Cunjo Carl
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Posts posted by Cunjo Carl
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What kind of bearings does turbo machinery use? Like on a turbojet or a turboshaft. I heard somewhere they use plain bearings, but that sounds crazy. Thanks in advance!
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So good news for me, I have no idea for how long but my hands cleared up a bit, and the place I consult for is temporarily closed so I got to use some time to brush up on thermo and figure this out instead! It was surprisingly nice to just do something silly.
Also good news for me, my instincts were on point and the engine would work roughly as claimed. Despite this, both I and others pointed out that it probably won't be a terribly popular engine cycle for lack of a proper niche. But at the bottom line the pressure boost numbers wind up looking tantalizingly usable, and the engine would would need only the simplest conceivable turbopump from an engineering perspective (for 20 reasons), which is why I find it interesting. In all cases this cycle would use significantly lighter tanks and less Helium than an equivalent pressure fed, but it has to carry the turbopump. Pressure fed rockets are still considered and tested on occasion by the way, like NASA's 2017 ICPTA! I'll use it for the example. To recap, here's how the cycle works with letters for the subscripts used in the formulas later.
The Helium Pressurant Brayton Cycle (renamed
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The Helium follows this open cycle, in order:
i) Leaves the COPV, decompressing in some regulators (an ignored step for this)
h) Leaves the high temp heat exchanger or regenerator (isobaric expansion)
ad) Leaves the turbopump after doing work (adiabatic expansion)
t) Arrives at the prop tanks, after having gone through an optional second heat exchanger to raise/lower its temp again (isobaric volume change, probably expansion) If no second heat ex is used, T_t = T_ad and this step can be ignored.Meanwhile, the propellant follows this open cycle, in order:
t) Leaves the prop tanks
c) Arrives at the chamber with its pressure boosted from the turbo pumpWith the subscripts from above we can solve for the pressure boost our propellant gets from the turbopump between the tanks P_t to the chamber P_c. This assumes the prop tanks at roughly the same temp and pressure, like on the methalox ICPTA.
P_c = P_t*(1 + Efficiency*CCF*MW_He*(5/2)*(T_h/T_t)*(1-(P_h/P_t)^(-2/5)))
also,
P_c = P_t*(1 + Efficiency*CCF*MW_He*(5/2)*((P_h/P_t)^(2/5)-1)) special case, the formula simplifies when no second heat ex is used. If T_ad < ~180k it's probably smart to remove the CCF correction factor term for methalox.
T_ad = T_h * (P_h/P_t)^(-2/5)
m_He = ~ 100*CCF*MW_He*P_t*V_t/(R*T_t) (the factor 100 = 10^5 Pa/Bar * (.001 kgmol/mol) SI's weird sometimes)
Helium Savings Factor = m_He,pressureFed/m_He = ~ P_c/P_tMW is in Daltons, R=8.314, pressure in Bar, everything else is in SI. This is to 0th order. It assumes incompressible propellants and no pressure drops in the prop lines. To fudge factor in some of those losses and account for light weight cheap construction, an abysmal overall turbopump efficiency of 25% seems appropriate. To account for non-idealities, friction, injection pressure loss and tubing heat transfer in the He the actual P_h, T_h and CCF would all be a bit higher than on paper. CCF is the 'cumulative collapse factor', the fudge factor by which the high temp helium we send into the prop tanks is immediately cooled and shrinks... so we need more of it.
Let's check how well this pressurant brayton cycle would work with a real example. Fortunately NASA released some solid hints at numbers to their 2017 ICPTA pressure fed methalox lunar lander, so I plugged in those! Then, I chose a gentle T_h of 200C, which is cool enough to use things like an all-aluminum turbo/tubing, and common elastomeric gaskets/seals. And finally I back-calculated the tank pressure P_t we'd need to produce the ICPTA's same (approx) prop delivery pressure P_c. By pure serendipity, no second heatex is needed with this setup .
P_c = P_t*(1 + Efficiency*CCF*MW_He*(5/2)*(T_h/T_t)*(1-(P_h/P_t)^(-2/5)))
22 = 5*(1 + 0.25*1.5*4*(5/2)*(475/250)*(1-(25/5)^(-2/5)))The results: For the same 22bar propellant delivery pressure we can reduce the tank pressure and Helium usage by ~4.5x when switching to a pressurant brayton cycle, even when assuming a 25% efficiency turbopump running on only 200C, 25bar He. Meanwhile a 50% efficient turbopump would reduce the tank pressure and helium usage by ~9x and so on, though I think the 25% efficiency is more plausible. Is it worth it? It sure looks enticing, but probably not! Is it an interesting concept? Heck yeah!
