This week's "Fun with...": Photon Reaction Drives

[rhoenix]

After doing some research on STL sci-fi possibilities, only partially because designing a proper FTL tech mechanism that doesn't suck is nearly maddening, I came upon details for a photonic reaction drive.

Without the math, I'll distill the ideas down to a few points (and if you feel I've oversimplified, please leave a comment):

- Requires a rather beefy power source.
- Pretty efficient for power/thrust ratio, and has strong accelleration (in terms of G's)
- Don't be anywhere near the thrusters of the ship. Things wouldn't go well for you.

So far, it looks promising, and has good limits of it's own. However, this gave rise to a correlative idea - using this same engine as a weapon.

With this thought, my abstract concept for such a drive/weapon would be to emit scattered light as exhaust normally, as opposed to a direct beam. This would necessarily cut down on the efficiency of the drive, as I understand it, which would relegate such a change to "stealth" ships, assuming the drive doesn't create very much heat. (If anyone has comments about this step, I'd love to hear them)

However, refocusing lenses near the engines could gather the light before it is scattered, and send it out as a beam as a very tight beam from the exhaust, if one were to wish to use it as a weapon. My question about this supposition is: would doing such a thing create more thrust on the ship than normal, or less?

[darthdavid]

After doing some research on STL sci-fi possibilities, only partially because designing a proper FTL tech mechanism that doesn't suck is nearly maddening, I came upon details for a photonic reaction drive.

Without the math, I'll distill the ideas down to a few points (and if you feel I've oversimplified, please leave a comment):

- Requires a rather beefy power source.
- Pretty efficient for power/thrust ratio, and has strong accelleration (in terms of G's)
- Don't be anywhere near the thrusters of the ship. Things wouldn't go well for you.

So far, it looks promising, and has good limits of it's own. However, this gave rise to a correlative idea - using this same engine as a weapon.

With this thought, my abstract concept for such a drive/weapon would be to emit scattered light as exhaust normally, as opposed to a direct beam. This would necessarily cut down on the efficiency of the drive, as I understand it, which would relegate such a change to "stealth" ships, assuming the drive doesn't create very much heat. (If anyone has comments about this step, I'd love to hear them)

However, refocusing lenses near the engines could gather the light before it is scattered, and send it out as a beam as a very tight beam from the exhaust, if one were to wish to use it as a weapon. My question about this supposition is: would doing such a thing create more thrust on the ship than normal, or less?

Well assuming perfect efficiency, it should be the same 'cause it's the same number of photons with the same energy. But I'm no physicist so don't take that as certain.

[Wyrm]

So far, it looks promising, and has good limits of it's own. However, this gave rise to a correlative idea - using this same engine as a weapon.

That's the Kzinti Lesson. Interesting spaceflight requires mondo energy, energy able to wreck havoc and destruction.

With this thought, my abstract concept for such a drive/weapon would be to emit scattered light as exhaust normally, as opposed to a direct beam. This would necessarily cut down on the efficiency of the drive, as I understand it, which would relegate such a change to "stealth" ships, assuming the drive doesn't create very much heat. (If anyone has comments about this step, I'd love to hear them)

First of all, a beam of any breed is going to spread, period. Accept it, and move on. Secondly, the very operation of this drive means you are going to be emitting stuff that will potentially give you away, in the form of high-energy photons. Indeed a focused beam is superior in terms of stealth than a diffuse beam, because there's less chance of a far-away sensor catching photons from the drive. It also makes it a more horrifying weapon, because it's not as diffuse: the energy density of the beam is high if the cross section is small.

However, refocusing lenses near the engines could gather the light before it is scattered, and send it out as a beam as a very tight beam from the exhaust, if one were to wish to use it as a weapon. My question about this supposition is: would doing such a thing create more thrust on the ship than normal, or less?

The thrust increases, because the spread represents wasted power. The thrust you get from the engine is equal and opposite to the vector sum of all the momenta of photons you create per unit time and emit out the back. The momenta of each photon can be split into two components: a component aligned with the thrust vector of the engine, and a component perpendicular to that vector.

