KerikBalm Posted August 25, 2014 Share Posted August 25, 2014 Given how awesome the sabre engine is...and given how awesome a nuclear thermal rocket could be...and given that nuclear jet engines have been made...How feasible would it be to combine them into a single, nuclear SSTO?Given the stats that project Timberwind was hoping for (30:1 TWR, 1000 s ISP), I suspect that alone would be good enough for an SSTO.http://www.astronautix.com/engines/timnd250.htmOf course, hydrogen is rather expensive and hard to store... it sure would be nice to use the atmosphere as re-mass as much as possiblehttp://en.wikipedia.org/wiki/Project_PlutoI wonder if the pre-cooler technology of the SABRE engine could be made to work with a thermal-nuclear rocket core, to get an even higher payload fraction.Ie: Pump NH3 through the reactor during liftoff (more thrust, at expense of lower ISP, more mass efficient tankage, easier storage), accelerate to ramjet speeds, stop the flow of NH3, instead use liquified air as the saber precoolers are supposed to do, when air tapers off, pump H2 through it the rest of the way to orbit.The reactor wouldn't run that much for each trip, but would eventually get quite hot... reprocess the fuel rods? Once in orbit, transfer the reactor to a waiting tug for interplanetary or lunar missions (so the "tugs" are all running on old, "hot" reactors, the SSTOs are all running on newish, not-yet-that-hot reactors).Would such a craft be feasible with near-future technology?I've previously been really enthusiastic about fusion technology (Dense plasma foci, Polywell reactors, etc, NOT ICF or toruses like ITER)... but the more I think about it... the power density one can get with plain old fission is enourmous.. some of these compact reactor designs are putting out several gigawatts of power. Even under "ideal" or "optimistic" projections of power output for fusion ractors (like say... 1 gigawat reactor with a 3m diameter core) would be far worse, and would struggle to acheive orbit (either too little thrust, or increasing thrust lowers ISP too much, or chemical power would need to be used, like a LOX Augmented Nuclear Thermal Rocket - where the nuclear we're refering to is Fusion reactions).If you want to go interstellar with long burn times, out to significant fractions of the speed of light, fusion is what you want...But it seems fission is the way for getting to and from orbit... its a shame we can only launch from 1 inhabited planet where the dangers of flighing fission reactors around is so great. Link to comment Share on other sites More sharing options...
Streetwind Posted August 26, 2014 Share Posted August 26, 2014 (edited) The precooler that makes the SABRE so special is simply an air intake designed for hypersonic speeds. It could easily work with a thermal turbojet.The thing is though, while gas turbines are some of the most lightweight engines we have, nuclear thermal engines on the other hand are probably the heaviest by a large margin. This isn't something you can engineer away, either - it's the simple fact that any supercritical amount of uranium is heavy, and the radiation shielding is even moreso. You may be running with better Isp, but your whole spaceplane would have to be constructed differently than Skylon is, with higher structural masses for rigidity, which in turn requires more wing area, which weighs more and creates more drag... How much of the higher efficiency is lost to that? And getting it man-rated would be a nightmare, I imagine.Project Timberwind is one of those things that make me say "Where would we be today if Project Timberwind had finished successfully", but the thing is: it didn't. It was shut down after an audit was conducted on it, which is probably a telltale sign that something wasn't quite right under the hood. Also, the engine stats given for these Timberwind engines are paper stats at best, pure speculation at worst. No engine in flight configuration was built, to my knowledge, and there are no sources cited by this website... nor by Wikipedia, which is in my opinion the bigger indication that there's little to no substance to them. Wikipedia isn't always correct, but they tend to make every effort to cite sources. When they don't, that probably means there is no source in the first place.EDIT: Checking the SNTP (Timberwind followup program) final report cited on Wikipedia, as far as I can tell the SNTP program was canceled in the middle of qualifying the technologies required to build a NTR of the desired specifications. In other words, they weren't even ready to built a NTR yet, they were still researching how to build parts sturdy enough for the job. The report did say that it was believed the challenges would be solvable, but does so in a kind of petulant tone, never missing to complain about funding cuts in the same breath. Also, it seems that the specs given for the Timberwind engines were a project goals wishlist, much in the same way the politicians handed NASA a wishlist of "build us a 130 ton payload rocket using only 40 year old parts and manufacturing technologies". Except that the SLS is still a more realistic proposition than a 30 TWR nuclear engine, because all the congress-prescribed cost insanity aside, the technology has already been proven.In the executive summary, it is suggested that the closest the project came to a real NTR test article was a paper draft for an engine believed to be potentially possible, with an Isp of 930 and a TWR of 20 (as opposed to the Isp 1000 and TWR of 30 of the Timberwind engines). They didn't get around to actually trying to make it happen though, and thus we don't know if it would actually have been feasible. At the time of cancellation, they were still an estimated five years away from being ready to build it, and that's assuming no unexpected delays.The detailed portion of the document probably has more info, but it's also very long and I don't have that much time right now while at work. Edited August 26, 2014 by Streetwind Link to comment Share on other sites More sharing options...
