Jump to content

sevenperforce

Members
  • Posts

    7,618
  • Joined

  • Last visited

Everything posted by sevenperforce

  1. Ooh - I missed that. Somebody is being a special little snowflake - just in time for winter! That article is absurd. Sure, environmental racism is a real thing. The burden created by environmental and ecological waste/neglect often falls heaviest on low-income communities, particularly communities with many POC. But raising the spectre of environmental racism in an article about a white lady from Ohio who owns two vacation homes? Please.
  2. Satellites are small and space is big. It’s not a significant issue. Most orbital skyhook designs utilize relatively low orbital paths with solar-electric propulsion modules at each end and at the center of mass.
  3. It can be adapted to either, but the phasing issues will probably be solved more readily with a conjunction class mission, if the surface hab can support that long of a stay. If you want a larger surface hab, you can do orbital assembly with an inflatable heat shield and use either an expendable, naked Falcon Heavy or an EUS for TMI.
  4. Completely unnecessary. Skyhooks are very, very long; long enough that you don’t need high gees for a good launch.
  5. I don’t imagine you’d need more than 400 m/s for landing. Perseverance popped its chute at 420 m/s airspeed, and so a couple of drogues should do plenty to get the descent speed down to something manageable. Of course, you need to reserve RCS props for the descent. Having a single descent/ascent element has an abort mode advantage, because if you have a landing problem you can simply fire your ascent stage and return to orbit. But if you’re willing to accept a separate ascent and descent element, there are definitely some advantages. You can use solar power to crack LOX out of the Martian atmosphere and eliminate more than half of your landed mass. And perhaps even more interesting: you can make your descent element do double duty. Most Martian landing architectures call for a pressurized rover that can dock to your hab. If you’re going to have a pressurized rover, then why not land your crew IN the rover? The lander module docks with your orbiter and the crew crawls through to the rover. The landing progresses much like Curiosity or Perseverance, albeit with a much bigger rover. Then the crew simply drives the rover over to a docking with the hab.
  6. Oh, interesting. That is a cool concept. What's the limit of a skycrane landing? If it landed vertically, then presumably you'd have some sort of doors so you could drive a pressurized rover right out and onto the regolith. If horizontal, then you would drive the rover out the back end. For my more conventional approach, I was thinking of a two-stage skycrane. Put the hab and inflatable side modules behind an inflatable heat shield, and put a pressurized rover on top. The skycrane lowers the whole affair down to the ground, then severs the connection between the rover and the hab and lowers THAT to the ground next to it, then flies away.
  7. Yes, if you were launching a Harmony sized module and a Tranquility sized module at the same time, you could weld them together and put them on Falcon Heavy. The important question, though, is how we would go about stuffing a Martian surface hab into a 5-meter diameter.
  8. With the fairing stretch that Falcon Heavy is getting, it could loft both Harmony and Tranquility stacked end to end at the same time. I'm more curious about how deployment would work.
  9. Falcon Heavy is fairly limited by its 5-meter fairing. What kind of a Martian surface hab can you realistically fit in that kind of cross-section?
  10. I suggest you spend some time reading about skyhooks.
  11. Yeah, the Dragon would really only be doing a handful of actual things: Fly independently from the Falcon 9 launch vehicle to rendezvous with the transfer hab; dock (exactly what it does with the ISS right now) Assist the transfer hab in holding position while the EUS mates to it Stay attached to the transfer hab during coast After Martian orbital insertion, assist the transfer hab in holding position while the return propulsion unit mates to it Assist the transfer hab in holding position while the descent/ascent element docks to it Stay attached to the transfer hab during the Martian surface mission After the Martian surface mission, assist the transfer hab in holding position while the descent/ascent element docks to it Stay attached to the transfer hab during coast Undock shortly before Earth entry interface, adjust course, and re-enter. Only a single docking-undocking sequence. No free flight. Of course, that's for an eyeballs-out burn. For a more pleasant eyeballs-in burn, the Dragon would need to carry the EUS mating adapter in its trunk. But I don't know whether the docking port on the nose of the Dragon is up for pushing an 18-tonne payload through TMI. The Red Dragon proposal would have landed on Mars using SuperDracos and I don't think there was any particular concern about how long the hypergolics would last. I suppose they call them storables for a reason.
