sevenperforce
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Posts posted by sevenperforce
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On 2/29/2024 at 8:33 PM, DAL59 said:
There's been a lot of discussion on this thread of shrinking Orion and/or adding a small lunar lander to SLS, but isn't there another option to get around the "too weak to go to LLO and back" problem?
Why not have Falcon Heavy or an equivalent launch vehicle launch a kicker stage into LEO, Orion docks with it, gets boosted into a moon transfer, then uses its own fuel to capture into LLO and return. This would enable smaller lunar landers to be used for Artemis.
Orion can't use its own props to capture into LLO and return. It doesn't have enough. Getting into NRHO and back is already tough for it.
Remember that Orion was designed around the Constellation mission architecture. In Constellation, Orion would have docked to the lander launch stack in LEO for the TLI burn, and then the LLO capture burn would have been performed by the hydrolox engine on the Altair lander. So Orion only needed enough props to get back from LLO.
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28 minutes ago, Nuke said:
early fusion drives wont be break even, they dont really have to be self sustaining. it will be nice to eventually tap them for electrical power, say for life support. that wont be until were at second or 3rd gen fusion power. but for now you just want fast charged particles. thus it has the same problem as any other electric thruster. its going to require some wattage. wheres do you get the power? we presently can get a few hundred kw using a really heavy bit of kit that doesn't work too far from the sun (the solar array on the iss). or we got rtgs which last decades on the order of half a kw and decreasing with age.
The ever-useful Project Rho has a whole page dedicated to realistic fusion designs. The Firefly Starship is a particularly nice design. It's a true interstellar cruise spacecraft powered by Z-Pinch D-D fusion.
Z-Pinch fusion has the advantage of being useful in the near-term. We should be able to do it with only a little more theoretical work, really. You don't have to have a self-sustaining fusion reaction since you are doing it in pulses.
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1 hour ago, farmerben said:
https://en.wikipedia.org/wiki/Kilopower
The kilo power reactor uses sodium as a working fluid, and radiates heat into space.
Certainly, but again that's to get around the whole "lack of open-cycle working fluid" problem. If your goal is to use hydrogen as propellant, then there's no reason to use the reactor to heat the sodium to convert to electricity to run through resistors to heat tungsten to heat the hydrogen; just heat the hydrogen with the reactor directly.
The conversion of heat to electricity and back to heat is extremely inefficient.
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6 minutes ago, farmerben said:
What do you call electrothermal propulsion? You could run a kilopower reactor as primarily an electric generator then run tungsten heating elements with hydrogen propellant. It would still be combined cycle, as the waste heat from the reactor would warm the cryogenic propellant into a warm gas before running it though the primary heating element in the nozzle. Would some sort of ion drive be better?
Any nuclear reactor needs two things: a working fluid and a heat sink. Otherwise, it can't generate power. To spin a turbine, you've got to move the heat out of the reactor and into a fluid, and then you've got to allow that fluid to expand through the turbine, and then you've got to do something with that fluid. Usually that means condensing the fluid back down by cooling it with some outside heat sink so you can run it back through the loop.
Space doesn't have any readily-available heat sinks, though; any heat rejection has to be done via radiators, and that is limited by surface area (not to mention mass). If you're in space and you're using liquid hydrogen as a propellant already, then you should just be running the liquid hydrogen through the reactor and using it as your working fluid, then venting it through the engine as your engine exhaust. If you're doing that, though, you should just skip the turbine altogether and boom, you've got an ordinary nuclear thermal rocket.
Is there any use for adding something further? Maybe. The limiting factor on the efficiency of a nuclear thermal rocket is typically the temperature of the reactor. You don't want it to melt, after all. The temperature inside the RS-25 is actually higher than the temperature inside a nuclear thermal rocket for this reason; the NTR is just more efficient because it doesn't need to use any of that heavy oxygen in the reaction. In theory, I could imagine a design in which the waste heat from an NTR is pulled off by a propellant preheating loop which turns a turbine before sending the preheated propellant into the engine, and that turbine somehow sends electricity into the propellant downrange of the reactor, heating it up beyond the melting point of the reactor before it is exhausted. But that's a LOT of complexity for not a lot of utility.
