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Post by apophys on Feb 13, 2017 21:39:33 GMT
- The ship relies on cooling by evaporating hydrogen. This hydrogen needs to be pumped through the whole ship. If you don't your bubbles of boiling hydrogen won't get anywhere, they will remain where they originated spreading heat around and creating hot spots you want to avoid. I very much doubt that it takes only two kw to move all that hydrogen around through piping and gas separators and provide power for the crew module and on-board systems like the author claims.
- I don't understand where the author gets that 8km/s of exhaust velocity, he just says that by heating hydrogen to 3000K exhaust velocity can reach 8km/s. If you were wondering, you can find the formula for exhaust velocity here (equation 1.22). We don't know all the variables though, like pressure in the furnace. However, it is quite obvious that the engine is 100% efficient in the article (I mean the lenses and the furnace). All sunlight that strikes the lenses get into the furnace. There are no internal reflection and no heating of the lenses. All energy that got into the furnace is used o heat hydrogen. The walls don't store a single watt. I guess that the author figured that all the boiled hydrogen that was used to cool the components of the system will get dumped into the furnace anyway and then...
- Well, as you correctly point out the second law of thermodynamics says heat cannot be destroyed, only moved around. So if the hydrogen exhaust is cooled to 22K, all the heat is moved to the nozzle and since the nozzle is cooled it gets dumped into the remaining hydrogen. The problem is clear now: since your hydrogen reserve is dwindling and the amount of heat you need to store is ever increasing at some point you will have all your hydrogen boiled off, but you still haven't spent it all for thrust and your ship is getting above 22K since you no longer have anything colder than that. The former has to occur before the latter or you will break thermodynamics, but how much hydrogen you will have spent for thrust by then is worth calculating. And don't forget the heat from other sources like on-board nuclear reactor and habitat is stored into the hydrogen as well.
In other words, the whole concept can be rephrased as "the ship uses its remaining propellant as a heat sink". There is nothing more to it. 1. Consider this setup. The walls of the cylinder rotate for (low but nonzero) artificial gravity. The hydrogen propellant is right up against the walls. In this case, the less-dense hydrogen gas boils off and rises toward the spin axis all on its own. The pressure of gas in this large chamber pushes it towards the nozzle. If you are not actively thrusting, this pressure builds up a bit. There is no pumping whatsoever involved (except the injector at the nozzle). The power produced onboard is only for systems like life support, sensors, and controls. 2. The hydrogen NTRs that we have get 9 km/s. So 8 km/s is actually a fairly low estimate, and I suspect you could hit 11 km/s (around our resistojets) without much issue. Engine walls would of course have to be regeneratively cooled with the gas that is about to be spent, probably with multiple layers of regenerative cooling. 3. Expanding gas cools due to its expansion (work is performed). How much heat is transferred to the bell, idk, but certainly not all of it. A very high expansion ratio will be needed. I normally make things with ~50 K exhaust, and quick testing shows that 22 K is feasible. I have no idea how visible that plume would be.
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Post by apophys on Feb 13, 2017 22:00:36 GMT
2. The heat source is irrelevant and the Fresnel lens unnecessary. A nuclear heatsource could be substituted, assuming it can handle the high temperature. While technically true, you need to deal with the sunward surface somehow, so putting the sun's heat into the exhaust and avoiding the need for some of the cooling makes a lot of sense. If you use a resistojet or NTR, you will be boiling off your hydrogen significantly faster.
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Post by shiolle on Feb 13, 2017 22:04:27 GMT
2. The heat source is irrelevant and the Fresnel lens unnecessary. A nuclear heatsource could be substituted, assuming it can handle the high temperature. In other words you're saying that the hydrogen steamer is unnecessary and you can use a regular hydrogen NTR, right? 3. The rocket nozzle is isobaric in operation. Much like a CO2 or Air Tank will get very cold when discharged quickly. The nozzle doesn't need to accept anything but incidental radiant heat during the process. The nozzle and engine must be insulated from the rest of the craft and actively cooled externally for obvious reasons. You are right of course. However the scenario I've described is still valid since you can't make all the heat do useful work. The heat will still accumulate within the spacecraft, only the amount of propellant left when that all the hydrogen turns to gas will be lower than I thought. Still, the amount of heat transferred to the nozzle won't be trivial at all because in reality it isn't a perfectly efficient isentropic process. It seems you are mistaken that this transfer will be done only through radiation. Since there is no vacuum separating the nozzle from the flow of propellant the heat will be transferred through convection and, if I'm not mistaken, generated through friction as well.
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Post by ross128 on Feb 13, 2017 22:13:08 GMT
The problem with the nozzle is not whether the gas will cool as a result of expansion. It will, the problem is it won't do so instantaneously. After all, if rocket exhaust expanded and cooled instantaneously as soon as it had the opportunity, you wouldn't have a hot exhaust plume in the first place and wouldn't even need any kind of special nozzle (space is quite voluminous and low-pressure).
As a result, that "incidental radiant heat" will be quite a lot. It's likely to be exactly as much as a naked exhaust plume would have radiated, since that nozzle isn't allowing any more expansion than what the vacuum of space would have allowed anyway.