The vector sum of the aligned components add up to the momentum of the photon exhaust. The vector sum of the perpendicular components add to vector zero by linear algebra. Yet the perpendicular components contribute to the energy consumption of the engine, therefore, the more the beam spreads (indicating larger perpendicular components), the larger the energy consumption per unit thrust. Therefore, focusing the beam results in a more efficient engine.

One word of caution, however: be sure that whatever you're using to focus the beam does not absorb but the tiniest fraction of the photons. Not only does each photon absorb mean wasted energy, but it will also tend to do nasty things to the lens. An ordinary glass lens operating on the end of a visible-light photon drive would be slagged instantly.

[rhoenix]

That's the Kzinti Lesson. Interesting spaceflight requires mondo energy, energy able to wreck havoc and destruction.

Yes, I was reading about that over at the Atomic Rocket. After doing further reading on the Battleship Yamato, with it's "wave-motion engines" which can be redirected to power a "wave-motion cannon," I got the idea for a slightly less handwavy photon engine, and wanted to run it by the minds here for consideration.

First of all, a beam of any breed is going to spread, period. Accept it, and move on.

That's fine - the only reason I brought up "light diffusion" was in the hopes that it would make the exhaust trail not quite so bright, or quite so damaging to other ships nearby. After all, one wouldn't want to recreate the Kzinti Lesson on friendly ships on accident.

Secondly, the very operation of this drive means you are going to be emitting stuff that will potentially give you away, in the form of high-energy photons. Indeed a focused beam is superior in terms of stealth than a diffuse beam, because there's less chance of a far-away sensor catching photons from the drive. It also makes it a more horrifying weapon, because it's not as diffuse: the energy density of the beam is high if the cross section is small.

I hadn't thought of that aspect of the photon trail - I had conceived that the diffused light would be harder to track, but it didn't occur to me that it would be more difficult for sensors to detect a direct stream, rather than diffused photonic exhaust.

The thrust increases, because the spread represents wasted power. The thrust you get from the engine is equal and opposite to the vector sum of all the momenta of photons you create per unit time and emit out the back. The momenta of each photon can be split into two components: a component aligned with the thrust vector of the engine, and a component perpendicular to that vector.

The vector sum of the aligned components add up to the momentum of the photon exhaust. The vector sum of the perpendicular components add to vector zero by linear algebra. Yet the perpendicular components contribute to the energy consumption of the engine, therefore, the more the beam spreads (indicating larger perpendicular components), the larger the energy consumption per unit thrust. Therefore, focusing the beam results in a more efficient engine.

With this explanation, your earlier point in the post I'm replying to makes far more sense. Thank you for that.

One word of caution, however: be sure that whatever you're using to focus the beam does not absorb but the tiniest fraction of the photons. Not only does each photon absorb mean wasted energy, but it will also tend to do nasty things to the lens. An ordinary glass lens operating on the end of a visible-light photon drive would be slagged instantly.

With that kind of power behind the beam, I would imagine so. Do you have any suggestions for what to use as a lens? Is it possible to have a finely-polished and ground lens?

To attempt to answer my own question, I would guess that yes, it is possible, but a lens wouldn't work more than once. This limitation would necessitate great care when using the cannon, as one wouldn't get a follow-up shot until some brave soul replaces the lens.

[TheLemur]

To attempt to answer my own question, I would guess that yes, it is possible, but a lens wouldn't work more than once.

A lens wouldn't work at all. Conventional glass lenses absorb around 4% of the light they transmit. At 1 TW, rather low for a thousand-ton-plus starship's photonic engine, you're talking about a power of 40 GW, which is enough to melt the lens after some milliseconds. Even a ten-ton glass lens would melt after half a second or so. Even if you shot the weapon for a hundred milliseconds and then replaced the lens, what's going to accelerate the starship to relativistic speeds in the first place?

