# Playing With Space In SciFi

Bab5space
Space is space with the virtual absence of mass, with the presence of radiation and whatever stray particles that pass over it.

From a scifi perspective space can be used for a type or even types of scifi propulsion.

Since space is volume and the virtual absence of mass, any scifi drive that was capable of compressing and expelling space itself would at the very least be able to protect a vessel from kinetic collisions. Why? Space is volume, and if the vessel is expelling lots of space volume all at once, then kinetics would have to travel through however many hundreds of kilometers of space the vessel is expelling to even reach it.

Before that happens, the vessel could also dodge out the way.

Another thing one can utilize by compressing space is the radiation present in it. Who needs solar panels when you have an engine that can compress hundreds of kilometers of space... with radiation stored in it?

The real question is whether or not expelling space as propellant would impart any momentum.

I tend to think yes and no.

No: Space has virtuallt no mass and no mass means no thrust. So no help there.

Yes: Light has momentum, and if the vessel was able to compress perhaps several light hours or days worth of radiation, then ecpelling that out the back could be useful for propulsion. One drawback is that that's a photon rocket. And any photon rocket of useful thrust is also a death ray.

Think of a laser beam that can blow stuff up like a bomb. That's the level of energy we are talking. In the exhaust.

Anyone have anymore to add that I may have missed?

Mentor
This is science fiction so soft that you could equally call it magic. It provides propulsion exactly when you want it.

The closest thing to "expelling space" would be the emission of gravitational waves, which have energy and momentum.

Bab5space
This is science fiction so soft that you could equally call it magic. It provides propulsion exactly when you want it.

The closest thing to "expelling space" would be the emission of gravitational waves, which have energy and momentum.

Gravity is not something you could propel a ship with. Although you could do some gravity sling shots with it, or ecen mdke a gravity gun of sorts.

Gravity is far better at a launching or slowing objects other than the one posessing the gravity generation.

Gold Member
2022 Award
Gravity is not something you could propel a ship with. Although you could do some gravity sling shots with it, or ecen mdke a gravity gun of sorts.

Gravity is far better at a launching or slowing objects other than the one posessing the gravity generation.
Yeah, but it's still better than your "concept", which is pure magic and has no possible basis in reality.

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Gravity is not something you could propel a ship with.

Why ever not? There's lot of mass around to latch your gravity motor onto and tractor your ship along! The sun is about 1.989 × 1030 kg, that's a pretty solid anchor to pivot around. Further out, Jupiter is about 1.898 × 1027 kg, it's another solid anchor to ply your wares against out in the dark.

Anyone have anymore to add that I may have missed?

Yes, the concept of a 'space drive' makes exactly as much sense as the author needs it to. You are presenting a scenario as if it's physics, but it isn't. It is possibly a plot mechanism, but you're not writing a story, are you @Bab5space.

Staff Emeritus
There's lot of mass around to latch your gravity motor onto and tractor your ship along!

This seems simple enough. You need an asteroid, a spaceship and a stick. The asteroid pulls the spaceship towards it, and the spaceship pushes the asteroid in the direction it wants to go with the stick. What could be simpler?

member 656954 and phinds
Bab5space
Yeah, but it's still better than your "concept", which is magic pure and has no possible basis in reality.

I suppose you prefer scifi hard corrosponding to IRL limits and current understanding?

If so know this:

Neither antimatter nor fusion are the panacea scifi makes them appear to be, as both require a healthy helping of fiction to even work at all as commonly depicted in scifi media.

As for gravity itself, yes, one could use it almost exclusively for matching orbits, slowing and speeding up.

All you need is a rapid jump drive.

The cool thing is this, some places you can match orbital speed in minutes. The moon if I read correctly only orbits at 1 kilometer per sec! That's lower than LEO velocity!

Jump out and let gravity make you fall again and again till you get the speed you want.

Then jump near where you want to land. Done.