Though actually, if they are looking to use gas law effects to alter the plume's temperature, a rapid drop in pressure would be more desirable than an increase in volume at a constant pressure. Partially because, as PV=nRT implies, temperature is positively proportional to both pressure and volume. If you try to hold pressure constant but raise the volume, the PV product will get *larger*. The situation with a CO2 tank is caused by a fixed volume (the tank) that has a rapidly dropping pressure, causing the PV product to shrink. The energy of that heat of course is going into the work of pushing the gas out of the tank.
So to expand the volume of the exhaust gas by 150x while holding pressure constant, they would actually have to dump quite a bit of heat *into* the gas, meaning it will be quite hot when it leaves. Once it leaves the nozzle its pressure will drop precipitously (as it would have anyway, entering the vacuum of space) and the gas will rapidly cool of course... by means of radiating that heat into space.
They're going to have to try harder if they want to mask a 3000K exhaust plume.
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Post by apophys on Feb 13, 2017 22:15:47 GMT
Since there is no vacuum separating the nozzle from the flow of propellant the heat will be transferred through convection and, if I'm not mistaken, generated through friction as well. This probably can be significantly reduced using magnetic repulsion of the hot hydrogen plasma.
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Post by shiolle on Feb 13, 2017 22:31:35 GMT
This probably can be significantly reduced using magnetic repulsion of the hot hydrogen plasma. I think you mean using permanent magnets rather than magnetic coils. Those will have to be massive. Also, the whole nozzles are supposed to be vectoring. I'm not sure it's worth it.
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Post by apophys on Feb 13, 2017 22:33:22 GMT
So to expand the volume of the exhaust gas by 150x while holding pressure constant, they would actually have to dump quite a bit of heat *into* the gas, meaning it will be quite hot when it leaves. Once it leaves the nozzle its pressure will drop precipitously (as it would have anyway, entering the vacuum of space) and the gas will rapidly cool of course... by means of radiating that heat into space. They're going to have to try harder if they want to mask a 3000K exhaust plume. Where are you getting the idea that pressure is constant? The pressure drops dramatically as it passes through the nozzle, not simply after it leaves. Please take a look at any ingame thruster. The color gradient is the temperature drop. The exhaust that leaves is much cooler than the heated portion. 22 K exhaust is achievable. I'm certainly not an expert on the subject, but the game is clearly contradicting your statement.
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Post by apophys on Feb 13, 2017 22:35:21 GMT
This probably can be significantly reduced using magnetic repulsion of the hot hydrogen plasma. I think you mean using permanent magnets rather than magnetic coils. Those will have to be massive. Also, the whole nozzles are supposed to be vectoring. I'm not sure it's worth it. No, I mean superconducting magnetic coils. After all, you have cryogenic coolant at 22 K freely available.
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Post by ross128 on Feb 13, 2017 22:38:16 GMT
So to expand the volume of the exhaust gas by 150x while holding pressure constant, they would actually have to dump quite a bit of heat *into* the gas, meaning it will be quite hot when it leaves. Once it leaves the nozzle its pressure will drop precipitously (as it would have anyway, entering the vacuum of space) and the gas will rapidly cool of course... by means of radiating that heat into space. They're going to have to try harder if they want to mask a 3000K exhaust plume. Where are you getting the idea that pressure is constant? The pressure drops dramatically as it passes through the nozzle, not simply after it leaves. Please take a look at any ingame thruster. The exhaust that leaves is much cooler than the heated portion. 22 K exhaust is achievable. I'm certainly not an expert on the subject, but the game is clearly contradicting your statement. I generally expect the pressure to drop in the nozzle as well, of course. The claim that the nozzle is somehow isobaric (they never explain what mechanism they use to achieve this, by the way) is theirs, not mine. It is also, if I am understanding correctly, counterproductive.
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Post by shiolle on Feb 13, 2017 22:47:47 GMT
No, I mean superconducting magnetic coils. After all, you have cryogenic coolant at 22 K freely available. What is not free is heat to electricity conversion which makes you expend that cryogenic propellant, so it's an open question which is more effective: to cool a regular nozzle or to cool a reactor powering those coils. Edit: I generally expect the pressure to drop in the nozzle as well, of course. The claim that the nozzle is somehow isobaric (they never explain what mechanism they use to achieve this, by the way) is theirs, not mine. It is also, if I am understanding correctly, counterproductive. I think Easy meant isentropic, not isobaric since it's a common assumption about de Laval nozzles.
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Post by apophys on Feb 13, 2017 23:08:13 GMT
The claim that the nozzle is somehow isobaric (they never explain what mechanism they use to achieve this, by the way) is theirs, not mine. It is also, if I am understanding correctly, counterproductive. Direct quote from that hydrogen steamer page: "A de Laval nozzle allows the propellant to expand before it leaves the engine." en.wikipedia.org/wiki/De_Laval_nozzle (basically the same as ingame nozzles, pressure and temperature drops normally) Referring to it as isobaric is a mistake by them. I.e. the design is sound; the explanation is poor. EDIT: Space ninja'd. What is not free is heat to electricity conversion which makes you expend that cryogenic propellant, so it's an open question which is more effective: to cool a regular nozzle or to cool a reactor powering those coils. As far as I'm aware, the electricity required to maintain a field in a superconductor is vanishingly low. Point taken, though.