[Rekkon]

If you are going hard sci-fi, does it pay to try to work in 'stealth' ships at all? According to the Atomic Rockets page, the concept is basically worthless since there is no way to shield enough of your emissions and even the heat from the life support would give it away at extremely long ranges.

[Starglider]

If you are going hard sci-fi, does it pay to try to work in 'stealth' ships at all?

Stealth ships require handwavium to be useful. Stealth weapons platforms are another matter entirely. A mass-driver-launched missile bus, with low power electronics and a liquid helium cooled outer shell, is quite practical. You shoot it at where you expect the enemy to be and it releases a surprise volley of missiles at the closest point at which they can still make the intercept. Whether this is tactically effective and cost efficient depends on the specifics of the setting.

[rhoenix]

A lens wouldn't work at all. Conventional glass lenses absorb around 4% of the light they transmit. At 1 TW, rather low for a thousand-ton-plus starship's photonic engine, you're talking about a power of 40 GW, which is enough to melt the lens after some milliseconds. Even a ten-ton glass lens would melt after half a second or so. Even if you shot the weapon for a hundred milliseconds and then replaced the lens, what's going to accelerate the starship to relativistic speeds in the first place?

This would be the idea - shoot long enough so you don't utterly melt the lens. As for achieving ship's propulsion after the shot, that's simply a matter of engineering and planning so that both beam and engine don't use the same port.

If you are going hard sci-fi, does it pay to try to work in 'stealth' ships at all?

Stealth ships require handwavium to be useful. Stealth weapons platforms are another matter entirely. A mass-driver-launched missile bus, with low power electronics and a liquid helium cooled outer shell, is quite practical. You shoot it at where you expect the enemy to be and it releases a surprise volley of missiles at the closest point at which they can still make the intercept. Whether this is tactically effective and cost efficient depends on the specifics of the setting.

Essentially, I'll "me-too" what Starglider said. There are a number of ways to make a ship give off less heat, thereby making the ship more difficult to detect to the given sensors of the time. Trapping the heat between the inner and outer hulls and venting it in controlled bursts, the cooled outer shell as Starglider mentioned, and a number of other tricks as well.

This sort of ship might be equipped with a photon cannon, but I'll leave it in the realm of possibilities instead of probabilities for now. Not only would a ship rather dramatically give away it's position with the shot, it would also be unable to make a second shot until the lens is replaced, and that's not counting practical concerns of redirecting most of the power generated by the ship from the engines to the photon cannon, and back again. Missles are a far more friendly solution for stealth platform weaponry.

[Ender]

- Requires a rather beefy power source.

As it takes 300 MW to get a single newton, you better believe it.

- Pretty efficient for power/thrust ratio, and has strong accelleration (in terms of G's)

You have that exactly backwards. you are spending 300 megawatts to get a single newton - your power/thrust ratio is abysmal. And your acceleration even moreso. Frankly, if you can generate that kind of power and handle that kind of waste heat, you would be better off building clusters of ion engines or something similar

[rhoenix]

- Requires a rather beefy power source.

As it takes 300 MW to get a single newton, you better believe it.

Yes, I kept the OP abstract, though I used the Atomic Rocket as a reference. However, I must have missed this:

- Pretty efficient for power/thrust ratio, and has strong accelleration (in terms of G's)

You have that exactly backwards. you are spending 300 megawatts to get a single newton - your power/thrust ratio is abysmal. And your acceleration even moreso. Frankly, if you can generate that kind of power and handle that kind of waste heat, you would be better off building clusters of ion engines or something similar

Thank you for bringing this up. If the photon engine idea is unfeasable due to the shocking power requirements, then as an alternative, ion engines are an excellent secondary choice - far less messy, too.

Due to the topic of the OP, I'll make a new thread concerning ion engines, thanks to the information you pointed out.