  12. It can take 210 days docked to the ISS before the radiation environment starts to degrade the electronics, particularly the solar arrays. A beefed-up Crew Dragon for the Martian return would presumably also have beefed-up radiation protection and rad-hardened electronics. Costly but not a lot of mass growth.
  13. Fortunately this question has already been answered: Full explanation here.
  14. If the question is "how do we send stuff to Mars without Starship?" then that might actually be a viable use case for SLS...not as a launcher, but as a way of throwing heavy stuff to Mars. It doesn't have the cadence to support orbital assembly but it might be able to finish everything out at the end. The new RL-10C-3s for the EUS are supposed to get 460.1 seconds of specific impulse. This familiar image outlines some of the capabilities that the EUS is supposed to have in comparison to Block 1: And here is this (which seems like a good an estimate as any for EUS): Trans-lunar injection is 3.2 kilometers per second. If EUS on SLS Block 1B Cargo can push 42 tonnes (plus its own 13.13-tonne dry weight) to TLI, then it would need to leave LEO with an m0 of 112.1 tonnes. Given that it would mass 155.4 tonnes at staging, this means it stages just 1,474 m/s shy of LEO. Now, the SLS core would stage at a higher velocity if it wasn't pushing a 42-tonne payload on top of the EUS. The SLS has a dry mass of 85.3 tonnes and a prop mass of 987.5 tonnes, for a total wet mass of 1,072.8 tonnes. Add the EUS and a 42-tonne payload, and that's a launch mass of 1,228.2 tonnes and a burnout mass of 240.7 tonnes. With the 452 second vacuum specific impulse of the RS-25s, that's a total of 7,224 m/s. We can use this since gravity drag, aerodynamic drag, and pressure drag will be roughly the same. Yes, I'm ignoring the boosters; they don't make much of a difference in this scenario. If you take off the 42 tonnes, then your launch mass becomes 1,186.2 tonnes and your burnout mass becomes 198.7 tonnes, for a total of 7,920 m/s. So a naked EUS would stage around 696 m/s faster than an EUS carrying a 42-tonne TLI payload, meaning it would only need to burn 778 m/s to reach LEO. It would have some sort of docking/berthing adapter, of course, but let's not worry about that just yet. It would only need to burn 18 tonnes of propellant to reach LEO, leaving it with 82.3 tonnes of residuals. Now we turn to our Mars payload, assembled in LEO using some combination of commercial launch vehicles. How big could it be? Well, at 460.1 seconds of specific impulse, a stack needs to burn 65% of its total weight to get the 3.6 km/s of dV required for a Hohmann transfer to Mars. 82.3 tonnes divided by 65% gives you a total stack weight of 126.6 tonnes and a burnout mass of 44.3 tonnes. Once we subtract the dry mass of the EUS, that's about 31 tonnes injected to Mars. Assuming you have to brake in with storables, that should give you about 23 tonnes to a high, eccentric Martian orbit (a la the "podsadka" approach referenced upthread) or 15 tonnes to low Martian orbit, before subtracting the dry mass of the propulsion unit. What does that give us? Well, not much. For a best-case scenario with the eccentric Martian orbit, that's probably around 21 tonnes of useful payload. Architecture would need to look something like this: Pre-positioned hab, 16.8 tonnes launch mass. Launched direct to TMI by FHe, performs direct EDL at Mars. Descent/ascent vehicle, 31 tonnes launch mass. Launched to LEO by FH(ce), then sent to TMI by EUS, then burns its own engines to enter eccentric Martian orbit. Return propulsion module, 16 tonnes launch mass. Launched direct to TMI by FHe, uses about 5 tonnes of its propellant to enter eccentric Martian orbit. Transfer hab and Martian orbital