Just now, darthgently said:Depending on temperatures, that could be (nearly?) an ion engine. Sort of, right? Not sure, but would plasma be involved?
If you can get up to around 5000 K, most of the hydrogen molecules will be disassociated into neutral hydrogen atoms, cutting the exhaust molecular weight in half and thus increasing exhaust speed and specific impulse. This is already used in other situations; the RS-25 actually runs extra fuel-rich by adding more hydrogen than is required to decrease the average molecular weight and thus increase specific impulse even though this drives down temperature a little.
Up over 7000 K you get ionization of the individual hydrogen atoms. You can get near-complete ionization north of 10,000 K.
A true ion engine, however, operates by electromagnetically accelerating charged particles. Because the acceleration is electromagnetic rather than thermal, an ion engine can "skip" the thermodynamic limitations of the Carnot cycle and achieve much higher particle energies. There's no real gain from having thermally ionized exhaust in a thermal rocket.
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4 hours ago, Exoscientist said:
Keep in mind such an expendable launch is only in the $90 million cost range. While not at the $10 million aspirational rate Elon wants for full reusability, considering the amount of payload it could loft, it still would be a major improvement over what we have now, and literally orders of magnitude cheaper than the SLS.
I don't think anyone is dispute that using Starship is better than using SLS for...well, basically anything.
4 hours ago, Exoscientist said:By the way, suppose as a SpaceX exec once suggested the Starship HLS would need “10ish” orbital refuelings. At the $10 million per launch rate for a fully reusable SH/SS that’s still $100 million. Then the single launch expendable approach would actually be in the same cost range as the fully reusable approach, anyway.
A single-launch architecture could certainly have cheaper launch costs than the current planned Starship HLS, but that would require that Congress and NASA pursue a lunar mission profile that avoids SLS entirely, and they evidently don't want to do that.
You'd also need to dev a lunar lander and a crew vehicle that would fit on top of an expendable Starship. The amount of time that it would take to rework Orion for a single-stack launch architecture is...prodigious.
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On 3/14/2024 at 9:43 AM, Exoscientist said:
It was the reentry thermal control purpose for the plug nozzle for which Bono was granted the patent. If you look at the interviews of Andy Lapsa of Stoke Space such as by Everyday Astronaut and NasaSpaceflight it is this purpose for which Stoke Space is praised for their “innovativeness”. My opinion, Phil Bono should have been given credit for this innovation.
No, the innovation for which Stoke Space has received praise and attention is the use of a thermally-powered, actively-cooled heat shield. The fact that this heat shield also happens to function (somewhat) as a plug nozzle is secondary, and not that rare of an idea anyway. The Chrysler SERV and the Phoenix SSTO were designed to use the same approach.
Saying that Stoke Space is getting undue credit for "copying" Phil Bono's idea is like saying that Reaction Engines Limited gets undue credit for "copying" the Space Shuttle, because both designs were spaceplanes that used hydrolox for the main push to orbit, circularized with hypergolics, and landed horizontally. The thing that makes the Skylon concept unique is the super-cold heat exchanger in the SABRE engine, not the brute notion of a spaceplane. Likewise, the thing that makes the Nova rocket concept unique is the use of re-entry heat to operate the pumps for the actively-cooled heat shield using the same system that powers the engines.
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29 minutes ago, Exoscientist said:
SpaceX still has not demonstrated the Raptor can relight reliably in flight.
"SpaceX is just a private company; it still has not demonstrated it can launch a liquid-fueled rocket!"
"SpaceX can launch a liquid-fueled rocket, but it still has not demonstrated it can reach orbit!"
"SpaceX can put a liquid-fueled rocket in orbit, but it still has not demonstrated it can deliver a useful payload!"
"SpaceX can deliver useful payloads to orbit, but it still has not demonstrated it will be able to bring a booster back through re-entry!"