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Post by deltav on Feb 15, 2017 0:15:00 GMT
The main problem I see with the much-vaunted "hydrogen steamer" is that it's not a practical rocket. Although the article claims 8km/s of exhaust velocity (with no math to back it up), I find it extremely unlikely that such a velocity can actually be achieved without a correspondingly high exhaust temperature. They claim to solve this problem by using an extremely large nozzle to give the gas room to expand and cool before leaving the craft, but all that achieves is giving you a white-hot nozzle the same size as what your exhaust plume would have been. After all, expanding and cooling is exactly what the gas would have done *after* leaving the nozzle anyway. Covering the necessary expansion volume with the nozzle only shifts the problem from a 3000K exhaust plume to a 3000K nozzle, one way or another you're going to have to radiate that heat eventually. It stands to reason that either the hydrogen steamer isn't actually as stealthy as advertised during a burn, or its exhaust velocity isn't nearly as high as advertised. And that's besides the fact that a significant factor in detection is time. Even with the best stealth imaginable the question usually isn't *if* you get detected, but *when*. If you're going to need several months of burning to initiate a transfer and several more months of burning to complete an insertion, that's a huge window where you can get caught up in an asteroid-monitoring sweep while your engines are on. And after all that, even if it's lucky enough to evade detection for all the years it will take to get to its destination, the war might be over by the time it arrives because the side with conventional ships has already attacked and won. Yep Ain't no stealth in themthere otta space. www.projectrho.com/public_html/rocket/spacewardetect.php
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Post by deltav on Feb 15, 2017 0:27:14 GMT
Assuming the traders use an MPD drive, which necessitate a fairly strong nuclear reactor, I can see them becoming the escorts themselves - by buying and adding a laser to their merchant ship, making use of that spare reactor power. In fact, since there is this synergy between trade ships and both laser (MPD drives necessitate high reactor outputs anyways) and kinetic (MPD drives allow one to accelerate a lot of mass to really high speeds) combat modes, the combatants themselves may very well be rival merchant trade federations, seeking a monopoly over the solar system... This may result in a fairly militarized society, where the distinction between civilian and military is fairly thin.... This is what I was thinking. I started playing around with militarized tankers as many of my heavy carrier ship designs trade delta v to have more armor and capacity. They have heavy armor and much upgraded engines so they can keep up with their military ship, but they have minimal weapons, really just enough for them not to run away. I found that to keep your tankers with your warships, you HAVE to armor them, and give them extra radiators as well as a small gun (or they run away as soon as the shooting starts). Attachments:
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Post by Easy on Feb 15, 2017 0:29:08 GMT
I generally expect the pressure to drop in the nozzle as well, of course. The claim that the nozzle is somehow isobaric (they never explain what mechanism they use to achieve this, by the way) is theirs, not mine. It is also, if I am understanding correctly, counterproductive. I think Easy meant isentropic, not isobaric since it's a common assumption about de Laval nozzles. I used the term isobaric because it was the term used in the article. I welcome the correction that the flow is isentropic and and adiabatic. Or it would be adiabatic, but there will always be some heat transfer.
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Post by Easy on Feb 15, 2017 0:41:00 GMT
Assuming the traders use an MPD drive, which necessitate a fairly strong nuclear reactor, I can see them becoming the escorts themselves - by buying and adding a laser to their merchant ship, making use of that spare reactor power. In fact, since there is this synergy between trade ships and both laser (MPD drives necessitate high reactor outputs anyways) and kinetic (MPD drives allow one to accelerate a lot of mass to really high speeds) combat modes, the combatants themselves may very well be rival merchant trade federations, seeking a monopoly over the solar system... This may result in a fairly militarized society, where the distinction between civilian and military is fairly thin.... This is what I was thinking. I started playing around with militarized tankers as many of my heavy carrier ship designs trade delta v to have more armor and capacity. They have heavy armor and much upgraded engines so they can keep up with their military ship, but they have minimal weapons, really just enough for them not to run away. I found that to keep your tankers with your warships, you HAVE to armor them, and give them extra radiators as well as a small gun (or they run away as soon as the shooting starts). I don't think kinetics are a wise choice on the necessarily large cross section tankers and cargoships. They might work as a point defense with lasers, but the very last thing you want to do is get in gun range of a hostile. Drones and missiles are very much like docked escorts and they can do very convenient things like forcing an engagement away from the fat transports. Especially useful if the cargoship is already transporting a military shipment of drones and missiles. In regards to armor, aerogel is good against lasers and nuclear flashes, but for a very large vessel you may choose to only armor the crew, power and propulsive modules. The crew might even appreciate something that can stop a few sandblaster rounds. In general your need for escorts will depend on what you expect your opponent to bring. But if the transports are carrying something important, the last place you want them to be is on the firing line.
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