Mentor
Fusion and antimatter as fuel are just engineering challenges - we can fuse atoms and we can produce and annihilate antimatter, just not on the scale to use it for spacecraft . They don't require anything completely new, and most importantly they don't require breaking the laws of physics as we think we know them.
The moon if I read correctly only orbits at 1 kilometer per sec! That's lower than LEO velocity!
Higher orbits have a lower orbital velocity, this is a general rule.

Bab5space
Fusion and antimatter as fuel are just engineering challenges - we can fuse atoms and we can produce and annihilate antimatter, just not on the scale to use it for spacecraft . They don't require anything completely new, and most importantly they don't require breaking the laws of physics as we think we know them.Higher orbits have a lower orbital velocity, this is a general rule.

The challenges of fusion I have discussed at length and won't do so again.

But the main reason or showstopper, even if we had a ton of antimatter and a safe way to store it (we have partially a safe way to store it), we are not out of the woods yet.

Why? Radiator mass. The greater the heat generated by a high performance rocket, the greater the radiator mass is required to radiate waste heat.

None of the popular scifi vessels look like this, and a sustained fusion vessel would likely have huge rad fins. Same goes for AM.

The way to avoid massive rad fins IRL is to increase the propellant mass flow, letting your propellant shed the waste heat away from the engine directly. The trouble with this is space is mostly devoid of propellant, so one cannot afford to burn it all up constantly.

Ways of getting high thrust with long burns and low fuel mass required tend to have high energy levels, that requure massive rad fins.

The conclusion is everything has a cost and that balances out any pwrceived advantage.

So what if I have a beam core rocket that allows me to thrust for hours on end... my cooling and rad fin mass cuts into my max thrust anyway. Lowering it.

Ironically the only spaceship designs that have highhigh thrust without huge rad fins are pusher plate propulsion types. The engine heat happens outside the vessel.

It is polite, @Bab5space, to acknowledge where you have completely changed your mind on a topic, as you appear to have done with using gravity for propulsion. You've outright rejected it in Post #3, then seemingly enthusiastically embrace it in Post #7.

I suppose you prefer scifi hard corrosponding to IRL limits and current understanding?

It is also polite that where you open a thread that is essentially magic - your OP can't be done with our current understanding of physics or engineering - is to not then get narky with PF members who reply on that basis.

The Science Fiction Writers subforum is a rare resource for authors but it is a privilege, not a right, and it behooves us all to keep that in mind when using it.

Staff Emeritus
wanna

Some advice to an aspiring writer:

Use words. Readers like them. Editors like them. Publishers like them. Wanna is not a word.

One carefully considered idea is better than forty half-baked ones. When I buy a book, it's not to read forty half-baked ideas. I can get that elsewhere for free.

It is true that #7 contradicts #3. It's bad when the middle of a story contradicts the beginning.

There is a member here who has actually published. You might be a little slower to dis him. As they say, "Your shower shoes have fungus on them. You’ll never make it to the bigs with fungus on your shower shoes. Think classy and you’ll be classy. If you win 20 in the show, you can let the fungus grow back on your shower shoes and the press will think you’re colorful. Until you win 20 in the show, however, it means you’re a slob.”

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essenmein, Bystander and phinds
Bab5space
There is a difference between being informed, misinformed, or being ignorant of the science behind common scifi staples.

Ignore that if you wish, but it won't change anything.

essenmein
Why? Radiator mass. The greater the heat generated by a high performance rocket, the greater the radiator mass is required to radiate waste heat.

None of the popular scifi vessels look like this, and a sustained fusion vessel would likely have huge rad fins. Same goes for AM.

So in the story I'm picking away at, they are fission powered at first, then AM. They have radiating plates, and having done some calculations I don't think you are aware of just how effective this is in space.

Radiated power per area is given by the Stefan-Boltzmann law:

You can see that power increases by temperature to the power of 4.