[Kuroneko]

300MW per newton per kilogram of payload, to be more precise. But although the power/thrust ratio is abysmal (if we exclude fuel rest mass as usual), the specific impulse is potentially immense. This is a very significant advantage to very long-term missions, and at subluminal speeds, every interstellar mission will be long-term. Perhaps a two-stage design would be best--a high-thrust burn to get to an appreciable velocity first, followed by a low-thrust photonic engine, since at very high specific impulses one can afford to keep it burning for a few years at a time.

[TheLemur]

This would be the idea - shoot long enough so you don't utterly melt the lens. As for achieving ship's propulsion after the shot, that's simply a matter of engineering and planning so that both beam and engine don't use the same port.

And so what's focusing the light coming out of the engine? Black magick? Whatever it is, why not use it on the weapon port?

300MW per newton per kilogram of payload, to be more precise.

What? How much force each photon generates has nothing to do with the mass of the payload. You mean 300MW per m/s^2 of acceleration per kilogram of payload.

you would be better off building clusters of ion engines or something similar

(laughs)

Ion engines operate at an exhaust velocity of around 30 km/s, which while far superior to chemical rocketry, is nowhere near enough to get you to relativistic speeds. Using Tsilokovsky's equation, in order to get to a mere 1% of lightspeed, for every kilogram of payload (including engine mass), you need 2.688*10^43 kg of fuel. To make it sound really impressive, that's 2,688,000,000,000,000,000,000,000,000,000,000,000,000,000 kg of fuel, or 1,344,000,000,000 solar masses worth of fuel, or ~134 Milky Way galaxy masses worth of fuel. Per kilogram.

[Solauren]

Just use spacial folding

[Kuroneko]

What? How much force each photon generates has nothing to do with the mass of the payload. You mean 300MW per m/s^2 of acceleration per kilogram of payload.

Bloody hell; one of these days I'll have to start remembering to pay more attention when posting after beer. Mea culpa.

[Starglider]

And so what's focusing the light coming out of the engine? Black magick? Whatever it is, why not use it on the weapon port?

You might have better luck using X-rays and grazing-incidence optics to focus the beam; for one thing they're denser and will heat up more slowly. There are designs based on total external reflection (such as the Kumakhov lens) which are theoretically lossless given perfect construction (i.e pure materials and nanoscale assembly techology).

Ion engines operate at an exhaust velocity of around 30 km/s, which while far superior to chemical rocketry, is nowhere near enough to get you to relativistic speeds.

Current practical ion engines yes. However we accelerate heavy ions up to near lightspeed in particle accelerators all the time, so an exhaust velocity of 99%+ c isn't implausible. The problem is that producing those kind of velocities with current technology is an extremely power inefficient process and only works on relatively small bunches of particles, so you'll get minute thrust and lots of waste heat. If you invoke advanced tech to solve the mass and efficiency problems, note that particle accelerator cannons also make frighteningly effective weapons.

[TheLemur]

You might have better luck using X-rays and grazing-incidence optics to focus the beam; for one thing they're denser and will heat up more slowly. There are designs based on total external reflection (such as the Kumakhov lens) which are theoretically lossless given perfect construction (i.e pure materials and nanoscale assembly techology).

Neat. But saying the lens will heat up more slowly because it's denser is the equivalent of saying that a rifle bullet will do more damage to 120 mm DU tank armor than a pistol bullet. It's still going to fry after a second or less if it absorbs even 1% of the incident energy.

Current practical ion engines yes. However we accelerate heavy ions up to near lightspeed in particle accelerators all the time, so an exhaust velocity of 99%+ c isn't implausible. The problem is that producing those kind of velocities with current technology is an extremely power inefficient process and only works on relatively small bunches of particles, so you'll get minute thrust and lots of waste heat. If you invoke advanced tech to solve the mass and efficiency problems, note that particle accelerator cannons also make frighteningly effective weapons.

Again true, but I'm not sure that's what Ender meant when he said "ion engines". The 30 km/s ones are far more commonly mentioned, as they are proven technology and have been tested in space already.

[Starglider]

You might have better luck using X-rays and grazing-incidence optics to focus the beam; for one thing they're denser and will heat up more slowly. There are designs based on total external reflection (such as the Kumakhov lens) which are theoretically lossless given perfect construction (i.e pure materials and nanoscale assembly techology).

