the project.  But that was impossible.  Fission products are nasty stuff for the human body.  

Under a fusion scenario, you would need a safety exclusion zone.  But people could probably re enter the exclusion zone within days and detect hardly anything.   

 

 

 

[farmerben]

If you can throw deuterons at 2% c, then you could use carbon, nitrogen, and oxygen in the atmosphere for fusion.  

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