So plug in some numbers, assume its an ideal black body radiator (emissivity = 1) with a temperature of 1200C (1473K), ie glowing red-orange. With this radiating into space (~0K) from one side, radiates 270kW/m^2. So a 25m x 25m cooling rad on the ship requires 170MW to maintain a temperature of 1200C. Increase that to 1300C and power loss increases to 221MW.

If I hand wave my system to be 95% efficient, then this gives me available power of 3-4GW from a fairly small rad plate.

Bandersnatch
Bab5space
So in the story I'm picking away at, they are fission powered at first, then AM. They have radiating plates, and having done some calculations I don't think you are aware of just how effective this is in space.

Radiated power per area is given by the Stefan-Boltzmann law:

View attachment 257242

You can see that power increases by temperature to the power of 4.

So plug in some numbers, assume its an ideal black body radiator (emissivity = 1) with a temperature of 1200C (1473K), ie glowing red-orange. With this radiating into space (~0K) from one side, radiates 270kW/m^2. So a 25m x 25m cooling rad on the ship requires 170MW to maintain a temperature of 1200C. Increase that to 1300C and power loss increases to 221MW.

If I hand wave my system to be 95% efficient, then this gives me available power of 3-4GW from a fairly small rad plate.

Interesting, but the handwave is the fiction part, just like so many scifi media spacecraft s have fiction working in league with known science to make them work.

Unless you guess or assume it will be possible to get 95% efficient radiators some day. It just might be.

Either way it favors small vessels instead of large ones, since as I mentioned, high mass lowers the high thrust you would get otherwise with super drives like AM and fusion.

essenmein
Interesting, but the handwave is the fiction part, just like so many scifi media spacecraft s have fiction working in league with known science to make them work.

Unless you guess or assume it will be possible to get 95% efficient radiators some day. It just might be.

Right, but it is science fiction, not an engineering dissertation on the design of a real space ship, if I could do that, I wouldn't have to write a story, I'd be flying around the stars. So somewhere under full analysis it will not hold up, but this should not be a surprise.

Also, radiators are 100% efficient today, no work needs to be done there. The 95% efficiency comes from the conversion of power from fission or AM to useful work (ie moving the ship or shooting stuff). This is absolutely handwavey, no argument from me there, although I hand wave with magnetohydrodynamic generators and heat conversion (ie high temp heatpumps) to do the cooling.

Bab5space
Right, but it is science fiction, not an engineering dissertation on the design of a real space ship, if I could do that, I wouldn't have to write a story, I'd be flying around the stars. So somewhere under full analysis it will not hold up, but this should not be a surprise.

Also, radiators are 100% efficient today, no work needs to be done there. The 95% efficiency comes from the conversion of power from fission or AM to useful work (ie moving the ship or shooting stuff). This is absolutely handwavey, no argument from me there, although I hand wave with magnetohydrodynamic generators and heat conversion (ie high temp heatpumps) to do the cooling.

Better efficiency would be nice, but the easiest way I know of doing that is by running through lots of propellant and mixing it with antimatter.

Granted you can fly for days at 1g that way, and a magnetic nozzle is likely a must as well as plenty of radiators. Magnetic nozzles are bad in the air, since air conducts the heat to the nozzle we are trying to avoid, so any landings on Earth would need other types of nozzles.

A scifi version of the old project Orion provides arguably more spaceship cargo than fuel tank, compared to rocket spaceships which... unless AM beam core require them to be mostly fuel tank.

And that has to do with the fact that Orion can do higher energy thrust than one can do witj rocket engines, which would melt or vaporize under those energies.

By the way, while we are talking scifi, I think it would be really cool to be able to have the ability to reaarange and move subatomic particles and atoms as we see fit. If we had a type of field technology that could do that, we could hover and lift big spaceships on Earth, just by sucking lots of air downward. Since for every reaction there is a reaction, the vessel would be lifted upward, at least until the air became too thin to provide enough propulsive force. By then the vessel could flip over and engage an orion pusher plate style propulsion.

With AM bombs.

By the way, while we are talking scifi,

Weren't we always? Oh that's right, the OP was magic, my mistake