K^2 Posted August 26, 2014 Share Posted August 26, 2014 Ions are much heavier than NTR. But yeah. I don't think NTRs have the necessary specific thrust to be useful for SSTOs. And once you're in orbit, SABRE isn't practical.Russians are currently working on an NTR-powered ship. Albeit, with a twist. They have an electrothermal rocket powered by a nuclear reactor. Apparently, they've had some success, but they're still as far from actually flying it as NERVA ever was. And, of course, it's not designed for exiting atmosphere, but rather be delivered to orbit, fueled up, and used for interplanetary exploration.But it's a Russian project, so it might be just vaporware. Link to comment Share on other sites More sharing options...
KerikBalm Posted August 26, 2014 Author Share Posted August 26, 2014 "I don't think NTRs have the necessary specific thrust to be useful for SSTOs"Specific thrust? I think you mean specific impulse?Even going with the old NERVA from the 70s... upwards of 800 ISP, TWR of what.. 3:1? (just for the engine?, not the entire proposed nuclear stage)http://en.wikipedia.org/wiki/NERVA#NERVA_rocket_stage_specificationsISP vacuum: 850M_0 / M_1 = 5.24189.8*850 * ln 5.2418 = 13800 That is more than enough to achieve orbit, but its the low sea level ISP and the low TWR that are the problem.Adding SABRE precoolers and running it as a combined thermal rocket/turbojet will take care of the first ~1,000 m/s of dV, get you up where the ISP is better, and a pair of wings should allow you to get around the relatively poor TWR... Maybe develop it into a LANTR (LOX-augmented Nuclear Thermal Rocket.) for takeoff... The NERVA of the late 60's could be significantly improved too. Even the Dumbo reactors would be great.You wouldn't even need half the TWR of timberwind for this to work, no?"This isn't something you can engineer away, either - it's the simple fact that any supercritical amount of uranium is heavy, and the radiation shielding is even moreso."Well, you can engineer a lot of it away... the more enriched the uranium is, the less of it you need to go critical, add berrilium neutron reflectors to make subcritial masses go critical... the radiation is a bit of a problem, but sitting on the launch runway, with the reactors never fired up yet, there'd be no need for radiation shielding.Running it for a few minutes at high altitude would really only require a shadow shield for the occupants, no radioactive particle would be thrown out.However, after repeated use, it would be quite hot... as I mentioned in the first post. Link to comment Share on other sites More sharing options...
K^2 Posted August 26, 2014 Share Posted August 26, 2014 (edited) KerikBalm said: "I don't think NTRs have the necessary specific thrust to be useful for SSTOs"Specific thrust? I think you mean specific impulse?No, I most certainly do mean specific thrust, which figures into rocket's overall TWR. (Not to be confused with specific thrust of a turbine, which is something completely different.)Specific X means "X per unit of ...". The blank part can be a little ambiguous, but given units, weight/mass of the rocket engine is the only thing that makes sense in the context.I'll just say "TWR" next time to avoid ambiguity. Edited August 26, 2014 by K^2 Link to comment Share on other sites More sharing options...