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Oof, I wish I could interact more. I'll be back in a week or two!
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5 minutes ago, mikegarrison said:
120K is *way* below the freezing temperature of RP-1. Won't that be a problem?
If we warm up the He somewhere where the RP1 is flowing, the RP1 will never cool down that much because it has so much more thermal mass.
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2 hours ago, sevenperforce said:
The biggest consideration (and possible stymie) to your idea is that heating the helium in an exchanger at the engine and then dumping it directly into the tank will immediately do Carnot-cycle work against the propellant, without the need for any turbopump at all.
Er, you shouldn't be able to get anything like Carnot efficiency by dumping a high pressure gas straight into a low pressure chamber! Consider the reverse process.
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38 minutes ago, sevenperforce said:
For reference, the "standard" pressure-fed cycle assumes that a heat exchanger will be used at the engine to increase efficiency:
The Apollo Descent Module engine tanks used a helium tank that circulated through the propellant tanks, as you propose, to bring them to ambient temperature before exhausting to increase efficiency.
I'm digging around to try and figure out how much helium is used in a pressure-fed stage that doesn't already run it through a chamber heat exchanger.
Interesting. I didn't know that was standard! If you know of a reasonable choice of temperature for the heated He entering the prop tanks, please feel free to use it! I agree Helium is probably the most conceivably benign fluid to have hot.
Edit: (with the temp of the He entering the turbine raising accordingly as well, I'd assume)
Edit 2: If T1/T2 stays the same, I think physically we should get the same work per tank volume. So I guess further heating the helium doesn't matter too much except that it lowers our dry mass. No complaints there!
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4 hours ago, sevenperforce said:
What you need to know is how much helium pressurant a pressure-fed stage needs in comparison to a pump-fed stage of equivalent size. Is it twice as much? Ten times as much? This tells you how much helium you have to work with, and then you can apply a Carnot cycle to determine how much total mechanical energy you can extract from the pressurant mass at your disposal. You can then divide by the total propellant mass to figure out what kind of pressure drop you can produce from that amount of energy, which tells you the pressure reduction (and associated weight reduction) in your tanks.
The amount of Helium (mass-wise) should just be found from doing PV=nRT on the propellant tank pressures and temperatures, for a given O:F ratio. Let's rephrase the problem backwards as: for a 12bar propellant pressure at the turbopump outlet, how low can we make the prop tank pressure? For adiabatic isentropic expansion of Helium (monatomic), Work = mass*Cp*(T1-T2), or in context Work = mass*(5/2)*R*T1*(1-(P2/P1)^(2/5)) . I forgot how to do the VdP work for a turbopump though, and can't surf enough to find out
4 hours ago, sevenperforce said:Note that there is a positive but diminishing feedback loop: the mechanical energy available from your Carnot cycle is an inverse function of the outlet pressure, and the more energy you have, the lower that outlet pressure becomes.
It's true! But there will also be a much stronger negative feedback from: As we lower the prop tank pressure, we'll need to bring less He (mass wise), and so our available energy will decrease. A slightly higher prop tank pressure (2-4 bar) isn't so bad, because it makes the turbopump way simpler for cavitation reasons, and we can balloon tank the propellants. A 2-4bar prop tank is kinda what I'm hoping for if we have 12 bar propellants coming out of the turbo pump... Not that anyone's having troubles with their balloon tanks at the moment!
4 hours ago, sevenperforce said:It's entirely possible that pressure-fed kerolox stages already utilize heat from the kerosene to their advantage, at least on one side. If helium pressurant is exhausted into the RP-1 tank directly at LOX temperatures, it will warm and expand, adding supplementary pressurization, meaning you need less of it to get the job done. As a corollary, if you're exhausting helium into the LOX tank after running it through an RP-1 heat exchanger, you're going to warm the LOX, boiling small amounts of it and causing it to self-pressurize.
The helium pressurant system on the Falcon 1, with the Kestrel engine, already used a titanium heat exchanger around the engine to transfer heat to the helium pressurant -- far more than you could get from the RP-1 tank.
Absolutely for sure on both accounts! For rocket simplicity, I wanted to avoid having a hot He stream or autogenous pressurization. They'd both help the rocket's overall efficiency *tremendously* though and would make interesting follow-on problems!