"SpaceX can bring boosters back through re-entry, but it still has not demonstrated it will be able to land!"
"SpaceX can land boosters, but it still has not demonstrated it can refly them!"
"SpaceX can land and refly boosters, but it still has not demonstrated economical reflight!"
"SpaceX can make booster reflight economical, but it still has not demonstrated a design for a recoverable second stage!"
"SpaceX has built a recoverable second stage, but it still has not demonstrated that the engines work!"
"SpaceX got Raptor engines working, but it still has not demonstrated that it can land on them!"
"SpaceX completed low-altitude hop tests, but it still has not demonstrated relighting the Raptors!"
"SpaceX completed high-altitude hop tests with successful Raptor relight, but it still has not demonstrated landing successfully!"
"SpaceX completed high-altitude hop tests with landings, but it still has not demonstrated simultaneous firing of enough Raptors for Superheavy!"
"SpaceX completed static-fire tests, but it still has not demonstrated it can actually launch without blowing up!"
"SpaceX launched without blowing up, but it still has not demonstrated stage separation and in-flight lighting of the second-stage engines!"
"SpaceX achieved stage separation and lit the second-stage engines, but it still has not demonstrated Raptor can relight in flight!"
"SpaceX achieved Raptor relight in flight, but it still has not demonstrated the Raptor can relight reliably in flight!"
See where this is going?
5 minutes ago, PakledHostage said:Anyone have any thoughts on Everyday Astronaut's hypothesis that the liquid oxygen was freezing around the cold gas thruster nozzles, partially blocking them and affecting the resulting thrust force and direction sufficiently to screw up the control system? He figured SpaceX may have to add heaters on the nozzles.
That seems like a pretty likely possibility. That's part of the inherent design of cold-gas thrusters. Alternatively they could go to hot gas-gas thrusters.
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Just now, JoeSchmuckatelli said:
Very late to the party - just watched with my students.
Why was Booster so much above terminal so deep into the atmosphere?
It was supersonic at 5km
Terminal velocity is the stable point at which gravity and drag are equal, assuming infinite time to slow down or speed up. The booster was falling from space and was still actively decelerating under drag all the way down.
Most meteorites don't actually hit the ground with any of the speed they had in space; they reach terminal velocity and then fall under the influence of gravity alone. It's only the really big ones that can punch all the way through the atmosphere while still maintaining some of the original interplanetary velocity. Basically, terminal velocity is reached when the mass of air that has been pushed out of the way by the falling object equals the mass of the object. A meteor that is about 2.4 meters in diameter can reach the ground with interplanetary velocity if it is mostly iron; if it is a rocky meteor then it needs to be closer to 4.4 meters in diameter. Of course that is the size that has to survive ablation. Any meteorite smaller than that will have reached terminal velocity before hitting the ground.
16 minutes ago, JoeSchmuckatelli said:18 minutes ago, sevenperforce said:performance obligations
Goals. They have no obligations, just intentions.
Ofc you know that.
Well they do have certain performance obligations for their HLS contract with NASA. That's what I was talking about (because that's what a certain someone keeps bringing up, as if a contractual obligation to provide a particular service somehow creates a public interest in HOW they test the components with which they intend to provide that service).
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2 hours ago, Exoscientist said:
If SpaceX wants to prove the Raptor can relight reliably they need to...
That's just it -- SpaceX doesn't need to prove anything about their engines, certainly not to forum-goers. They test as they fly. They need to know internally how to meet their performance obligations. That is all they need to prove.
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1 hour ago, tater said:
BTW, relight for boostback was fine. They skipped relight on ship, or rather the SHIP skipped relight itself. My guess is that the lack of attitude control resulted in the ship not lighting engines. Had it been pointing the right way, it would have relit just fine.
I'm guessing you're guessing right. At first I mistook RCS firing for the engine light plume, but it just kept going and going for quite a while. This may be part of the inherent limitations of "best part is no part" engineering: they might just have to add in a separate thruster system after all.
