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Everything posted by sevenperforce

  1. For anyone who might find it useful, I went ahead and did a pixel count with known values (total vehicle height and second stage height) to estimate the dimensions of everything in Neutron so far: Interesting that the "5 meter diameter" fairing appears to be the true internal envelope diameter (as this is the diameter of the upper stage) rather than a measurement of the outside of the fairing, which is notably larger. The 7 meter diameter does not include the landing legs. I'm sure someone can drum up tank volumes from this.
  2. It's fun and dancy and full of joy, as JLo would say.
  3. This is clearly just preliminary concept art, but I spy four reasonably large engine bells peeking out from what appears to be the service module. Given that there is no apparent intention to make the upper stage reusable, it might make sense to give the service module a ring of simple, high-thrust, low-efficiency methalox engines (preferably ignited with a solid pyro) plumbed through the PAF to the upper stage itself. Then, in a low-altitude abort, those engines would drain the upper stage in a matter of moments, pulling it and the capsule free of the booster: This would eliminate the need for carrying the abort propellant on the service module like Starliner (which means less total mass to orbit), and also avoids plumbing the capsule itself with the same propellant used for abort like Crew Dragon. The abort motors would be pretty thoroughly oversized but they'd certainly have no lack of propellant.
  4. Interesting...from this article, it's stated that the very hefty first stage will use methalox, not hydrolox: https://universemagazine.com/en/aerospace-startup-tests-reusable-rocket-engines/ Not particularly well-written so I don't know how much I trust it, but it might have been something discussed in one of the presentations that I missed. I'm still pining for that fully-reusable sustainer-architecture TSTO of lore, though. I suppose that once they have it flying as a proper TSTO, they could start experimenting with making the upper stage an SSTO using hydrogen drop tanks like the ROMBUS concept. Or, you know, make the drop tanks crossfed side boosters that use the interface with the side of the main ship as an intake ramp for air-augmentation. . . .
  5. I'm eternally sad that they jettisoned the drop tank plan (pun intended). It always seemed like an electric-pump-fed methalox lander with solar-rechargeable batteries and drop tanks was a really clean solution.
  6. I know this is kinda off topic, but while you remember right about the dual turbines, each also had their own separate preburner: Oh, dang, that's even worse than I remembered. You'd think that with completely separate preburners, they would have worked on varying the mixture ratios to optimize thrust and Isp since the pumps are completely independent, but evidently the RS-25 maintains a single mixture ratio at all times. Weird. And technically the RS-25 is both a FRSC engine *and* a closed expander cycle engine, since the fuel is pumped around the chamber and engine bell to cool it, then that supercritical fuel expands through a low-pressure turbopump to step up the propellant pressure before it enters the main turbopump: Because the RS-25 uses a fuel-rich preburner to operate the oxidizer turbopump, it had to have a separate helium tank to provide a constant flow of helium to continually purge the cavity between the fuel and the oxidizer. The only thing that doesn't need the constant helium purge would be a proper FFSC engine like Raptor. The engine I was thinking of was not actually the RS-25, but the RS-68. It has a single burner (although it's a gas generator, not a preburner) that exhausts into separate turbines which run the pumps independently: The two gas generator exhausts also provide single-axis roll control. The Chinese YF-77, which is essentially a clone of the RS-68 (except that it uses active dump cooling on the nozzle instead of ablative cooling like the RS-68) does the same thing, with a single gas generator and two separate turbines with two separate exhausts. They fly the YF-77 in pairs on the Long March 5 core, though, so I don't know whether they use the nozzles for roll control at all. As always, I highly recommend reading this amazing blog post about the many possible complexities of just one type of engine cycle. The engines on Neutron's first stage will always keep the center of gravity extremely low, just like with Falcon 9's first stage. Opening the fairings would definitely pull the center of lift/drag further back, but that's not going to be a major issue since the center of gravity will already be low enough. I was just thinking in terms of additional drag.
  7. Illustrating again why "start with the coldest possible substance" is not a useful exercise in active cooling. You need the same amount of delta-v. You need more thrust. . . . Well, to be pedantic: if you have a vehicle with just enough Δv to make orbit, and then you add mass to that vehicle for heat shielding, then that vehicle no longer has enough Δv to make orbit. So you need more Δv. And you might not need any more thrust at all; you might have wholly sufficient thrust already. There are any number of ways to get more Δv and "more thrust" or "more power" are not the whole.