propulsion module, 18 tonnes launch mass. Launched to LEO by FH. Beefed-up Crew Dragon, 13 tonnes launch mass. Launched to LEO by Falcon 9, docks with the transfer hab, and then the whole stack is sent to TMI by EUS. The transfer hab would be spartan -- only about 8 tonnes. The orbital propulsion module attached to the transfer hab would brake it and Crew Dragon into eccentric Martian orbit, where the vehicle would rendezvous with the waiting descent/ascent vehicle and the return propulsion module. Checkouts would ensure that both were fully operational; if the return propulsion module showed issues necessitating an abort, the descent/ascent vehicle would have ample dV to perform the TEI burn. Crew would enter the descent/ascent element, reach the surface and the hab, have their Martian stay, and then return to the waiting transfer hab, Crew Dragon, and return propulsion module. Then the return propulsion module would burn for TEI with Crew Dragon and the transfer hab. Required launches: 2X Falcon Heavy Expendable 1X Falcon Heavy (core expended) 1X Falcon Heavy 1X Falcon 9 2X SLS Block 1B The two SLS launches would be two years apart, which solves the cadence problem. I believe all these mass budgets are roughly in line with what @RCgothic proposed upthread. The difference is that I'm using Falcon Heavy's expendable configuration where necessary and I'm using an eccentric Martian orbit rather than a low Martian orbit. I can use eccentric Martian orbit because using EUS for TMI allows for a very large monolithic descent/ascent element with the necessary dV to make up for needing to go past LMO. I'm also splitting the propellant budget up so that the crew vehicle doesn't have to brake all its return propellant into Martian orbit when it arrives.
  15. Block 1B is supposed to be able to throw 42 tonnes to TLI. What can it throw to Mars? And what is our best estimate of what kind of propellant residuals EUS would have if launched empty to LEO?
  16. "Approximately" because it could be varied realtime by the ballast sled. I don't know what L/D ratio TKS had, but Apollo's was around 0.35, not 0.4, and the 7K-L1 Zond only need an L/D of 0.2 for skip entry reducing the gees from 10-15 down to 4-7 (although this was increased to 0.3 to give better landing accuracy). In any case, an L/D of 0.24 is enough for a skip entry, so it's fine.
  17. Approximately 0.24. It has a 120-kg ballast sled that can be moved back and forth behind the heatshield to adjust the L/D ratio in realtime.
  18. But this is NASA we're talking about, so they would want to use their precious Orion. Probably with the tanks largely drained. I will point out that SpaceX offers both a 1,575-mm PAF and a 2,624-mm PAF, and for the latter, the Falcon User's Guide explicitly provides for payloads up to 19 tonnes. The User's Guide also says that Falcon can accommodate heavier payloads on request. Plus, if I recall correctly, Bridenstine talked about Falcon Heavy being able to launch both ICPS and Orion in a single stack, which would come to about 52 tonnes to LEO. Of course that would be flying expendable. But I don't think there is any reason to think Falcon Heavy couldn't do a 21-tonne monolithic payload to LEO (which is about what we would expect it to be capable of given its 8 metric tonne payload to GTO). Or Falcon Heavy can send a 16.8-tonne hab module to Mars in a single launch, flying expendable. Now, a 16.8-tonne hab might not be enough. They might want to do orbital construction of something a little bigger. But even so, a naked Falcon Heavy (either expendable or partially expendable) would be able to dock with it and push it to TLI. You can do a lot with 68 tonnes of residuals. What physical characteristics of Crew Dragon are you are unfamiliar with?
