Laser light heat exchanger for an SSTO?

[SomeGuy12]

If you wanted an SSTO with non embarrassing mass fractions, you need higher ISP. You can't do much better than 500 with the most exotic chemical propellants. Nuclear has the problems of the reactor itself being heavy, the shielding, if there is any, being heavy, and the reactor has to be constructed very carefully and there is always a risk of it crashing and leaving a radioactive mess that is difficult to clean up.

So why not construct an enclosed chamber. Quartz or other transparent material is used as a window along one side. There's a stack of plates with holes in them. The plates closer to the window have larger holes, and around each hole there is a ring of material with a surface that absorbs the frequency of laser light you are using (so it probably appears black).

The plates also have heat radiator fins on the back side, opposite where the laser light comes in.

Essentially, you use this apparatus to super-heat hydrogen flowing through the chamber and you send it out the bottom. The plates are made of the same material they make NERVA reactor tubes, so you should be able to reach temperatures that can produce 1000 ISP. You might have an air-breathing phase of flight where ram air intakes are opened to send ordinary air through the chamber instead of hydrogen, so that you don't have to consume onboard propellant.

[fredinno]

[quote name='SomeGuy12']If you wanted an SSTO with non embarrassing mass fractions, you need higher ISP. You can't do much better than 500 with the most exotic chemical propellants. Nuclear has the problems of the reactor itself being heavy, the shielding, if there is any, being heavy, and the reactor has to be constructed very carefully and there is always a risk of it crashing and leaving a radioactive mess that is difficult to clean up.

So why not construct an enclosed chamber. Quartz or other transparent material is used as a window along one side. There's a stack of plates with holes in them. The plates closer to the window have larger holes, and around each hole there is a ring of material with a surface that absorbs the frequency of laser light you are using (so it probably appears black).

The plates also have heat radiator fins on the back side, opposite where the laser light comes in.

Essentially, you use this apparatus to super-heat hydrogen flowing through the chamber and you send it out the bottom. The plates are made of the same material they make NERVA reactor tubes, so you should be able to reach temperatures that can produce 1000 ISP. You might have an air-breathing phase of flight where ram air intakes are opened to send ordinary air through the chamber instead of hydrogen, so that you don't have to consume onboard propellant.[/QUOTE]
How would you get the power to power something like that? Nuclear reactor seems the most reasonable- which brings us back to the NTR problem of being insanely heavy.

[PB666]

[quote name='SomeGuy12']If you wanted an SSTO with non embarrassing mass fractions, you need higher ISP. You can't do much better than 500 with the most exotic chemical propellants. Nuclear has the problems of the reactor itself being heavy, the shielding, if there is any, being heavy, and the reactor has to be constructed very carefully and there is always a risk of it crashing and leaving a radioactive mess that is difficult to clean up.

So why not construct an enclosed chamber. Quartz or other transparent material is used as a window along one side. There's a stack of plates with holes in them. The plates closer to the window have larger holes, and around each hole there is a ring of material with a surface that absorbs the frequency of laser light you are using (so it probably appears black).

The plates also have heat radiator fins on the back side, opposite where the laser light comes in.

Essentially, you use this apparatus to super-heat hydrogen flowing through the chamber and you send it out the bottom. The plates are made of the same material they make NERVA reactor tubes, so you should be able to reach temperatures that can produce 1000 ISP. You might have an air-breathing phase of flight where ram air intakes are opened to send ordinary air through the chamber instead of hydrogen, so that you don't have to consume onboard propellant.[/QUOTE]

Just to correct you, the thermnuclear gas heated rocket is limited by its internal temperature limit of about 4000 degrees and the nuclear fuel density required to prevent overheating. The problem is that limits the amount of gas that can be heated and accelerated, and although it creates a higher ISP the thrust is much lower. In addition because it has a higher operating temperature it is rather restricted to heavier metals. The atm ISP is a much lower ratio og vacuum ISP and the engine irself is not safe for atmospheric use. THerefore as a launch engine its a nogo for just about every reason.

The drive you describe, much better designed is the VASIMR is a deep space engine.


As has been discussed before energy production densities in space are petty relative to land versions, exceptionally are oxidation reduction reaction which produce uncoverted heat-thermal expansions. The VASMiR with its 5 newtons of thrust is a 200 kw. That is essentially 40kw per newton, this will effectively limit any addon production of thrust to the electrical energy production density per unit mass. Since solar panels cannot be deployed during launch phase, batteries are not weight effective, you are left with piddly nuclear reactors for craft that would have to have nuclear during deep space flight stage. Therefore you still have risk. if you are considering tracking lasers and craft solar panels you are still limited to 50 kw per meter of panel. if you had 100 square meters per ship that is 5000 kw or 125 N, consider how many newtons the average launch engine produces. It would not even suffice to lift the panels.

