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I'm really in doubt if a fusion thruster could be used inside a planet (populated places). As i saw in the TV show "The Expanse", their fusion engines are really, really hot, hotter than the surface of the Sun and they can't use it close to stations and on planets. So i ask: in a VTOL spaceship that uses fusion as a way of traveling, would its fusion thrusters and engines be prohibited from being used on a planet? Aside from the main engines that can be changed to atmospheric mode, what about its thrusters? Chemical reaction thrusters aren't efficient because it uses too much propellant. How do i solve this dilemma?

How does the Tempest doesn't destroy everything with its million-degree thrusters??

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    $\begingroup$ Because everything it might touch is thermally inert? $\endgroup$ – Pᴀᴜʟsᴛᴇʀ2 Oct 22 '17 at 23:47
  • $\begingroup$ @Pᴀᴜʟsᴛᴇʀ2 Except for the air.... Which would then absorb ALL of the thermal energy. $\endgroup$ – Andon Oct 22 '17 at 23:49
  • $\begingroup$ @Andon - Sorry, that was meant a little tongue-in-cheek, playing off the other question. :o) $\endgroup$ – Pᴀᴜʟsᴛᴇʀ2 Oct 22 '17 at 23:57
  • $\begingroup$ Depends fusion thrusters typically have a specific impulse of 130,000 seconds compared to chemical of 450s. Specific impulse is the measure of 1 pound of thrust from 1 pound of fuel for a certain duration in seconds. To hover you can redirect the airway to allow atmospheric air to go around your fusion core and let it go out in a controlled manner, I repeats do not engage fusion thrusters! $\endgroup$ – user6760 Oct 23 '17 at 0:13
  • $\begingroup$ @Pᴀᴜʟsᴛᴇʀ2 I figured as much. It's just amusing, to me, how much that would backfire. $\endgroup$ – Andon Oct 23 '17 at 2:36
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Really hot is not as big a deal if it is really small.

Consider the experimental tokamak fusion reactors. They are real. They are hotter than one million degrees. They are sitting on the earth. How?

from https://www.euro-fusion.org/faq/new-what-is-the-temperature-generated-in-a-tokamak-reactor-how-can-the-inner-wall-material-resist-that-temperature/

In order for fusion to occur in the very hot gas – or plasma – we create inside JET, the plasma must be heated to temperatures in excess of 150 million degrees Celsius. In order to achieve this, the plasma is actively held away from the walls of the tokamak container by using powerful magnetic fields.

However, it does sometimes touch the walls: what material could withstand that temperature? The key here is that there is only a very small amount of plasma there( ~0.1 g). So although it is exceptionally hot, this is counteracted by the very small amount, compared with the wall, which is many hundreds of tonnes in mass. Therefore the wall can withstand impact without getting seriously damaged.

So too the thrusters of your ship. A really hot reactor like this could work like a ramjet in the atmosphere, ramming in atmospheric gas and spitting it out the back under pressure. The gas flow will keep things from heating up too much.

In fact a hot little fusion reactor seems to me to pose more trouble in space, where you do not have ready mass at hand for coolant, or to throw behind you for propulsion. But that was not your question.

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    $\begingroup$ I am loving that green check but there is no hurry. Sometimes the check might scare people off. Leave it unawarded and play the field for a bit. It is a cool question. Smart people with good ideas might wake up in the morning and make their own contributions. $\endgroup$ – Willk Oct 23 '17 at 1:39
  • $\begingroup$ Very well then, gonna leave it off for a while. $\endgroup$ – Felipe Andrade Oct 23 '17 at 2:46
  • $\begingroup$ You answered temperature part, but not radiation part. Any chance you'll expand your answer? $\endgroup$ – Mołot Oct 23 '17 at 6:28
  • $\begingroup$ @Molot - I am not seeing radiation part of this question. Is it your understanding that non-thermal radiation is a component of the problem? $\endgroup$ – Willk Oct 23 '17 at 23:25
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The main reason fusion thrusters wouldn't be used in a planet's atmosphere is gravity. Will's answer already provides the key element. A fusion exhaust will be hot and will be small.

The small part is the real problem. The thrust will be small too. This means the acceleration will be very small too. Estimates are around a rate of one centimetre per second squared. This is barely enough to lift a spaceship off the surface of the Moon, let alone any of the planets of the solar system, and this would include the moons of the planets too.

Fusion propulsion systems blazing away like thermonuclear versions of chemical rockets are science-fiction. A fusion rocket will be essentially a high-energy stream of particles generating a low acceleration. Their main advantage is the long acceleration times that can propel a vehicle to high velocity.

Fusion thrusters won't used in the atmospheres of planets or even planets without atmospheres because the spaceships could neither land nor takeoff. While the jet from a fusion thruster would be effectively a deadly weapon and highly destructive that isn't the main reason for their non-use on a planet.

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  • $\begingroup$ But what about antimatter engines? Would its thrust be enough? $\endgroup$ – Felipe Andrade Oct 24 '17 at 0:01
  • $\begingroup$ @FelipeAndrade Too dangerous. An antimatter thruster could have an exhaust jet of pure gamma radiation. If they could produce enough thrust to fly it would be a self-propelled laser cannon emitting energy equivalent to multiple nuclear weapons per second. A form of 'magic' space technology powered by antimatter might work especially if it does generate a lethal jet. This does involve retreating from plausible & realistic technology and embracing super-scientific hand-waving. $\endgroup$ – a4android Oct 24 '17 at 0:46

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