Discussion of Non-relativistic Interstellar Space-Flight at Light-Speed

[CrazyJebGuy]

"Non-relativistic" as in this hypothetical assumes no relativistic effects. (e.g. relativity is wrong, or it's a fictional setting without it.) It's just Newtonian physics here. So, what challenges are there in going faster than light, even with relativity's absolute ligh-speed limit removed? Want to hear thoughts/problems with this. Here's the one I thought of:

HEAT PRODUCTION

Space, even interstellar space, still has stuff. One hydrogen atom per cubic centimetre is the usual figure I've heard. But when you're going at light speed, that creates issues with cooling. For one square meter of cross-sectional area facing forward, it will get hit by v*10⁶ atoms per second. In weight, 1.7e^-18 grams of matter per second. The kinetic energy of stuff hitting it will be 0.5mv², and putting the mass/s in, we get a kinetic energy absorption = 8.5e^-19v³. Assuming velocity is c = 3x10⁸, energy per second is (27*8.5)*10⁵ = 2.3x10⁷ watts. Presumably, most of this is going into heat, so let's assume the wall on this spacecraft is 5 inches of steel. Steel is about 8 tonnes/m^3, so this square metre space-ship front weighs near enough 1000kg. Steel melts at 1510c, and let's assume it's 0c. How long could we travel at light-speed before it reaches 1000c? Specific heat capacity of 452 J/kg, so 452,000 J to heat the ton one degree, and 4.52x10⁸ to heat it to 1000 degrees. We have 2.3x10^7 watts of heat, so it will last about 20 seconds.

That's a bit of a problem if we want to go interstellar, or even to Mars. The sun is 8 minutes away, so anything further than the moon is more or less out of the picture.

I want some input on these next things: what effect would angling the plate have? Currently we've assumed 100% of the collisions' energy goes into heat. But if we angle the plate, wouldn't the incoming atoms bounce off, potentially sparing a lot of the heat? I don't know how to calculate this, but what fraction of heating might be avoided if the plate were angled away by 60, 70 or 80 degrees?

Also, what cooling might there be? We're assuming really advanced propulsion, but for fun, how much heat can KSP tech radiate? The wiki says the radiator panel (large) can dissipate a max of 3.64 MW. Since 1c generates 23 MW, we would need 6.4 for every square metre facing frontally, just for 1c. That sounds pretty managable, though I don't have KSP installed right now and I can't check how big the panels are. Still, it will place a heavy restriction on any potential spaceship's design, because it will need a lot of sideways/rear surface area per frontal surface area, and it means the spaceship must face the direction of travel.

What if we wanted to go faster than 1c? The formula I got for kinetic energy absorption goes up with v cubed, so 2c has 8x the heat, 4c has 64 times, and we're still looking at over a year to get to Alpha Centauri. We'd need about 420 large radiators for each frontal square metre, and that's beginning to get absurd.

Aside from other issues/thoughts, any solutions to the heating problem at over about 2-3c?

[Spacescifi]

5 hours ago, CrazyJebGuy said:

"Non-relativistic" as in this hypothetical assumes no relativistic effects. (e.g. relativity is wrong, or it's a fictional setting without it.) It's just Newtonian physics here. So, what challenges are there in going faster than light, even with relativity's absolute ligh-speed limit removed? Want to hear thoughts/problems with this. Here's the one I thought of:

HEAT PRODUCTION

Space, even interstellar space, still has stuff. One hydrogen atom per cubic centimetre is the usual figure I've heard. But when you're going at light speed, that creates issues with cooling. For one square meter of cross-sectional area facing forward, it will get hit by v*10⁶ atoms per second. In weight, 1.7e^-18 grams of matter per second. The kinetic energy of stuff hitting it will be 0.5mv², and putting the mass/s in, we get a kinetic energy absorption = 8.5e^-19v³. Assuming velocity is c = 3x10⁸, energy per second is (27*8.5)*10⁵ = 2.3x10⁷ watts. Presumably, most of this is going into heat, so let's assume the wall on this spacecraft is 5 inches of steel. Steel is about 8 tonnes/m^3, so this square metre space-ship front weighs near enough 1000kg. Steel melts at 1510c, and let's assume it's 0c. How long could we travel at light-speed before it reaches 1000c? Specific heat capacity of 452 J/kg, so 452,000 J to heat the ton one degree, and 4.52x10⁸ to heat it to 1000 degrees. We have 2.3x10^7 watts of heat, so it will last about 20 seconds.

That's a bit of a problem if we want to go interstellar, or even to Mars. The sun is 8 minutes away, so anything further than the moon is more or less out of the picture.

I want some input on these next things: what effect would angling the plate have? Currently we've assumed 100% of the collisions' energy goes into heat. But if we angle the plate, wouldn't the incoming atoms bounce off, potentially sparing a lot of the heat? I don't know how to calculate this, but what fraction of heating might be avoided if the plate were angled away by 60, 70 or 80 degrees?

Also, what cooling might there be? We're assuming really advanced propulsion, but for fun, how much heat can KSP tech radiate? The wiki says the radiator panel (large) can dissipate a max of 3.64 MW. Since 1c generates 23 MW, we would need 6.4 for every square metre facing frontally, just for 1c. That sounds pretty managable, though I don't have KSP installed right now and I can't check how big the panels are. Still, it will place a heavy restriction on any potential spaceship's design, because it will need a lot of sideways/rear surface area per frontal surface area, and it means the spaceship must face the direction of travel.

