18 thoughts on “Stoke”

  1. Even the initial version has enough payload you could mount a crew cabin and docking port for it. I’d be willing to risk being blown to bits for the ride. I wanted to ride in a cargo Dragon when I was 60. Now I’m 72. What’ve I got to lose?

  2. Some extremely clever engineering is embodied in the Stoke spacecraft. There also seems no obvious reason that it can’t scale upward. A larger successor craft might just be the first effective competitor to Starship.

  3. It’s a clever design, but I wonder about the LNG vs LH2 trade-off for the upper stage. Sure, LH2 has been successful for 2nd stages, but with LNG-Ox in the 1st stage, they have the complexity of different fuels for the 2 stages. I wonder if they could make the 2nd stage work with LNG, albeit with a reduced mass to orbit. Operationally, it would make things simpler.

    I also wonder if Stoke’s 2nd stage, scaled to match diameters, might not make an excellent 2nd stage for Super Heavy.

    1. The ring engine is made up of smallish thrusters. Without LH2 maybe they cannot get enough ISP to make it worthwhile? They are keeping the IP close to the vest as they should. I guess we’ll see.

      1. I’m skeptical. Wouldn’t the important factor be specific heat and heat of vaporization? If you pour enough LNG through, you should be able to cool things pretty well. It’s not as if the temperature difference between the point where stainless steel gets too hot to be useful structurally and that of LNG is very much less than for LH2.

      2. No, the trick for stage 2 is the open-loop expander cycle- a small portion of the LH2 is vaporized and used to power the turbine, then vented overboard to provide the base bleed. Any other fuel would take a big hit in total Isp, but hydrogen is ok.

        1. I show CH4 liquid to gas volume ratio and heat of evaporation per kg to be within 20% or so of H2, which means it’s far better volumetrically. Seems like those would be the figures of merit for a good expander cycle fluid. Why is hydrogen the better choice?

    2. I get why they don’t want to be dependent on a competitor, but it would save a ton of development if their upper stage could just ride a Falcon first stage.

      1. It would take a ton of development to make it suitable to ride as S2 on Falcon, not to mention the development and construction of LH2 infrastructure that does not exist at Falcon pads.
        All to ride on a competitor?

  4. I saw this video earlier in the week. Very impressive. I like the ring engine. A worthy successor to the aerospike. .

  5. This is so cool. I’m not an engineer, but the combination of spike like engines with the cooling system it awesome. In reading and viewing Musk and videos like this, it reminds me of the huge aerospace advancements during the 1930’s to 50’s.

    I sure hope they have the funding to follow this through.

  6. >> The whole point of using LH2 in the upper stage is reentry heatshield cooling. CH4 is not cold enough.

    From critical to 1000K (about as high as stainless can retain good properties) LH2 takes away about 3x as many J/kg as water. I have done a similar calc in the past CH4, and it’s similar to water for that range (Water’s liquid specific heat and enthalpy of evaporation is almost unbeatable, but its specific heat in the gas phase is pretty poor). Usually the problem with that is hot spots boil, raising backpressure in those channels and causing thermal runaway. The solution is either to jack up pressure in the cooling circuit so the mass flow difference is small between phases, or pre-boil using an evaporator and design the cooling channels for all gas, or give up on 2-phase cooling. They’re probably going for option 3, since they’ll get a lot of the gas film cooling effects by using the cold GH2 as a plug, but option 2 seems like a potential scaling improvement in the future.

    >>There also seems no obvious reason that it can’t scale upward.

    They will run into square-cube issues pretty quickly. The way the 1st stage is flared may just be for passive reentry stability, but my guess is they had to scale the 2nd stage heat shield area to keep heat levels reasonable during reentry. If they scaled this up to starship payloads this probably needs to look like a flying saucer.

    1. Even if LH2 is a better performer on a mass basis for reentry cooling, it’s not clear to me that for the overall design it might make sense to use LNG instead. You save enormously on tank volume, tank mass, tank insulation, and on thrust to weight ratio for your engine. Might you not have a net gain using LNG, even if you have to throw a little more mass at the cooling problem?

      1. Same problem as scaling in general. You actually want a big fat tank for reentry vehicles because fluffier vehicles have lower peak heating. Methane stage would either have lower incident surface area (bad for reentry heating) or a much shorter stage (bad for stability and potentially structural efficiency).

  7. My weird roller idea for heat transfer could possibly be applied for a re-entry system.

    If you take a bunch of tapered hollow rollers, such as you’d see on a factory conveyor system’s curves, and mesh them together like a bunch of spur gears, you end up with a big circle of rollers that are all in contact, with each roller turning the opposite direction of its neighbors. If you spin one, you spin all, though I’d add actual spur gears on the ends to make sure.

    If you heat one side and cool the other, the surface of the rollers are heated and cooled with a 50% duty cycle, and since they’re being directly cooled on the outer surface, instead of from behind, the heat flow through the roller material doesn’t much matter.

    If your roller centers are 3″ in diameter and rolling with a surface speed of 30 mph, the roller gets 9 millisecond heating pulses and 9 millisecond cooling pulses. From there it’s pretty easy to calculate how much transient heating you’ll have, and how deeply the heat will penetrate into the roller.

    But you can beat a 50% duty cycle by using two circular roller plates, threading a piece of stainless steel foil around each roller on the exterior roller section, then running it around a mating roller on the interior roller section, like film threaded through a motion picture camera. So you’d still have a 9 millisecond heating pulse, but the length of the cooling pulse would be set by the long path length between the two roller sections, which of course is filled with coolant.

    If the heat pulses are short enough, which is set by the roller speed, no heat even makes it through the stainless steel foil and the rollers’ thermal properties become irrelevant.

    The big challenge is keeping the coolant from leaking around the ends of all the rollers, and keeping the rollers from bending under the applied external pressures.

    1. Rennie: “I’d thought you’d be further along by now.”
      Jaffre: “Well with the first order effects, yes, but what about the second and third order effects?”
      Rennie: “Almost negligible. With proper variation of parameters, this is the answer.”
      Jaffre: “I see. Tell me, have you tested this theory?”
      Rennie: “I find that it works well enough to get me between planets.”

      1. The thermal performance is outstanding, but the design exercise becomes one of lots and lots of rotating end seals to eliminate leakage.

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