The sun as a photon rocket

[Spacescifi]

Consider a scifi spaceship that literally has the photon energy of the SUN.

Would that give it enough thrust to get to orbit or not? I know photons are weak for thrust, but a sun's worth of photon's is quite a bit to throw behind your vessel.

The ship weighs 200 tons and is fully loaded with 25 tons of cargo. So it has to lift 225 tons into orbit. Can it do it?

What do you think? I think it would work, given that the old plans called for antimatter, which still would provide less photons overall than the photon equivalent of the ENTIRE sun.

Yet that also means making a rocket plume from the ground itself at liftoff. The photonic exhaust is going to burn for miles.

Edited May 24, 2019 by Spacescifi

[cantab]

The power required for a photon rocket is 300 MW/N.

The total luminosity of the Sun is around 3 * 10^26 W.

So that would provide 10^18 Newtons of thrust.

However the mass-energy equivalent of 200 tonnes is only about 2 x 10^22 Joules. Meaning that even assuming ideal mass-to-energy conversion, the spacecraft will use up its fuel in about 60 microseconds.

In any case,  what you've created isn't really a rocket. It's a bomb. It's not going to lift into orbit, it's just going to be sent flying to kingdom come and leave a giant crater behind it.

[Spacescifi]

8 minutes ago, cantab said:

The power required for a photon rocket is 300 MW/N.

The total luminosity of the Sun is around 3 * 10^26 W.

So that would provide 10^18 Newtons of thrust.

However the mass-energy equivalent of 200 tonnes is only about 2 x 10^22 Joules. Meaning that even assuming ideal mass-to-energy conversion, the spacecraft will use up its fuel in about 60 microseconds.

In any case,  what you've created isn't really a rocket. It's a bomb. It's not going to lift into orbit, it's just going to be sent flying to kingdom come and leave a giant crater behind it.

So what you're saying is, if fuel was not a concern (in scifi it often is'nt), this would make a viable albeit destructive way achieve orbital speed?

Edited May 24, 2019 by Spacescifi

[cantab]

Indeed, if you have bottomless power sources and heat rejection a la Star Wars, then photon rockets work. And you don't need anywhere near the power equivalent of a star unless you're flying something the size of a planet or moon. But so does whatever other sci-fi magic you write.

Beamed power solves the onboard fuel problem in real life; at the simplest level, the spacecraft is just a giant mirror. (Which, by the way, means the beam only needs 150 MW/N, because the photons are reflected back rather than just emitted). However I suspect launching to orbit will be a problem because of heating. No mirror can reflect all radiation, but some do get impressively close.

A top-tier dielectric mirror can reflect 99.999% of light, albeit for a limited frequency range and incidence angle. (So we need to shine a laser beam at the bottom of the rocket). That means that 1 Newton of thrust will heat the mirror by 150 MW * 0.0001 = 15 kW. To lift 200 tonnes against Earth's gravity requires 2 MN of thrust, and that still means 30 gigawatts of heating of the mirror.

That kind of power might be manageable. It's comparable to the power produced by real-world heavy launchers like the Saturn V and Space Shuttle. But it's going to be a tall order. Chemical rocket engines circulate propellant around the chamber and nozzle which is then ultimately jetissoned away from the vehicle. If your photon rocket uses an open cycle cooling system, then dumping the coolant means you have to deal with a finite resource. If you want to handle 30 gigawatts of heating without open cycle...good luck.

[Spacescifi]

11 minutes ago, cantab said:

Indeed, if you have bottomless power sources and heat rejection a la Star Wars, then photon rockets work. And you don't need anywhere near the power equivalent of a star unless you're flying something the size of a planet or moon. But so does whatever other sci-fi magic you write.

Beamed power solves the onboard fuel problem in real life; at the simplest level, the spacecraft is just a giant mirror. (Which, by the way, means the beam only needs 150 MW/N, because the photons are reflected back rather than just emitted). However I suspect launching to orbit will be a problem because of heating. No mirror can reflect all radiation, but some do get impressively close.

A top-tier dielectric mirror can reflect 99.999% of light, albeit for a limited frequency range and incidence angle. (So we need to shine a laser beam at the bottom of the rocket). That means that 1 Newton of thrust will heat the mirror by 150 MW * 0.0001 = 15 kW. To lift 200 tonnes against Earth's gravity requires 2 MN of thrust, and that still means 30 gigawatts of heating of the mirror.

That kind of power might be manageable. It's comparable to the power produced by real-world heavy launchers like the Saturn V and Space Shuttle. But it's going to be a tall order. Chemical rocket engines circulate propellant around the chamber and nozzle which is then ultimately jetissoned away from the vehicle. If your photon rocket uses an open cycle cooling system, then dumping the coolant means you have to deal with a finite resource. If you want to handle 30 gigawatts of heating without open cycle...good luck.

Well I will just go the route of good heat resistance and an infinite fuel source.

Basically the only real limits to ship size with this scifi method involve whether or not you care about ever landing again without bombing your own ship repeatedly with tbe blowback of the ground's blast wave.

[kerbiloid]

Spoiler

We should color one half of the Sun with coal, and fly to the stars with all our planets.

 

[Spacescifi]

17 minutes ago, kerbiloid said:

  Hide contents

We should color one half of the Sun with coal, and fly to the stars with all our planets.

 

LOL. If only it were that simple.

You would do better by laser induced coronal mass plasma ejection though.

Blast the sun with an effective laser and you just made a giant plasma rocket!

May be hazardous to earth though, what with basically a constant solar flare going on.

[Starhawk]

This thread been moved to Science and Spaceflight.

KSP Moderation Team

[Nuke]

drill a hole in the ground, add water and a nuke, cover with a steel plate. bolt on a lawn chair. instant spaceship!

Edited May 29, 2019 by Nuke

[wumpus]

On 5/24/2019 at 11:01 AM, Spacescifi said:

So what you're saying is, if fuel was not a concern (in scifi it often is'nt), this would make a viable albeit destructive way achieve orbital speed?

If you have anything like the output power of the Sun, you should be thinking interstellar flight and not the wimpy little delta-vs needed for orbital velocity and hohmann transfers.

Change the albedo of the Sun (or ideally some other expendable star) and you might get it to move in a specific direction.  Wait long enough and it should achieve relativistic velocities (hopefully your albedo changing doping didn't cause the fusion process to change too drastically) under somewhat iffy control.  What you have is a single-use death star, only one that likely gives away its approach long before it crashes into your sun/planet/whatever.

PS: you want to give the bright/exhaust side a "dark albedo" and the other side a "white albedo".  Why?  Because black bodies in black body radiation radiate the most energy.   Colors absorb and transmit light at similar rates.

[kerbiloid]

Spoiler

5 hours ago, wumpus said:

Change the albedo of the Sun

to make it reflect the moonlight in desired direction.

 

[Guest]

A little heavier than 200T:https://en.wikipedia.org/wiki/Stellar_engine

[wumpus]

On 5/29/2019 at 4:39 AM, kerbiloid said:

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to make it reflect the moonlight in desired direction.

 

The Sun basically emits light two ways, first by direct fusion which pretty much goes however it will.  Second through blackbody radiation of its 15million degree Celsius heat.  Convince a plasma to somehow "turn black/white" in opposite direction and the sun will have significant thrust (while the fusion energy will certainly far exceed the blackbody radiation, the blackbody radiation should be more than enough to  get it moving fast).

While this only uses a small fraction of the Sun's energy, it at least is an idea of how to harness such power.

[DAL59]

https://www.centauri-dreams.org/2013/11/26/moving-stars-the-shkadov-thruster/