[SomeGuy12]

It's a heat exchanger. There's a gigantic array of lasers on the ground with mirrors that can keep the spot size small enough to keep the heat exchanger illuminated during ascent. This is for reaching orbit. The payload of the spacecraft contains stuff for maneuvering in space.

You leave the spacecraft power source on the ground...

[PB666]

[quote name='SomeGuy12']It's a heat exchanger. There's a gigantic array of lasers on the ground with mirrors that can keep the spot size small enough to keep the heat exchanger illuminated during ascent. This is for reaching orbit. The payload of the spacecraft contains stuff for maneuvering in space.

You leave the spacecraft power source on the ground...[/QUOTE]

Huh, lasers don't need to be lensed. Thats not your primary problem, a heat exchanger needs a to undergo thermal conversion to electricity. The most efficient method we have is steam conversion, each step in the process adds mass (steam generator, steam lines, return flow pumps, exchanger), slows down the process, produces inefficiencies, and reduces the maximum transfer rate. steam engine works on a cooldown zone on the low pressure side, so there is a maximum amount of hv you can add to the high pressure evaporator side before you exceed the cooling capacity on the low pressure side. And the big problem is that that the condensor plate needs to operate below 100'F. And your rocket works most efficiently below 30000 meters close to the speed of sound, if you examine the ideal gas equation, ambient thermal gas velocity plus mach one and your surface temperature for convectionsl heat exchanger is already up there.


If you are going to heat the exhaust gas directly just remember light carries momentum if you target the exhaust gas you will slow down the differential building pressure and heat back into the nozzle, which will probably explode. Most modern engines drive the fuel through the nozzle to cool the casing down and preheat the gas to the decay point if you laser the gas at the nozzle it overheats and the engine explodes.

If you are going to go this direction, by far a light oil burning turbine is a much much better choice and if you are going to go that route just put the platform on a ring of jet power engines with an electric fan driven exhaust gas removal system, get your thrust directly at least you can get the craft to 20 km and keep the launch boosters. You have turbofans out there that can produce 100,000 ft/lbs of thrust and work to 15,000 meters. You are even going to get remotely close to this level of thrust with a remote powered system, the conversion systems are too too slow. Edited November 18, 2015 by PB666

[Rakaydos]

isnt the OP asking for a Microwave Thermal Engine?

[SomeGuy12]

PB666, what are you talking about?

Lasers do need to be "lensed" (lens and mirrors are 2 alternate ways to do the same thing). In fact, they obey the same laws as any other light source. What do you think [URL="https://en.wikipedia.org/wiki/Laser_Weapon_System"]that [/URL]is? Inside the laser turret, that shiny thing you see is a mirror. Longer range needs a bigger mirror, and you need a range of around 100 kilometers for an orbital launch.

The lasers shine through a window onto a heat exchanger. The (ceramic, metal, whatever) plates in the heat exchanger get hot from absorbing laser light. The propellant flowing over the plates in turn heats up. Rocket roar comes out the bottom. It's the same principle as NERVA, except minus the nuclear reactor.

[PB666]

[quote name='SomeGuy12']PB666, what are you talking about?

Lasers do need to be "lensed" (lens and mirrors are 2 alternate ways to do the same thing). In fact, they obey the same laws as any other light source. What do you think [URL="https://en.wikipedia.org/wiki/Laser_Weapon_System"]that [/URL]is? Inside the laser turret, that shiny thing you see is a mirror. Longer range needs a bigger mirror, and you need a range of around 100 kilometers for an orbital launch.

The lasers shine through a window onto a heat exchanger. The (ceramic, metal, whatever) plates in the heat exchanger get hot from absorbing laser light. The propellant flowing over the plates in turn heats up. Rocket roar comes out the bottom. It's the same principle as NERVA, except minus the nuclear reactor.[/QUOTE]

Refocusing laser is the least of you problems, its a red herring inyour argument. Its like saying you can fix fusion energy using a better water filter on the cantinas kitchen refridgerator. Glass and quartz start melting around 800, they would never survive the heat of explosion. Your idea is far fetched and implausible.

[NuclearNut]

[quote name='fredinno']How would you get the power to power something like that? Nuclear reactor seems the most reasonable- which brings us back to the NTR problem of being insanely heavy.[/QUOTE]
Presumably you would get the energy for the laser from a nuclear reactor on the ground. This of course brings up the problem of "what if we miss".

[Aghanim]

[quote name='PB666']

If you are going to heat the exhaust gas directly just remember light carries momentum if you target the exhaust gas you will slow down the differential building pressure and heat back into the nozzle, which will probably explode. Most modern engines drive the fuel through the nozzle to cool the casing down and preheat the gas to the decay point if you laser the gas at the nozzle it overheats and the engine explodes.