  8. Brittle? As in easy to break? A metal can be either brittle or ductile. A more ductile material will be able to handle expansion and contraction without cracking, while a more brittle material will not. Put simply, you can make a very springy spring out of steel; you cannot make a very springy spring out of tungsten. Having a ductile construction material is important for a vehicle like Starship because the tanks will expand and contract as they go through temperature cycles. Tungsten is also very difficult to work because it is so brittle. It is very hard to bend it at all, but even if you can get it to bend, it will probably crack. Its melting temperature is so high that it is also extraordinarily difficult to weld, and in fact it must be welded in a pure argon atmosphere (or another inert gas) or other stuff will get into the weld and weaken it. Its high melting point means you can't even smelt it; you have to extract it and refine it chemically. Fine... coat a layer of tungsten over the steel. If you have a steel inner tank wall with something that will keep the steel cool, then you don't need the tungsten at all. To that point, though, there's something fundamental you need to understand about cooling. Suppose you have a back injury and you want to put an ice pack on your back (which we will assume normally has a surface temperature of 35°C). What will keep your back colder for longer: a pouch of water at 0°C, or a pouch of rubbing alcohol at -10°C? Assume that both pouches contain the same weight of coolant. You might think the rubbing alcohol would work better, because it's starting at a lower temperature. But you'd be wrong. The heat capacity of water is 74% higher than the heat capacity of rubbing alcohol, so by the time the alcohol temperature goes from -10°C to 35°C, the water will only have gone from 0°C to 25.8°C. That's because water is just better at absorbing heat than rubbing alcohol. Keeping something from overheating is not a function of how cold your coolant starts out, but rather a function of how fast your coolant can get rid of heat. If you want to use active cooling with liquid hydrogen or liquid helium or even water, that's perfectly fine. Just know that you'll have to carry that extra liquid hydrogen or liquid helium or liquid water into orbit. No, we don't. You don't need more power; you need more delta-v. And simply adding mass is one way to fix it but not the only way. You don't even need a bunch of mass. The design by Stoke uses an actively-cooled heat shield.
  9. Tungsten has a cross-sectional ultimate tensile strength that is 90% greater than stainless steel. However, its density is 145% greater than stainless steel. So a tungsten tank with the equivalent strength of a steel tank will be around 30% heavier. It's also very brittle and becomes more brittle as it gets cold, which makes it VERY poor for holding cryogens. And before you suppose that a layer of tungsten will work: tungsten can handle re-entry heat just fine, but it will also transfer that heat to whatever it is touching, so you'll still need insulation between the tungsten and everything else.
  10. Good idea. Unfortunately: Dreamchaser's heat shield isn't rated for high-energy returns from the moon Dreamchaser doesn't have enough propellant to enter or return from lunar orbit There aren't any upper stages with storable propellants that have enough dV to push Dreamchaser into TLI.
  11. There was a time when we had the capability to keep an extra shuttle ready on the other pad for a rescue mission every time we launched. No reason we couldn’t have maintained both pads, transitioning one to Shuttle-C and one to DIRECT in order to ultimately set up for EOR lunar missions. Shuttle-C would have been able to put 71 tonnes into LEO. That, surely, is sufficient for an all-hypergol lunar lander including LOI braking propellant. Combined in LEO with Orion launched on Jupiter-246, with the JUS upper stage performing the TLI for the whole stack, you surely would have had enough performance for the whole mission.
  12. Shuttle C at least keeps your pad running long enough to come up with a smooth transition to DIRECT.
  13. That’s counter-intuitive, though, because a longer first stage burn (to enable the fairing drop and staging to be simultaneous) means a hotter re-entry, which you’d think would require a braking burn. Maybe if they leave the clamshell open for initial shuttlecock entry, like Stoke is planning, that will help. Well oxygen is denser so you can extract more torque in a smaller turbine. That’s probably the first reason. Using ORSC also gives you a higher overall O:F ratio which reduces both your tank volume and your propellant costs. Finally, while coking isn’t typically a problem with methane, ORSC is much better-studied and has a longer legacy, and it certainly obviates any possibility of coking from any impurities that end up in the CH4. IIRC, the RS-25’s FRSC cycle actually uses dual turbines running off a single fuel-rich preburner because trying to build a single-shaft FRSC engine is just too complicated.
