A better photon rocket...

[Cunjo Carl]

15 minutes ago, Spacescifi said:

 

So lemme get this straight.

A mass conversion rocket that converts it's fuel mass 100% into slower than light photons would have rocket thrust but expend the fuel mass at chemical rocket rates?

Which means why even bother?

 

Direct mass conversion would be a little different. If we have energy from true mass conversion (like an antimatter drive) at unlimited power then we'd want the speed of light to stay as it is or become higher for the better Isp. At a higher speed of light we'd get much more energy from our mass!

Meanwhile for almost anything else that doesn't take its energy from direct mass conversion (fusion, fission, chemical energy, batteries, etc.) you'd want the lower speed so you can get better thrust and more momentum from the power and energy you do have available.

[Spacescifi]

1 hour ago, Cunjo Carl said:

Direct mass conversion would be a little different. If we have energy from true mass conversion (like an antimatter drive) at unlimited power then we'd want the speed of light to stay as it is or become higher for the better Isp. At a higher speed of light we'd get much more energy from our mass!

Meanwhile for almost anything else that doesn't take its energy from direct mass conversion (fusion, fission, chemical energy, batteries, etc.) you'd want the lower speed so you can get better thrust and more momentum from the power and energy you do have available.

 

Hmm... so slow speed light rocket really are a waste of breaking physics?

Unless the thrust of slow light speed rocket is dramatically greater than chemical rockets. Then you could just pulse fire it to orbit like an orion battleship.

 

The only issue I have with the direct matter to photon conversion drive is... it would nuke all in it's wake.

 

Ideally I want high thrust that can last for hours while not nuking everything behind the ship.

 

Seems like a direct conversion to superluminal photons is the only solution.

I think one could use less superluminal photons than regular photons as exhaust, therefore avoiding nuking all in it's immediate wake.

[Cunjo Carl]

10 hours ago, Spacescifi said:

The only issue I have with the direct matter to photon conversion drive is... it would nuke all in it's wake.

It's a convenient hyperdrive and galaxy rending death beam all in one convenient package, what's not to like?

I wouldn't worry too much though. Space is pretty big. If you had the beam defocused to a 10 degree cone, you'd only lose a few percent of your Isp, but the beam would diffuse very quickly over stellar distances. It would be eminantly noticeable but not planet ending for modestly sized ships.

[VincentS]

Another approach to enhance photon rocket : use rocket engine nozzles.

Yes, I know that photon don't expand. But what if you pump some gaz into the concentrated light beam (or even fire ice pellets into harm's way) ? You basically get some kind of thermal rocket. While in atmosphere, you could pump gaz for free. While in space, you could use unwanted junk to get some increase in TWR

[Ol’ Musky Boi]

19 hours ago, VincentS said:

Another approach to enhance photon rocket : use rocket engine nozzles.

Yes, I know that photon don't expand. But what if you pump some gaz into the concentrated light beam (or even fire ice pellets into harm's way) ? You basically get some kind of thermal rocket. While in atmosphere, you could pump gaz for free. While in space, you could use unwanted junk to get some increase in TWR

That's the concept behind the lightcraft, and these engines have actually been flight tested: https://youtu.be/KtH-SxqdtaA?t=110

Not sure if they are still in development though.

[Starstruck69]

Your on dangerous ground here because a photon cannot go any faster unless you go against Einsteins GR and SR, regardless of your relative position. 

[Starstruck69]

On 6/21/2019 at 10:14 PM, Cunjo Carl said:

It does though! Because of einstein's mass-energy equivalence, any form of energy storage actually has mass. It's pretty paradoxical but when you solve for mass-energy equivalence as you say, despite the photon being massless the Isp of a photon rocket is exactly equivalent to a normal rocket shooting reaction mass out at the speed of light.

Mass-energy equivalence applies to all forms of stored energy from radioisotopes to batteries to chemical propellants. For a fun case-and-point, let's consider a photon rocket that operates by burning and holding onto 1kg of methalox, collecting the energy and using it to shoot photons out the back. What we'll find is our rocket becomes lighter by a tiny mass 'm', and our rocket gains a momentum of m*c. Again, this is the same mass efficiency as what we'd get from a normal rocket with an Isp of c/g0.

Getting started, 1kg of burning methane will look like:

CH₄ + 2O₂ ->  CO₂ + 2H₂O + 16.7MJ of heat

Assuming by magic we collect all the energy, the products of the above reaction will be lighter than the reactants by 16.7MJ/c^2  (1.9E-10kg) because of the mass energy equivalence. I think that's amazing! The chemical reaction actually changes the total mass, despite no particles ever leaving the system. Moving on to the next step, launching the collected energy out the back of the rocket as photons will provide us a momentum of:

Momentum Gained = Energy Used / c
            .056 kg*m/s = 16.7MJ / c   
            .056 kg*m/s = 1.9E-10kg*c   (alternatively)

And finally, we notice that our rocket has lost 1.9E-10kg of mass, and gained 1.9E-10kg * c of momentum! (.056 kg*m/s = 1.9E-10kg*c) Like we claimed at the start despite the photon being massless the photon rocket has the same mass efficiency as a normal rocket with an exhaust velocity of c (or an Isp of c/g0) . Crazy stuff, right?

I love these fun problems and a good post. Apart from the obvious thermo losses i really like this. Carbon dioxide for the plants, water for us and the plants and very approximate 16.7MJ of energy for the propulsion. Great post:) 

[KerBlitz Kerman]

Ask @nyrath (Nyrath the Nearly Wise)

[WinkAllKerb'']

tl.dr matteer is full of void, same apply to (yet) seeable universe(s) §+§ = primitive soup
 

 

Edited July 13, 2019 by WinkAllKerb''
typos ....