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Cunjo Carl

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  • About me
    Rocket Fancier
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    Staring out the window
  • Interests
    Science. All of it!

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  1. 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!
  2. 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!
  3. 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 ) 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 pump With 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_t MW 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!
  4. Oof, I wish I could interact more. I'll be back in a week or two!
  5. 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.
  6. 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.
  7. 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!
  8. 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 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! 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!
  9. 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.
  10. 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!
  11. @JonnyOThan Congrats on taking the record!! I look forward to watching your run.
  12. Well, sad news. I got a secondary infection of hand foot and mouth disease which has more or less taken my hands out of commission for a while now. I can't really browse or type, so I can't really interact in the forums any more. If I improve I'll be back! In the meanwhile, be well. Keep healthy! And if you can't fly safe, fly fast.

    1. Ultimate Steve

      Ultimate Steve

      Oof. I hope you get better soon, but better for real.

    2. Cunjo Carl
    3. xendelaar


      that's sad news indeed. i hope you get well soon! 

  13. 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!
  14. 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!
  15. 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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