  14. It's also very interesting that they provided the expendable, reusable, and RTLS payloads specifically (15 tonnes, 13 tonnes, and 8 tonnes respectively). The difference between the downrange recovery and expended recovery is remarkably small. Definitely enough information that we can play around with it and maybe come up with some dry mass numbers.
  15. So they're going with oxygen-rich staged combustion rather than the gas generator we had originally anticipated. Looking at 303 seconds of sea level specific impulse, 329 seconds of vacuum specific impulse for the sea level engines, and 367 seconds of specific impulse for the vacuum engine. The "not a capsule announcement" is about what I would have expected for not really having much of a plan yet. But it would be cool if they were thinking about making the service module integrated with the second stage. Very interesting that the staging structure is NOT what I had originally thought. I had imagined that the upper stage and the payload was all contained within the moveable fairing, but in actuality the upper stage is contained within the whole fixed upper half of the launch vehicle.
  16. The Kerbal Diana program (hint: Diana is the Roman version of the Greek goddess Artemis) is plagued with cost overruns, schedule snafus, and recurring problems with legacy hardware. However, the Kerbal Space Center is confident that their KLS Megarocket will eventually be functional, and they're looking for commercial solutions to get crew and cargo down to the Munar surface from the Munar Gateway. For this project, the powers that be at the KSC are looking for two things: reusability and flexibility. They want the same autonomous vehicle to be able to deliver crew on one trip and cargo on the next trip, or vice versa. For cargo deliveries, like rovers or surface stations, the vehicle should drop the payload off on the surface of the Mun and return back to a space station; for crew deliveries, the vehicle needs to bring the crew back. You can do drop tanks or refilling, but obviously the lower mass required from Kerbin to the space station, the better. Show me what you've got!
  17. That seems odd to me as well. The booster lands approximately five miles downrange so that's a pretty vertical re-entry nominally. Maybe the abort kicked the capsule back toward the launch site which made it more vertical?
  18. Again, these numbers aren't right; you need to factor in the additional engines that an expendable Starship Lite SSTO would need to get off the ground. But even if they were right, what of it? If SpaceX is willing to fly Starship upper stages expendable, then they would just put those expendable upper stages on top of the reusable Superheavy booster. Then they only need to throw away three vacuum engines and they get 175 tonnes to LEO.
  19. Here's what that absolutely horrific monstrosity would look like, just in case you've got extra eye bleach you need to use:
  20. We must always and forever lament that Orion Lite wasn't flying ISS missions this whole time. If it had been, we'd be accustomed to Orion reuse and adaptable Orion service modules. Things like upgrading the heat shield and doing an extended service module for cislunar missions would be natural evolutions rather than clean-slate challenges. But, alas, it was not to be. Orion is a little bloated for a capsule, but given its additional capabilities and carrying capacity, the bloat over the Apollo CM isn't THAT bad. It really just needs a meaningful service module. Lunar Gateway makes no sense absent Orion's current SM, but let's suppose we give Orion's new service module a total of ~2.1 km/s of dV. That's enough to shuttle between TLI and LLO with one stopover in NRHO (but not 2). Current injected lunar mass is 26.5 tonnes with 8.6 tonnes of propellant, giving it the measly 1,216 m/s it has right now. Multiplying the propellant mass by 2.3 (and adding on another ~2 tonnes for tank mass growth and associated margin) gives it 2,150 m/s and gives an injected TLI mass of just under 40 tonnes. Is there any commercial upper stage big enough to push 40 tonnes to TLI? You've got to have some reasonable T/W ratio at staging, after all. Here's what existing and near-future upper stages would do for a 40-tonne evolved Orion (note, some values estimated): Stage Stage m0 Stage mf Total thrust & Isp T/W ratio w/OrionX dV w/OrionX Centaur III DEC 23.3 mt 2.5 mt 198 kN, 451 s 0.32 1,762 m/s Centaur V 57.2 mt 5.2 mt 214 kN, 453.8 s 0.22 3,407 m/s New Glenn US 190 mt 15 mt 1,420 kN, 449 s 0.63 6,300 m/s Starship Lite 1240 mt 40 mt 7,825 kN, 378 s 0.62 10,160 m/s ICPS 34.2 mt 3.5 mt 110 kN, 465.5 s 0.15 2,438 m/s Ariane 6 ULPM 34.6 mt 3.6 mt 180 kN, 457 s 0.24 2,407 m/s Falcon 9/H US 111 mt 4.5 mt 934 kN, 348 s 0.63 4,170 m/s Any upper stage for an evolved Orion would need to do the