Nuke Posted August 26, 2014 Share Posted August 26, 2014 (edited) specific thrust is thrust/mass flow. how much thrust you get for each unit of propellant mass injected into the engine. this being high allows things like rockets that go up instead of just sit on the pad and go nowhere. source: google specific thrust and click the first link (its nasa). Edited August 26, 2014 by Nuke Link to comment Share on other sites More sharing options...
K^2 Posted August 26, 2014 Share Posted August 26, 2014 Nuke said: specific thrust is thrust/mass flow. how much thrust you get for each unit of propellant mass injected into the engine.That's specific impulse. As in, that's precisely the definition of specific impulse. Link to comment Share on other sites More sharing options...
Sky_walker Posted August 26, 2014 Share Posted August 26, 2014 Specific thrust: https://en.wikipedia.org/wiki/Specific_thrust Link to comment Share on other sites More sharing options...
Red Iron Crown Posted August 26, 2014 Share Posted August 26, 2014 Project Pluto is relevant here, a nuclear powered ramjet. Link to comment Share on other sites More sharing options...
meve12 Posted August 26, 2014 Share Posted August 26, 2014 Red Iron Crown said: Project Pluto is relevant here, a nuclear powered ramjet.I think a nuclear jet that didn't irradiate everything under it's flight path would be slightly more desirable.On topic: I'm not sure pumping so many different gasses through one engine over the course of the flight would be good for it's health. Is the precooler incapable of operation at subsonic speeds? Link to comment Share on other sites More sharing options...
KerikBalm Posted August 26, 2014 Author Share Posted August 26, 2014 meve12 said: I think a nuclear jet that didn't irradiate everything under it's flight path would be slightly more desirable.On topic: I'm not sure pumping so many different gasses through one engine over the course of the flight would be good for it's health. Is the precooler incapable of operation at subsonic speeds?The precooler would be fine, the NH3 wouldn't be needed.The precooler is just to allow it to operate in air breating mode for longer/to higher velocities.Project Pluto (linked in the first post) was a ramjet that needed to be accelerated to a certain speed before it can work. Thermal turbojets are more complex, given that you need combressors and such.http://en.wikipedia.org/wiki/Aircraft_Nuclear_Propulsionhttp://en.wikipedia.org/wiki/General_Electric_X-39#The_nuclear-powered_X39A nuclear ramjet would be lighter, but it also requires an initial "boost".So (somewhat ambiguous terms here, using the same terminology as in KSP for the rapier): Closed cycle takeoff, open cycle cruise phase to high altitude and hypersonic speeds, closed cycle to orbit.This is what Skylon would do with its SABREs, if I understand correctly (the sabers can't operate in open cycle from zero airspeed, correct)I was thinking NH3 initially, simply because its easier to store, and much denser than H2, *and* for a given power output, you can get more thrust with it (at lower ISP) for the same flow rate (in liters/sec, not mass /sec).Heck, maybe you could use Neon instead for "moar thrust" on takeoff, I don't know if the pressures in the reactor would be a problem (as I'm guessing the gasses deviate quite a bit from the ideal gas laws in there), but at least you know it won't be clogging or corroding anything.As I understand it, you can do direct cycle engines without leaking radioactive particles, but in practice its hard to make sufficiently durable coatings.Still, if you wanted to operate these things on anywhere but earth.... who cares about the radiation?Assuming a never before used reactor, it should be fine. Maybe not a reusable-on Earth SSTO, but a SSTO that can get large payload to orbit and beyond, but then never comes back to earth - detach precoolers and wings once in orbit, become a dedicated tug? be a reusable SSTO on Titan/crew return vehicle (run it on N2 gas taken there?)Lets send a nuclear thermal turbojet to Titan!I doubt Mars has sufficient atmosphere to run them as ramjets, though you could still condense the atmosphere for ReMass.Venus would probably be too harsh on it, and like mars its mostly CO2 atmosphere is not very good as a propellant. Link to comment Share on other sites More sharing options...
