Steam Propulsion to Space

Why don’t space agencies use steam jets in the thermo/ionosphere? Both layers are hot. Just compress the air until it’s dense enough to transfer heat, inject some water, and voila—steam. Seems simple, right? What could be the challenges with this approach?

Here's a question for you: How do rockets work if there's no oxygen in space?! Hell, have a second one, how is the sun on fire if there's no oxygen in space?!

>>11548 That's actually a plane.

>>11550 My idea was to build a space plane with a single stage to orbit scramjet with auxiliary compressors for low-speed flight (think Dyson fan), supplemented with oxidizers for higher altitudes.

>just compress it; its already hot because radiation its vanishingly thin. to collect a significant amount you'll have to already be moving incredibly fast through it. Your proposal doesn't even need collection of air, you need collection of energy from heat. Most rockets simplify things by carrying their energy with them. Why water ? There's a lot of things you could evaporate I'm not sure why water would be preferable to anything else. You'll want something, liquid or even solid, that can vaporize at a good temperature without too much energy and expand massively, but is light. Does water meet these requirements better than any other compound ? idk Maybe something like this would be useful as like a maneuvering engine during situations where you're gonna have a lot of excess heat anyway. Reentry craft or high supersonics or whatever where you're getting a tonne of friction. >>11553 aren't dyson fans just electric motors. Gonna fly an SSTO with a ducted fan starter engine to get it up to IDK mach four or something so you can kick in the scramjet cum rocket engine for the rest of the flight ? Gudluq >>11549 things ~they~ dont want you to think about

>>11548 Assuming you're serious and not trolling: The pressure at, say, 400km, is ~10^-11 kg / m^3. Specific heat capacity of air is ~1 J / g / K. So, how much total thermal energy is there per m^3, if the gas is at 1500K? Simple enough calculation: 10^-11 kg / m^3 * 10^3 g/kg * 1 J / g / K, giving 10^-8 J/m^3. Or 10nJ/m^3. Oops. Rockets fly supersonic, because they have to to achieve orbital velocities. Unfortunately what that means is that a rocket flies faster than the 'speed of information' in the atmosphere. There isn't really any way to suck in air from the side, because by the time the air at the side 'sees' that there's a vacuum to one side you're already long past. And the vast majority of the cost of a rocket is at supersonic speeds on the way to getting up to orbital velocity. Going high and slow is comparatively easy. It costs about 45 kilojoules to accelerate 1kg to 300m/s. It costs about another 30 megajoules to get from 300m/s to orbital velocity (7.7km/s). In other words, getting to 300m/s gets you about 0.15% of the way there. Even the cost of lifting that 1kg to 400km (~4MJ) isn't that significant compared to that 30MJ/kg to get to orbital velocity. So. You're supersonic, most of the way. While you're supersonic, you're limited by the frontal area of your intake. Maybe a little bit more, but not much. (You can cheat a bit if you can maintain a shockwave slightly outside the craft - but how far out you can maintain a shockwave is limited by how far information can travel between the front of the shockwave in front of your craft arriving and your scoop passing by. This is a minimal effect at best.) So. Starship has a cross-sectional area of ~65m^2. Let's be optimistic and say we have a scoop with 200m^2 of effective frontal area, somehow. Somewhere around the size of a singles tennis court when viewed from the front/above. Cancelling units, that means that we're getting ~2 uJ / m (micro-Joules per meter) of total thermal energy entering our scoop. Amusingly, because of the way the units cancel out, 1 J/m is equivalent to 1 Newton in this context. (If you're going slowly, you require only a little additional kinetic energy to go a little faster... but you're going slowly so you aren't scooping up that much per second. If you go quickly, you're scooping up more per second, but you require more kinetic energy to go a certain amount faster. As it turns out, these two effects cancel each other out.) So how much force are we getting out of our scoop, were we somehow to magically convert _all_ thermal energy entering the scoop perfectly to kinetic energy of the rocket? 2 uN. This is not quite enough force to lift a single small grain of sand against Earth's gravity.

>>11553 This is a project currently more or less being built right now, see picrel. I'm not going to delve into the real specifics of it, but most conventional rockets are like 80% fuel. 75% of the mass of the Saturn V was just the propellant to get it to space. The problem is that you can't get enough propellant mass to actually compress for a reasonable payload. The main problems are that >Water is heavy, so you still need propellant or fuel to reach altitude >Atmospheric density decreases with altitude so you get less compression the higher you go >Less compression means less efficiency, which means more propellant, see the first point The problem is that unlike aircraft, which use aerodynamic principles like lift to fly, rockets use brute force to push upwards. Rockets pretty much already produce steam as a byproduct of whatever Hydrogen or Liquid Oxygen type reactions they use. China just uses Hydrazine. >>11556 Is partially correct. There is a transitional phase in fluid dynamics which affects the effective exhaust velocity of the nozzle, where compression becomes supersonic instead of subsonic. There is also the concept that the ambient pressure affects the efficiency of the rocket exhaust stream, which is why you get different shape profiles, and we don't have adjustable nozzle shapes that we can use. If the geometry of the exhaust stream is too inefficient for the ambient pressure/altitude, you either loose propulsion, or you get detonations which can blow up the rocket. IIRC the soviets had this problem which hindered their program, I think the Americans solved this problem by putting grating into their engines to help smooth the propulsion profiles and eliminate undesired internal detonations. Thermodynamic engines are by far the most soykafty type of rocket. Ion and plasma thrusters are extremely efficient, but they produce little amounts of thrust. One of the more promising compromises is to just use rotating detonation in rockets, but that is also a poorly understood field. tl:dr, rocket propulsion is more of a fluid dynamics problem, and the specific impulse of a water heated engine would be too low.