same job as the S-IVB: completing orbital insertion and then providing the TLI burn. The requirement for TLI is 3.2 km/s, so that alone eliminates everything but Centaur V, New Glenn US, F9/H US, and of course Starship Lite. However, I'm skeptical that a staging T/W ratio of 0.22 will be sufficient, given the need for abort modes. For those four stages, this is the staging velocity you'll need: Centaur V: 7.6 km/s New Glenn US: 4.7 km/s Starship Lite: 0.8 km/s Falcon 9/H US: 6.8 km/s So, the question is whether there's a way to build a commercial frankenrocket capable of lofting any of the above configurations to those respective staging velocities. Note that Starship Lite is still just massively overpowered for this job. It would make more sense for SpaceX to build a Starship Ultralite with shorter tanks and only a single Raptor Vacuum if they were asked to send a 40-tonne payload to TLI in a single launch. There has already been some talk about a three-core Vulcan Heavy. It's horribly cursed, of course, but I would suspect that an expendable Vulcan core with two Falcon Heavy side boosters could surely loft the New Glenn upper stage to 4.7 km/s...possibly while preserving recovery of the FH boosters and perhaps even SMART reuse on Vulcan. The diameter change from Vulcan's 5.4 meters to New Glenn's upper stage 7 meters would be horrific, though.
  21. The trouble is that SH+SS is just so capable that any reasonable analysis ends up reducing to "just use Starship". Comanifesting is fun and all, but part of the point of the whole way Artemis is envisioned is that comanifesting isn't really the point since you're sending other stuff separately, because neither Orion nor SLS have the capability. If political considerations were nonexistent and you were willing to field a lego rocket that is truly accursed, I wonder what capability you'd get if you slapped two Falcon Heavy side boosters onto a New Glenn core topped with a Centaur V upper stage. But Orion's SM is ultimately the problem.
  22. Elon already said that a maximally slimmed-down Starship would only be around 40 tonnes. That's stretching the math. No, it would not, because your estimates for "reusability systems" aren't reasonable. There's already a reusability system for Starship: the heat shield and flaps and forward LOX tank. There's no re-entry configuration for Starship that would achieve meaningful payload at lower dry mass than the existing cargo Starship.
  23. Definitely takes much longer, as @mikegarrison notes. Which is not ideal for life support, obviously. However, it is a way to get a free-return while also reducing the size of the insertion burn. I've done it a number of times in KSP when I have extra margin on the TLI stage but I still want a free-return. Obviously in KSP I could leave the TLI stage attached since you get unlimited restarts and infinite throttling, but it doesn't look as pretty. IRL, it could have been used for something like Artemis 1 to increase an uncrewed Orion's cislunar capabilities while also setting up for a re-entry test. With the high-apogee prograde free-return, you actually are still being gravity boosted into a more energetic orbit, but your trajectory is being altered so that you end up going back through Earth's atmosphere with that higher energy. You can also use this kind of a maneuver to do a more energetic Oberth maneuver at perigee for a deep space mission. The faster you're going when you start the final ejection burn, the more help Oberth will give you, but there's an obvious limit to how high your apogee can be. Accordingly, you could use this kind of trajectory to get a lunar gravity assist added to your ejection energy before your final ejection burn. Don't know whether this has ever been done in real life. Yep. The rotational direction doesn't matter but the orbital direction does, obviously. That's where the gravity assist (positive or negative) comes from. Apropos of nothing (somewhat), this is essentially the maneuver depicted in the xkcd comic that first introduced me to Kerbal Space Program:
  24. You can also do a high-energy free-return where you circle the moon in a counterclockwise fashion but with a very high apogee, so you don't really get affected by the moon's gravity on the outward journey but you do a close pass on the return journey that will nudge you back into an Earth entry interface trajectory. This is useful if you want to take maximum advantage of the Oberth affect and you want to frontload some of that energy using your launch vehicle during the TLI burn and use less dV for a counterclockwise low lunar capture burn. But this sends you back into a free return with a much higher Earth entry velocity than you would get from a retrograde free-return, so your heat shield's mileage may vary.
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