K^2 Posted August 26, 2014 Share Posted August 26, 2014 Sky_walker said: Specific thrust: https://en.wikipedia.org/wiki/Specific_thrustThat looks suspiciously like the article I've linked to. Note that this is only relevant to turbines as defined, and is only necessary to differentiate it from specific impulse as defined with respect to stored fuel. None of that applies to a rocket. Thermal or conventional. Link to comment Share on other sites More sharing options...
Nuke Posted August 26, 2014 Share Posted August 26, 2014 K^2 said: That's specific impulse. As in, that's precisely the definition of specific impulse.well i was going by thishttp://www.grc.nasa.gov/WWW/k-12/airplane/specth.htmli was tired and maybe didnt interpret the equations correctly. Link to comment Share on other sites More sharing options...
K^2 Posted August 27, 2014 Share Posted August 27, 2014 Yeah, that's also just for the turbine engine.When you are dealing with a jet turbine, you have two relevant mass flows. Mass flow of the scooped up air, and mass flow of the fuel. As with rockets, you have the specific impulse. In SI, specific impulse is impulse the rocket acquires per unit of fuel mass. That's ISP = dp/dm. Using chain rule, we can obtain: dp/dm = (dp/dt)/(dm/dt) = F/m'. Or thrust per mass flow of the fuel.The term "specific thrust", similarly implies thrust acquired per unit of something. You can use specific thrust with respect to mass flow of the air through turbine. You still get F/m'. Because there are two different mass flows in a turbine, the two terms are used to differentiate between impulse per mass of fuel and impulse per mass of air.On the other hand, with rocket, there is only one mass flow, so I did not expect this sort of confusion to arise. So I've simply used the term "specific thrust", expecting it to be understood to be with respect to the mass of the engine. That is typical usage for rocket engines, though, it's much more common with specific power. Link to comment Share on other sites More sharing options...
meve12 Posted August 27, 2014 Share Posted August 27, 2014 OK, thanks for the explanation. Although... KerikBalm said: The precooler would be fine, the NH3 wouldn't be needed.As I understand it, you can do direct cycle engines without leaking radioactive particles, but in practice its hard to make sufficiently durable coatings.Still, if you wanted to operate these things on anywhere but earth.... who cares about the radiation?Assuming a never before used reactor, it should be fine. Maybe not a reusable-on Earth SSTO, but a SSTO that can get large payload to orbit and beyond, but then never comes back to earth - detach precoolers and wings once in orbit, become a dedicated tug? be a reusable SSTO on Titan/crew return vehicle (run it on N2 gas taken there?)Lets send a nuclear thermal turbojet to Titan!I doubt Mars has sufficient atmosphere to run them as ramjets, though you could still condense the atmosphere for ReMass.Venus would probably be too harsh on it, and like mars its mostly CO2 atmosphere is not very good as a propellant....if you're running a base, yes people are going to care about radiation.An oxygen "burning" jet would probably be better for Titan, because it can use fuel found in-situ. Link to comment Share on other sites More sharing options...
KerikBalm Posted August 27, 2014 Author Share Posted August 27, 2014 (edited) People on an extraterrestrial base won't be going outside, breathing the radioactive exhaust. They will always be in airtight, shielded units. They won't care if you've spread fallout across half the cellestial body, because nothing lives there.And again, it shouldn't be shedding radioactive debris. When the reactor has been running for a while, it will be quite "hot", but you can fly over the cellestial body without irradiating anyone, because noone is there.land near the base, orient the craft such that the "shadow shield" is between the reactor and the direction of the base... and everything would be fine.I am assuming EVAs between the base (likely mostly underground) and the transport, as opposed to the transport going inside the base.As far as the oxygen burning jet..... no....A nuclear thermal jet could run for years with no "fuel" Your O2 will run out very quickly, and you'd still need to collect the methane from the lakes (which aren't that widespread, the atmosphere is mostly N2)It would be, at best, an air augmented rocket. Edited August 27, 2014 by KerikBalm Link to comment Share on other sites More sharing options...