>>11555 >aren't dyson fans just electric motors As are most consumer fans. I envision the compressors would be turbine-powered. >Gonna fly an SSTO with a ducted fan starter engine to get it up to IDK mach four or something so you can kick in the scramjet No, a full-sized compressor ducted to CD nozzles along the perimeter would not impede the operation of a ram/scramjet. I presume ramjets cannot operate at higher speeds due to overheating due to compressing the air to subsonic speeds, but water/methanol injection would indeed solve that issue.

>>11557 >some autist ITT was able to build a better than glowies nice to know

>>11557 > This is a project currently more or less being built right now, see picrel. Very different concept. SABRE burns O2 from the atmosphere with H2 on board at low altitudes and speeds, to somewhat oversimplify. 28 km up instead of 400+. Works out to something like a billion times the atmospheric pressure, which is why this isn't quite so batshit insane as OP. > There is a transitional phase in fluid dynamics which affects the effective exhaust velocity of the nozzle Absolutely; this can only reduce the effective thrust, not increase it. Thermodynamics sets the upper bound here. Also the Skylon D-1 is hilariously optimistic. When the fuarrrking AFRL calls your design "technically very risky" you've got a problem.

>>11559 That's not the main problem with ramjets at high speeds. A ramjet cycle is, roughly: 1. Adiabatic compression of intake air. 2. Inject a bunch of energy by burning fuel. 3. Adiabatic expansion of air (+ combustion products). 3 is essentially the reverse of 1. You do lose some energy, but for the most part things balance out nicely enough. The intrinsic problem with ramjets at high speeds is above mach 6 or so the energy release at the intake from slowing the air down to subsonic speeds is so high that you start breaking gas molecules. Which means that instead of nice efficient adiabatic compression you start wasting a lot of energy making e.g. NOX. (There are surmountable engineering challenges before this - designing a nozzle that'll slow mach 6 air to subsonic speeds without instabilities is challenging all on its own, for instance - but those aren't as much of an intrinsic problem.) Scramjets are better because they don't need to slow down the air as much, so they aren't dumping as much energy into the intake air, so they don't hit this problem in the first place. Of course, scramjets have other issues... Injecting water or methanol into the intake doesn't actually help here. Instead of wasting a lot of energy making NOX, you waste a lot of energy turning water into steam. It'll help with thrust, same as a jet engine. But it doesn't help efficiency of a ramjet.

>>11567 >start breaking gas molecules which is purely a heating effect that could be mitigated by water/methanol injection. >you waste a lot of energy turning water into steam >It'll help with thrust, same as a jet engine. But it doesn't help efficiency of a ramjet If compressing air hot enough to make NOx's without combustion, the engine's already running pretty efficiently. Add that the air-fuel mixture is probably being homogenizing by injecting in the intake ram, and you have the potential for NOx-free HCCI combustion. There you go, a ramjet that can fly even faster.

>>11569 >water is not combustible I think his idea is that a big hot thing can quickly boil water, causing it to rapidly expand in volume. You can make a rocket out of that, whether its a good idea is another matter, and obviously the air up there is pretty thin.

>>11569 As previously stated, I presume ramjets cannot operate above Mach 6 due to overheating issues. Water/methanol injection could overcome this issue. To get it up to Mach 3, I proposed a turbo-ramjet hybrid with a compressor housed out of the way of the intake ram. The compressor would feed jet nozzles around the perimeter of the intake ram that would direct the air through the ramjet.

>>11571 have you done the math for how much heat you need to bleed off, how much coolant you'd need to ablate to do it etc etc etc ? such coolants are heavy, you're getting a little thrust out of them but probably not that much, seems like it might be a lot of mass, both fuel and equipment.

>>11570 I found some research studies talking about steam rockets. I'm not sure why so many daredevils and stuntmen like using them, lmao Most of the projects just heat the steam, I could not find any reasonable use of water injection onto a radiative thermal surface while in flight. For this to work, you would need a lot of water, since the specific impulse of water is low, and water weighs a lot. I guess the best way to handle it would be to try to use atmospheric friction to collect heat, then somehow transfer that heat through a radiator or something to the water to heat it. You would probably get about as much force out of it as a tea kettle tho. >>11572 This, the volumetric expansion is not great. From doing a little research, it seems like the largest possible use is making do with the soykafty amount of thrust you get from steam by hooking it up to a nuclear reactor and boiling off water. Supposedly it would be practical for interplanetary travel because ice/water is easy to find in the solar system in a usable state for propellant.

>>11572 >Such coolants are heavy But the thrust still exceeds its weight at atmospheric pressure. >>11573 >For this to work, you would need a lot of water, since the specific impulse of water is low Not so, water has among the highest latent heat among the known compounds, so you don't need a lot of it to have a significant cooling effect. Add that the latent heat increases as the pressure in the ram increases (which increases with speed), and you've got water with an extremely high specific impulse.

>>11574 >but the thrust still exceeds its weight at atmospheric pressure you've got to lug it up tens of thousands of feet and to mach six first before its useful though.

>>11575 >you've got to lug it up to mach six first before its useful though. Space catapult