Neat. But saying the lens will heat up more slowly because it's denser is the equivalent of saying that a rifle bullet will do more damage to 120 mm DU tank armor than a pistol bullet. It's still going to fry after a second or less if it absorbs even 1% of the incident energy.

True, but the lossless designs might be a viable option. I'm not sure about quantum effects, but in principle the lens itself doesn't pick up any heat. The problem is that because the optical elements take up some of the area you need some kind of shadow mask/radiation shield to protect them, and that will absorb radiation. Based on my limited understanding of the theory the Kumakhov design permits the optical element cross-section to be less than 0.0001% of the total, but building and cooling an accurate x-ray mask is going to be a problem. The lens itself will suffer from the same extreme fragility as weapons grade laser mirrors - any tiny blemish or misalignment will start picking up heat really quickly, which will cause some of the tubes to melt, which will pick up more heat etc until a few milliseconds later your lens is a mass of superheated vapor .

[Starglider]

The problem is that because the optical elements take up some of the area you need some kind of shadow mask/radiation shield to protect them, and that will absorb radiation.

Actually thinking about this you might be able to avoid this problem with a spiralling arrangement of grazing-incidence mirrors somewhat like a jet engine compressor (at least that's what my doodle looks like). That's just idle speculation though - someone familiar with X-ray optics design would have to do the maths to see if it would actually work.

[Wyrm]

It's occured to me that, as long as we're not proposing really soft sci-fi stuff like gravitational lenses and whatnot to focus photon engine exhaust, there would be a tradeoff between efficiency and engine mass. Grazing incidence reflectors, for instance, look massive for the amount of focusing they are capable of, and can only decrease exhaust dispersal at the expense of greatly increased mass.

The exhaust focusing apparatus can only pay for itself in terms of increased thrust if the amount of thrust you add is not counterbalanced by the mass it adds to the engine. This means that the critters in rhoenix's universe will inevidably have to give up some efficiency in exchange for a lighter engine.

[Ender]

you would be better off building clusters of ion engines or something similar

(laughs)

Ion engines operate at an exhaust velocity of around 30 km/s, which while far superior to chemical rocketry, is nowhere near enough to get you to relativistic speeds.

And quasars and radio galaxies have Herbig-Haro Objects that are polar jets hurling planetary masses worth of ionized matter out at relativistic velocities via graviomagnetism. Like I said, if you have the stellar levels of power needed to do this, do things on a stellar level. Fuck, even if you don't we still have particle accelerators that get streams of ions up to relativistic speeds, with todays power and efficiency. Which is vastly superior to that of a photon rocket.

Using Tsilokovsky's equation, in order to get to a mere 1% of lightspeed, for every kilogram of payload (including engine mass), you need 2.688*10^43 kg of fuel. To make it sound really impressive, that's 2,688,000,000,000,000,000,000,000,000,000,000,000,000,000 kg of fuel, or 1,344,000,000,000 solar masses worth of fuel, or ~134 Milky Way galaxy masses worth of fuel. Per kilogram.

I find it interesting that someone trying to be condescending in a rocketry discussion doesn't even grasp either when or how to use scientific notation.

Again true, but I'm not sure that's what Ender meant when he said "ion engines". The 30 km/s ones are far more commonly mentioned, as they are proven technology and have been tested in space already.

Why the fuck would I be talking about something that uses kilowatts when my clearly stated point was that if you can harness massive amounts of power you should do things on a massive scale? Particularily since anyone who passed highschool knows that a NSTAR style ion engine is physically impossible to scale up? Oh wait, there was zero reason to do that unless you are some newbie trying to make himself out to be a badass know it all by taking on a senior person. Of course, it means strawmanning the senior person, but so what?

And the guys name was Tsiolkovsky. Try knowing the name of the fucking formula before laughing at someone. Particularly when you are all fucked up on a number of other levels.