Nuke Posted August 27, 2014 Share Posted August 27, 2014 (edited) there is no reason the exhaust need be radioactive. if the reactor is only providing the thermal energy neccisary to run the engine, it can be isolated from the propellant with a heat exchanger. you are going to loose some efficiency when you do that, but you get a safer engine. the engine also provides the cold side of a brayton cycle so you can use the engines in place of a radiator to generate power. so you might be able to augment the engine exhaust with an arcjet or ionic type device to increase the isp of the engine. if that is not viable, you have the power for other systems, such as life support.another thing you get is versatility as to where your thermal power winds up. if your engine is a stacked turbine/ram/scram/thermal configuration of sorts, you will likely need a number of different heat exchangers to deliver thermal power to the various stages of the engine. its beneficial to use as few heat exchangers as possible, because they are really heavy. you might combine the ram and scram portions of the engine into a dual mode system that only requires one heat exchanger for example. its hard to build a combined cycle engine around a solid core reactor.if your goal is to build an ssto that can fly from earth's surface to leo, you want clean exhaust the whole way. if you want an engine to use in space or on the surface of the moon, you are probibly better off with a more traditional nerva design. Edited August 27, 2014 by Nuke Link to comment Share on other sites More sharing options...
Iskierka Posted August 30, 2014 Share Posted August 30, 2014 A nuclear precooled rocket would be impossible, because the precooler requires a cold sink to cool the incoming air. In SABRE, this is the fuel it is burning in the engine, so there is no issue - but an NTR equivalent would only have a hot core to operate with, making precooling impossible. A SABRE type engine must operate with cryogenic fuel, and very likely would only work with hydrogen, due to the low temperatures and very high specific heat capacity of LH2. KerikBalm said: So (somewhat ambiguous terms here, using the same terminology as in KSP for the rapier): Closed cycle takeoff, open cycle cruise phase to high altitude and hypersonic speeds, closed cycle to orbit.This is what Skylon would do with its SABREs, if I understand correctly (the sabers can't operate in open cycle from zero airspeed, correct)False. The SABRE engines operate airbreathing from a standstill, and are even capable of self-ignition with no ground connection, which drastically simplifies launch operation. Having to burn in inefficient rocket mode for the initial acceleration phase would significantly reduce efficiency, so this is not done. KerikBalm said: As I understand it, you can do direct cycle engines without leaking radioactive particles, but in practice its hard to make sufficiently durable coatings.Doubtful, as the expelled gasses will be passed near a radioactive source, they will always be at least partially radioactive. KerikBalm said: Still, if you wanted to operate these things on anywhere but earth.... who cares about the radiation?Assuming a never before used reactor, it should be fine. Maybe not a reusable-on Earth SSTO, but a SSTO that can get large payload to orbit and beyond, but then never comes back to earth - detach precoolers and wings once in orbit, become a dedicated tug? be a reusable SSTO on Titan/crew return vehicle (run it on N2 gas taken there?)Lets send a nuclear thermal turbojet to Titan!I doubt Mars has sufficient atmosphere to run them as ramjets, though you could still condense the atmosphere for ReMass.Venus would probably be too harsh on it, and like mars its mostly CO2 atmosphere is not very good as a propellant.The people living there care about the radiation, as I'm presuming this far in the future we would have far-reaching manned missions, and any scientists collecting data from it would care, as it will interfere with results from samples. And if you're going to be flying a single-use vehicle, there's no benefit to making it single-staged, or carrying extra mass for components required to do so. Skylon only needs wings for return, a simpler vehicle based on the engine could carry a reduced payload on the engine, though there would be little point to going to all that cost.Running organic chemicals through a nuclear reactor sounds like a bad idea for Titan, and Venus should probably just be forgotten for any long-endurance missions. On Mars, if you were to bring a nuclear reactor, its weight would be very impractical for a transport vehicle - it would be smarter to use the reactor to electrolyse CO2 to CO+O2, which can act as a rocket fuel of 90% Isp as compared to kerolox. Build a simpler hopper-rocket, that returns to the reactor base to refuel, and that is a much more practical planet exploring vehicle. Link to comment Share on other sites More sharing options...
cantab Posted August 30, 2014 Share Posted August 30, 2014 Iskierka said: A nuclear precooled rocket would be impossible, because the precooler requires a cold sink to cool the incoming air. In SABRE, this is the fuel it is burning in the engine, so there is no issue - but an NTR equivalent would only have a hot core to operate with, making precooling impossible.Indeed, you hit the nail on the head there. Link to comment Share on other sites More sharing options...
