Thought: use particle beams (directed in a particle accelerator) for thrust?

[aoriginalusername]

The advantages of this would be the extremely high specific impulse (particles in an accelerator can get to high fractions of C), an adequate thrust (particles near the speed of light increase in thrust), and low fuel usage (you'd just need something like hydrogen or xenon). This would also need a lot of power, needing something like a polonium RTG or nuclear reactor), but not completely impossible.

Particle accelerators are big, but recent advancements have made them much smaller. Plasma acceleration and nanophotonic accelerators are reaching the size where the smallest functional ones can even fit on your thumb. Such high thrust would allow direct thrust trajectories between planets. A preliminary calculation, back of the napkin, shows that a 500 kg probe could accelerate at 1 g with one engine unit. This would need a lot of power, yes, but this could be supplied with a polonium RTG or solar panels for near-Sun operation.

[kenny008]

Interesting.  Would you mind sharing your back of the napkin calculations?

[Jim]

The advantages of this would be the extremely high specific impulse (particles in an accelerator can get to high fractions of C), an adequate thrust (particles near the speed of light increase in thrust), and low fuel usage (you'd just need something like hydrogen or xenon). This would also need a lot of power, needing something like a polonium RTG or nuclear reactor), but not completely impossible.

Particle accelerators are big, but recent advancements have made them much smaller. Plasma acceleration and nanophotonic accelerators are reaching the size where the smallest functional ones can even fit on your thumb. Such high thrust would allow direct thrust trajectories between planets. A preliminary calculation, back of the napkin, shows that a 500 kg probe could accelerate at 1 g with one engine unit. This would need a lot of power, yes, but this could be supplied with a polonium RTG or solar panels for near-Sun operation.

RTGs don't supply a lot of power

[aoriginalusername]

They don't, usually, but if the mission time is short enough Polonium produces 150 watts per gram.

[kenny008]

Again, your calculations would help us all better understand the magnitudes you're talking about here.  150w/gram sounds pretty tiny to me, unless your calculations show otherwise.

[aoriginalusername]

I did some calculations on how much a trip to Mars would cost, energy wise, and yeah. It'd take some 2.78 × 10¹⁵ J for the trip. I suck at math, so this might be off, but this means you'd need WAY too much polonium for 1g constant acceleration.

[kenny008]

Ah, OK, so you haven't really calculated this out.
Lots of things to take into consideration (mass of fuel, mass of energy source, Isp, thrust, etc., etc., etc.).  I'd recommend you take a look at all of that, and then let us know if this really looks like something that would work at all.  We're all willing to look over your calculations, but just throwing the idea out there without anything to back it up probably won't get you too many positive comments.

[InterestedEngineer]

I did some calculations on how much a trip to Mars would cost, energy wise, and yeah. It'd take some 2.78 × 10¹⁵ J for the trip. I suck at math, so this might be off, but this means you'd need WAY too much polonium for 1g constant acceleration.

The primary two equations that debunks most of these ideas is

1. the equation F=mdot * exhaust velocity = m * a

    mdot being the kg/sec of mass being thrown out the back, and a being the desired acceleration and m being the mass of your spaceship including fuel.

2. You then calculate energy rate (watts) = 1/2mdot * v²_exhaust and get the energy rate of such a system.  You'll find for any constant 1g acceleration you either quickly run out of fuel and limits imposed by the rocket equation, or you are generating terawatts of power from some mysterious power source and even worse most systems are at best 66% efficient so you are having to discard 100s of gigawatts of heat.

[InterestedEngineer]

The advantages of this would be the extremely high specific impulse (particles in an accelerator can get to high fractions of C), an adequate thrust (particles near the speed of light increase in thrust), and low fuel usage (you'd just need something like hydrogen or xenon). This would also need a lot of power, needing something like a polonium RTG or nuclear reactor), but not completely impossible.

Particle accelerators are big, but recent advancements have made them much smaller. Plasma acceleration and nanophotonic accelerators are reaching the size where the smallest functional ones can even fit on your thumb. Such high thrust would allow direct thrust trajectories between planets. A preliminary calculation, back of the napkin, shows that a 500 kg probe could accelerate at 1 g with one engine unit. This would need a lot of power, yes, but this could be supplied with a polonium RTG or solar panels for near-Sun operation.

You've basically just described an ion drive, which are in use all around our planet, aka Starlink, as well as some deep space missions currently under way.  They are all solar powered, if I recall correctly, as power/weight ratio is better for Solar out to I think the asteroid belt (I didn't look it up, just probably bad memory)