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2 hours ago, .50calBMG said:3 hours ago, Exoscientist said:
Actually, we do know definitely that the engines did not all relight during the landing burn for the booster. That proves something went wrong at this relight, and the damage could even have occurred during the first relight during boostback except that burn was not long enough to cause loss of vehicle.
If something goes wrong with the engines during flight, and it is a known problem to have occurred before during testing the first thing to suspect is that same issue is occurring again.
My best guess is that it's fuel slosh from the booster wobbling and spinning so much. That's solvable with tweaked control algorithms, not a failure of the engines. Seriously, with the number of test firings, I don't understand why you automatically jump to engine failure
Also possible that the booster grid fins simply don't have enough control authority to stabilize the booster. Which would mean they need larger grid fins, or more of them. Which also wouldn't be an issue with the engines.
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Just now, tater said:
Once it was getting some flap authority, seemed to calm down.
Agreed. There was a lot of concerning spinning and tumbling right there at the beginning of the ultrasonic regime, before there was significant visible heating but also before there was enough atmospheric mass to provide something for the flaps to bite into. But by the time we saw plasma, it seemed to be maintaining a pretty good attitude, at least in the roll axis. Impossible to tell whether there were issues in the pitch axis; that would be the next area of greatest concern.
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7 minutes ago, Ultimate Steve said:
Not necessarily tile loss, the thing appeared to enter the atmosphere in a tumble. There's been some speculation that there was minimal to no control after SECO, that roll may have been coincidental and not commanded, plasma on the silver side is not normal.
The views I was getting on video seemed pretty stable, though.
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Just now, Minmus Taster said:
All data was lost at the same time, that loss of signal could have been the end.
Yep, that makes sense.
RIP Starship -- you went farther than any before!
I'm worried that it could be burn-through at tile loss. I'm also worried because I have no idea how they would figure that sort of thing out.
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Still 3-4 minutes before the expected end of blackout.
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1 minute ago, Minmus Taster said:
Still nothing, you'd think if it was alive it would be done by now.
That's what I would have said, but they don't seem bothered or anxious yet. No noise coming from behind them.
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Just now, Minmus Taster said:
Telemetry is gone
I think this is the end. Goodbye S28
Yeah, the telemetry freezing doesn't look good. But it's also possible that the plasma sheath is just too much for any telemetry to get through.
Just now, Exoscientist said:Angry Astronaut noted, it is likely the problem of the relight of the engines of the booster for landing was due to the hot staging method.
Angry Astronaut has no legitimate reason to think this and that doesn't make any sense. The engines managed to get through the entire boostback just fine. Control authority appears to have been the problem prior to relight.
Just now, Exoscientist said:I just saw that SpaceX has decided not to try the engine relight test during the Starship return, which does support the idea there are still problems on relight of the Raptors.
It appears that literally anything that does or does not happen during any test will be interpreted by you to support your pet theory about Raptor reliability.
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THIS IS GORGEOUS
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OMG YES
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OOOOH the camera itself is on a flap.
Re-entry heating!!
HOLY excrements that is beautiful.
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Flaps are moving. Are we stable??
I'm seeing a persistent roll. Possibly not good.
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OMG, what gorgeous views.
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The onboard computer has decided not to perform the in-flight restart burn test.
Re-entry in 8 minutes!
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Operating the flaperons in the hypersonic regime is completely untested. There was some early talk of putting a subscale demonstrator on Falcon 9 just to make sure the flaps operate properly...who knows how that will go.
Looks like we have a good SES-2?
Wait, no, that was just a cold-gas flip.
How Come Nuclear Saltwater Rocket Engines Do Not Melt?
in Science & Spaceflight
Posted
I'm not sure if you can get enough power.
Since you were talking about tungsten before and tungsten melts at 3,400°C, I'm assuming you mean 4000°F. This paper has a lot of analysis on an electric resistance-based rocket engine. Figure 2 gives something on the order of 690 seconds specific impulse for hydrogen heated to that temperature. We know the biggest kilopower reactor would have generated ten kilowatts; ten kilowatts divided by ~6800 m/s is a thrust of 1.47 Newtons. That's not much.