Nuke Posted August 31, 2014 Share Posted August 31, 2014 i think the question should have been: 'can haz nuclear combined cycle engine?'. you cant use the sabre design because of the cold sink issue. but you can do a multistage thermal engine. turbine, ram and scram can operate purely on a heat source with no combustion. sabre does let you push your turbines further than they could go without the precooler which eliminates the need for complex ram/scram stages, but you still might be able to make mach one, and then isolate the turbines from the airflow allowing rams to take over. even an air augmented nuclear thermal design might be plausible if you can isolate the core from the ariflow without causing too much loss in efficiency. Link to comment Share on other sites More sharing options...
KerikBalm Posted August 31, 2014 Author Share Posted August 31, 2014 Nuke said: you cant use the sabre design because of the cold sink issue.Huh?You'd have a massive cold sink... your liquid H2 that you'll be pumping through the reactor very soon. Quote sabre does let you push your turbines further than they could go without the precooler which eliminates the need for complex ram/scram stages, but you still might be able to make mach one, I'm pretty sure the Sabre is not a conventional turbine, and is meant to operate past mach 5.... Link to comment Share on other sites More sharing options...
magnemoe Posted August 31, 2014 Share Posted August 31, 2014 Iskierka said: A nuclear precooled rocket would be impossible, because the precooler requires a cold sink to cool the incoming air. In SABRE, this is the fuel it is burning in the engine, so there is no issue - but an NTR equivalent would only have a hot core to operate with, making precooling impossible. A SABRE type engine must operate with cryogenic fuel, and very likely would only work with hydrogen, due to the low temperatures and very high specific heat capacity of LH2.This sound correct an cooler need a cooling medium. Iskierka said: False. The SABRE engines operate airbreathing from a standstill, and are even capable of self-ignition with no ground connection, which drastically simplifies launch operation. Having to burn in inefficient rocket mode for the initial acceleration phase would significantly reduce efficiency, so this is not done.Makes sense, however wonder if an sabre can add oxygen to the air then it start to get to thin or does it have to shift to internal hard as the KSP rapier engine? Iskierka said: Doubtful, as the expelled gasses will be passed near a radioactive source, they will always be at least partially radioactive.The atoms will get hit by neutron who can create radioactive isotopes, hydrogen is immune to this. Don't know how much problem it is with oxygen or nitrogen. You will also get particles from the reactor, note that running this in an oxygen atmosphere will also cause corrosion giving more particles. Iskierka said: The people living there care about the radiation, as I'm presuming this far in the future we would have far-reaching manned missions, and any scientists collecting data from it would care, as it will interfere with results from samples. And if you're going to be flying a single-use vehicle, there's no benefit to making it single-staged, or carrying extra mass for components required to do so. Skylon only needs wings for return, a simpler vehicle based on the engine could carry a reduced payload on the engine, though there would be little point to going to all that cost.Running organic chemicals through a nuclear reactor sounds like a bad idea for Titan, and Venus should probably just be forgotten for any long-endurance missions. On Mars, if you were to bring a nuclear reactor, its weight would be very impractical for a transport vehicle - it would be smarter to use the reactor to electrolyse CO2 to CO+O2, which can act as a rocket fuel of 90% Isp as compared to kerolox. Build a simpler hopper-rocket, that returns to the reactor base to refuel, and that is a much more practical planet exploring vehicle.You would not land on top of base anyway. This might be an idea for an Venus sample return mission, here you need dV hard and its unlikely to be an permanent base here. Same for an dive into an gas giant. On mars and Titan you can just as well use chemical as the dV requirements are moderate. Link to comment Share on other sites More sharing options...
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