What if we wanted to go faster than 1c? The formula I got for kinetic energy absorption goes up with v cubed, so 2c has 8x the heat, 4c has 64 times, and we're still looking at over a year to get to Alpha Centauri. We'd need about 420 large radiators for each frontal square metre, and that's beginning to get absurd.

Aside from other issues/thoughts, any solutions to the heating problem at over about 2-3c?

 

I will paraphrase what Elon is fond of saying, do not optimize what should not be optimized in the first place. You are trying to fix what should not be or cannot be fixed.

If you are writing scifi then you are limited to making stuff up if you don't want super long travel times.

Also, high thrust constant acceleration, while a possible means of WMD, can easily be countered with the exact same thing.

Or engine auto-shutoff when speed gets too high, or a bomb that goes off if you tamper with the engine

Most often to do FTL writers rely on tropes like hyperspace, warp, and jump drives. Adding their unique quirks or simply copying and pasting what others have done.

 

Me? I tend to look at video games for inspiration, along witj my knowledge of KSP and current physics.

Edited August 29, 2021 by Spacescifi

[K^2]

You know, usually, things are never as simple as just using a magnet, but in this case, the solution is literally just using a magnet. Lorentz force on a charge is proportional to velocity, so a magnetic deflection shield that works at 1c will work at 10c and 100c and 1000c. The magnetic field doesn't even have to be that strong, because that just determines the turning radius, and that can be hundreds of meters or even entire kilometers for an interstellar craft. And if you do happen across some neutral atoms, at high enough speeds, magnetic field will rip electrons clean off and your ship will just experience a tiny amount more drag than normal.

All of this works in relativistic case as well. The math is a bit more complex, but principle's the same. The higher your Lorentz boost, the more Lorentz force is applied, and the incoming particles are deflected safely around the ship. Problem of interstellar travel at light speed is trivially reduced to having enough propulsion and propellant to keep accelerating. That problem itself is the exact opposite of trivial, but if you have means of getting to near-light speeds, keeping the craft protected is actually the easy part. At hyper-relativistic speeds even small asteroids you might be risking running into will be turned into a cloud of plasma safely deflected around your craft.

[Rakaydos]

7 hours ago, K^2 said:

You know, usually, things are never as simple as just using a magnet, but in this case, the solution is literally just using a magnet. Lorentz force on a charge is proportional to velocity, so a magnetic deflection shield that works at 1c will work at 10c and 100c and 1000c. The magnetic field doesn't even have to be that strong, because that just determines the turning radius, and that can be hundreds of meters or even entire kilometers for an interstellar craft. And if you do happen across some neutral atoms, at high enough speeds, magnetic field will rip electrons clean off and your ship will just experience a tiny amount more drag than normal.

All of this works in relativistic case as well. The math is a bit more complex, but principle's the same. The higher your Lorentz boost, the more Lorentz force is applied, and the incoming particles are deflected safely around the ship. Problem of interstellar travel at light speed is trivially reduced to having enough propulsion and propellant to keep accelerating. That problem itself is the exact opposite of trivial, but if you have means of getting to near-light speeds, keeping the craft protected is actually the easy part. At hyper-relativistic speeds even small asteroids you might be risking running into will be turned into a cloud of plasma safely deflected around your craft.

Interesting- I've read early scifi involving "magnetic scoops" for Ramscoop fusion drives- it sounds like that's the same principle, in reverse?

[K^2]

2 hours ago, Rakaydos said:

Interesting- I've read early scifi involving "magnetic scoops" for Ramscoop fusion drives- it sounds like that's the same principle, in reverse?

I mean, yeah. A scoop also works as a shield. You can divert things around the ship or into a specific part of the ship designed for it. Ram scoops are a bit of a problem, though, as they do convert kinetic energy into heat by design. But a scoop can also be used as a pass-through of a linacc or fed directly into a black hole for power. In the later case, you'll still be getting the drag, but heat isn't a problem you have to deal with. So there are options there.

(I actually think I have an even better idea of what to do with it, but I need to run the numbers. Even in the best case scenario, diverting a charged beam produces synchrotron radiation, and I need an estimate on how much that contributes to drag.)

[magnemoe]

12 hours ago, K^2 said:

I mean, yeah. A scoop also works as a shield. You can divert things around the ship or into a specific part of the ship designed for it. Ram scoops are a bit of a problem, though, as they do convert kinetic energy into heat by design. But a scoop can also be used as a pass-through of a linacc or fed directly into a black hole for power. In the later case, you'll still be getting the drag, but heat isn't a problem you have to deal with. So there are options there.

(I actually think I have an even better idea of what to do with it, but I need to run the numbers. Even in the best case scenario, diverting a charged beam produces synchrotron radiation, and I need an estimate on how much that contributes to drag.)

One option is to use an much larger magnetic field to slow you down, way easier than an bussard ramjet. 

Isaak Artur mentioned that you would get problems going very fast because you blue shifted the microwave background radiation, but that is an relativistic effect