[/QUOTE]

Assuming the gas is relatively inert, like hydrogen, there is nothing for it to react with and hence the engine wouldn't explode, and the momentum of the laser light is negligible compared to the momentum of gas expansion in the heat exchanger nozzle

[SomeGuy12]

[quote name='PB666']Refocusing laser is the least of you problems, its a red herring inyour argument. Its like saying you can fix fusion energy using a better water filter on the cantinas kitchen refridgerator. Glass and quartz start melting around 800, they would never survive the heat of explosion. Your idea is far fetched and implausible.[/QUOTE]

I think you mistyped "[URL="https://en.wikipedia.org/wiki/Fused_quartz"]1700[/URL]" celsius.

Well, if no substance can both take the heat and pressure of a rocket engine and also is transparent to visible light, you could use something transparent to IR or microwaves...

Or don't have a window at all. Heat up a mess of tubes, exposed to the outside, and those tubes have the propellant flowing through. I just don't know how to evenly heat a cylinder of tubes when the laser only shines from one side. (presumably with really clever optical design, you'd make the tubes have all kinds of internal mirrors or something to evenly distribute light)

[Rakaydos]

[quote name='SomeGuy12']I think you mistyped "[URL="https://en.wikipedia.org/wiki/Fused_quartz"]1700[/URL]" celsius.

Well, if no substance can both take the heat and pressure of a rocket engine and also is transparent to visible light, you could use something transparent to IR or microwaves...

Or don't have a window at all. Heat up a mess of tubes, exposed to the outside, and those tubes have the propellant flowing through. I just don't know how to evenly heat a cylinder of tubes when the laser only shines from one side. (presumably with really clever optical design, you'd make the tubes have all kinds of internal mirrors or something to evenly distribute light)[/QUOTE]

I'm going to put this here: [url]https://en.wikipedia.org/wiki/Beam-powered_propulsion[/url]

[SomeGuy12]

[quote name='Rakaydos']I'm going to put this here: [url]https://en.wikipedia.org/wiki/Beam-powered_propulsion[/url][/QUOTE]

Yeah. Those 2 heat exchanger systems mentioned are what I'm talking about. I hadn't seen this wiki article, but this is what I meant.

[PB666]

[quote name='SomeGuy12']I think you mistyped "[URL="https://en.wikipedia.org/wiki/Fused_quartz"]1700[/URL]" celsius.

Well, if no substance can both take the heat and pressure of a rocket engine and also is transparent to visible light, you could use something transparent to IR or microwaves...

Or don't have a window at all. Heat up a mess of tubes, exposed to the outside, and those tubes have the propellant flowing through. I just don't know how to evenly heat a cylinder of tubes when the laser only shines from one side. (presumably with really clever optical design, you'd make the tubes have all kinds of internal mirrors or something to evenly distribute light)[/QUOTE]

Glass is an amorphous solid, it pours at ambient temperture and pressure, you dont see it however unless you are patient to stand still for a couple of hundred years. You can add atoms to glass that make it pour less quickly. Howevere, i work with glass in the lab so i can tell you that if you dim the lights, we have burners that with pressurized air burn an almost invisible blue, the moment it starts to glow the reddist of reds ever it can be deformed. In the high pressure of a rocket engine at 700 to 800'C it would begin to start swelling and eventually ball out and open up, not quickly, but for optical purposes it would destroy the lensing effect. In addition the glowing you see are ions, this will destroy transmission of the glass.

glowing red hot temperature of glass
[url]https://en.m.wikipedia.org/wiki/Incandescence[/url]

We can simluate that pressure in the laboratory which i routinely used to do for composition analysis, either by creating a vacuum, 14psi, or placing under nitrogen pressure. In fact it is quite difficult because you have to control the temperature of the glass to keep it from imploding, it has to stay or below the fusing temperature. Bizarre as it sounds if i tried to push the glass it would shatter, but under vacuum the small of motion that the vacuum creates on all the glass molecules allows one to slowly bend the glass without breaking it. And by turning the glass one can seal off one end of a tube from the other end creating a permanent vacuum.

[url]https://en.m.wikipedia.org/wiki/Glass_fusing[/url]

The important thing that the burner is not a rocket engine, its a tiny 1/4 inch burner using methane and less than 1 psi of air. IOW its on the low end of heat production.

[fredinno]

[quote name='Aghanim']Assuming the gas is relatively inert, like hydrogen, there is nothing for it to react with and hence the engine wouldn't explode, and the momentum of the laser light is negligible compared to the momentum of gas expansion in the heat exchanger nozzle[/QUOTE]
"Hydrogen"- "Inert" :D

[SomeGuy12]

[quote name='fredinno']"Hydrogen"- "Inert" :D[/QUOTE]

There has to be oxygen or it is inert. Yeah, during flight in the lower atmosphere, it's super bad.