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Yep! I guess engineering-wise it's like a halfway point in complexity and performance between the pressure fed and expander/bleed cycles, which is how I approached it. It fills a niche no one asked for, but hey that's why it's new!
For advantages, there's no need for a regenerator (so you can use a simple ablatively cooled chamber), no extreme temperatures or chemical conditions across the turbopump, no chance of saturation in the turbine, the He is inert so you can use leaky seals, and it can work with garden variety kerolox. Just futzing with numbers in my head, I'm thinking we'd get 0.5 - 2 MPa of chamber pressure boost out of it including losses?
I'd love some actual numbers though. I could easily be full of hot air
. Like I said, its probably not worth it, but I'm curious what it could do on paper.
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Hey! I'm still with it . It's been a rough couple seasons, but I'm very slowly improving again.
For long term health reasons I can't quite science like I used to. I've had an idea floating in the back of my head for months though, and I was hoping someone might help me run the numbers? The idea is to slightly boost the efficiency of a simple, cheap pressure fed kerolox engine like the Kestrel (or similarly with the Xombie) by using the Helium that's onboard anyways for pressurizing the prop tanks.
The Helium Bleed Engine
Cycle:
* Start with high pressure Helium (60 - 200bar) in a COPV stored in LOX (10bar, ~120K)
* Freely expand He to 50bar
* Warm He to T_h of ~240K using a heat exchanger in RP1 (~ room temp).
* Expand He in turbine to 10bar, and T_c ~ 120K
Use the turbine power to slightly boost the prop pressure using a turbopump
* 'Bleed' the 10bar He back into the prop tanks, like normal, equilibrating it to roughly the prop temps.Given the He volumetric flow rate = the prop volumetric flow rate, and the assumption of a chunky junky turbopump, what outlet pressure might we get for the propellants? Alternatively, assuming an 8bar chamber pressure with 12bar feed, how low can we get the prop tank pressure?
Is it worth it? Probably not, but curious minds want to know!
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@JonnyOThan Congrats on taking the record!! I look forward to watching your run.
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I'll put my money on nanoparticles. They can behave like semisolids even when anhydrous, and there's lots of opportunities to make them on the Moon. Definitely looking forward to hearing more!
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Could someone please make one of these fireworks in KSP and post a video for me to watch? I'd love to see what people come up with!
You can see a slow motion video of one taking off at 5:20. The ones in real life are more complex, but in KSP we can make something equivalent using only the tippe top effect. For extra challenge, maybe try making one with SRBs only. Part clipping is 100% ok, as are DLC and mods. Good luck, have fun, and thanks in advance!
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1 hour ago, M_Rat13 said:
Ok, I'm probably going to butcher real science, but what about some kind of push/pull system. As one planet gets too far from the 'rails', the other planet pulls in the veering planet, but this then pushes that planet away from the rails. The opposite then happens for each planet. Basically, the two planets allow some give and take, so they can change orbit a little, but then still ultimately right themselves before catastrophe strikes.
If someone understands what I mean, and can explain it better, please, do so. I'm really bad at this sort of thing.
Fortunately the planets themselves can be left on rails, despite orbiting eachother (see the video as an example). A space ship trying to fly between the planets though is another story...
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One other thing to keep in mind is changing SOIs at high timewarp is hard on KSP, where rounding errors can cause craft trajectories to diverge rather suddenly. I suspect they won't want closed orbits to constantly hop between multiple SOIs.
For a specific 2 body system acting on a craft, I think they can precompute trajectories and toss them into a massive lookup table, like they do in present KSP but with an extra set of variables involved. KSP 2 is going to hog easily 3+GB of memory I'll bet, so 150MB of lookup table won't break their bank.
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Perhaps the sun is just sideways?
.... /s
I'm taking notice of all the space clouds in the back, and thinking this may be in a nebulea? The nearby space clouds are lit only from the top, whereas the distant background clouds are lit diffusely from the back. If it's a stellar nursery, perhaps there's an interesting nearby feature temporarily lighting the otherwise nighttime side of Duna? Sounds good anyways! Gonna wreak havoc on the orbits though....
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Someone made a special Gold-Palladium alloy which is apparently so good at cracking hydrocarbons, that the surface builds up a layer of graphite 'coke' at room temperature. This tiny layer is so durable that the alloy becomes effectively one of the most abrasion resistant materials in the world. That's crazy because Gold Palladium itself is pretty dang soft.
https://www.sciencedaily.com/releases/2018/08/180816132009.htm
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Getting one of these into space would be an undertaking of Kerbal proportions!
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47 minutes ago, Nuke said:
if you use the tokamak yardstick. fringe fusion is always 5 years away.
Not sure if he counts as fringe, but here's a nice five year design along the lines of your statement. Hugely worth the watch in my opinion, he does a great job explaining the present day of fusion.
For present day 'hyperfission' on the other hand there's this reported 1H + 1H -> 3 Kauons + 300MeV reaction. Kinda crazy! Not sure I believe this guy's theory, but his experiment's an interesting one. https://iopscience.iop.org/article/10.1088/1402-4896/ab1276/pdf . It is definitely an easier goal than fusion! Whether it's real...? Probably not, but bizarrely not "definitely not". Still looking into it.
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3 hours ago, HebaruSan said:
I'm not a nuclear physicist, but I don't think they can design reactors by picking arbitrary numbers out of a hat like that. They have to work with known fuels / isotopes / reactions / decays, often in long complicated branching chains, each of which potentially has its own quirks and gotchas and tricks to make it work as desired.
Can confirm, this is exactly how it goes.
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My honest answer would be to go for both! Maybe build in an interesting tradeoff between the two, so the crew needs to make difficult decisions when confronted with a new challenge or one of the drives going offline.
Safe travels, come back and say hi some time if the mood strikes!
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On 7/22/2019 at 10:04 PM, mikegarrison said:
This takes me back about 30 years or so. I worked in the combustor group of a turbine engine manufacturing company. We used a little program called TSSST that was originally written to run on something like an HP-45 calculator. Anyway, all it did was make a few assumptions about luminosity and convection and it did a simple 1-d "thin-shell steady state (heat) transfer" calculation to come up with combustor temperatures. Interestingly enough, it worked really, really well considering the (lack of) computing power available back then.
Glory days of the RPN calculator right there! Also I totally get what you mean. If you know what assumptions to make it can feel rather cheeky what you're able to get away with!
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9 hours ago, Exoscientist said:
I've used the shareware program Rocket Propulsion Analysis, http://propulsion-analysis.com/index.htm.
Bob Clark
I took a quick peek at the web page for the program you found, and it looks like it does everything my spreadsheet would and then some! My plan of attack was:
1. Find an engine with known: Propellants, mix ratio, expansion ratio, Isp (ASL), chamber pressure, and exhaust exit pressure (guesstimate this last one by looking at exhaust expansion as it exits the nozzle)
2. Use the Isp (ASL) to back-calculate the temperature in the combustion chamber. Just use guess and check iteration until the right temp is found- the adiabatic flame temp can be used as a starting guess. I was planning on using JANAF thermodynamics values and an assumption of fast kinetics and adiabatic expansion (so all points are at equilibrium, and all of the losses are modeled as a simple reduction in combustion chamber temp). The one thing I wasn't sure about is how to deal with the non-ideality of the gas. I was planning to use Redlich-Kwong, but there's quite a few exhaust components I'm not confident I could find critical temps/pressures for. For starters, the ideal gas assumption may be 'close enough' though.
3. Using this combustion chamber temperature, calculate the Isp of the engine for different expansion ratios.
Of course real life doesn't really work like this, but the problems caused by the assumptions made here will tend to cancel eachother out. It should come out pretty close! Since you've got the program, maybe give this a go?
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6 hours ago, Exoscientist said:
Thanks for doing the calculation. I assume you take into account that with fixed nozzles there is also a backpressure term given at the end of the equation here:
F = q × Ve + (Pe - Pa) × Ae
No, the equation is just for the Ve term (v3 is the exhaust velocity). The Pe-Pa is something you'll need to calculate within your model/sim.
I went ahead and typed in the subscripts so it's a bit easier to read:
Cheers!
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My grandfather worked in the Mariner program, so he made sure to snatch up some 10yr Apollo Program anniversary stamps when they came out! They started at 8 cents, and became family heirlooms.
) 
. Like I said, its probably not worth it, but I'm curious what it could do on paper.
A great JPL talk about Cassini's Saturn discoveries
in Science & Spaceflight
Posted
Trina Ray (lead scientist of Cassini's Titan flybys) gives a first hand talk about a few of the fantastic findings of the Cassini-Huygens mission. She talks a lot about how people reacted to the data as it unfurled and slowly pieced together some rather amazing big pictures. It's lovely, figured I'd share!