  19. This is not limited to metallic hydrogen. Any modern orbital-class liquid engine would melt instantly if you tried to operate it without a coolant, which is why virtually all1 modern liquid rocket engines pump their fuel around the combustion chamber and nozzle to act as a coolant. However, if metastable metallic hydrogen existed, its properties would preclude it from being used as a coolant. So you would need an auxiliary propellant, like liquid hydrogen, to provide that coolant effect. 1. Some engines use ablative cooling, where the "coolant" is a solid layer coating the inside of the chamber and nozzle that burns off gradually. But that's not true. There are well-defined limits to everything. What you've done (and what you tend to do) is ask questions like, "If there wasn't a limit, where would the limit be?" And that's why these threads can be frustrating. I will point out that it is fine to go the hard-science route and get into the nitty gritty details. The Martian did a great job with this. But the problem arises when you try to mix nitty gritty hard-science details with handwavium. Because that breaks the reader's ability to maintain suspension of disbelief. See, this is exactly what I'm talking about. An anti-inertia drive is fine. Does it break physics fundamentally? Yes. Does it make any logical sense at all? No. But can you make one and put it into your story? Sure. Of course, what you're describing is WILDLY overpowered, but you are the god of your story, so you can fix that. If you don't your anti-inertia drive to be abused, then just say that it releases "graviton radiation" whenever it is used in a way that would make it too overpowered. You don't have to explain it. Your reader will accept the suspension of disbelief. But it makes no sense to talk about the nitty gritty details of material thermal properties in a metallic hydrogen engine when you've also got a universe with an anti-inertia drive. It's like trying to rewrite the story of Apollo 13, but with Moon Vampires. One of these things is not like the other. Agreed. It helps to know what you're talking about. And I agree. That's why I often show up in these threads. I remember when I was just learning all of this stuff. I will say, however, that I wish he'd just make one thread and stick with it. Otherwise it gets hard to see the rest of the threads I want.
  20. No, the melting point of materials is not a limiting factor for any modern rocket engine. Like I said before, there is really no realistic limit to the capacity of regenerative cooling. Modern orbital-class rocket engines run at temperatures far, far greater than the melting point of their constituent materials. For example, the RS-25 Space Shuttle Main Engine (that is going on SLS, someday) has a chamber combustion temperature of 3,300°C. Not only is that higher than the melting point of most metals, it is higher than the boiling point of most metals. The RS-25 combustion chamber is made of Inconel-718, which melts at 1,430°C, and it is lined with a copper-silver-zirconium alloy which melts at only 500°C. And yet, it persists. That is the power of regenerative cooling. Temperature is never, ever a problem.
  21. Again, Orion and NSWR are not remotely comparable. If you have access to an energy source like pure fusion, antimatter, or metallic hydrogen, it is TRIVIAL to build a very simple, bulletproof, impossible-to-mess-up engine. And you won't have to bother with making it "advanced" because your energy source is so OP that you don't need it to operate at the limits of its capabilities. If you want to have your science fiction spaceship powered by a pusher-plate spacecraft, then just propose a future where humans either (a) evolve immunity to radiation poisoning, or (b) invent super-lightweight radioactive shielding. And propose that massive uranium deposits are discovered, making nuclear charge propulsion cheaper than even a solid-fueled rocket. Then, boom (no pun intended), you've got your reason for a pusher-plate SSTO.
  22. OP tech that just works too well tends to break settings so much that arbritrary limits must be imposed to 'unbreak' them. Or your handwavium can be invoked on both ends: it can provide the "overpowered" characteristics and also provide their limits. For example, consider the beautiful Firefly-Class transport, the Serenity. Its main powerplant is the big thing in the back. It has extremely high specific impulse and boasts a respectable amount of thrust, enough to enable the ship to traverse the Verse quite quickly in quasi-Brachistochrone trajectories. It is, in essence, a torchship. Now, if such a drive were to exist, it would absolutely remove any need for the ship's twin VTOL engines, because everything in the series would be an instant tailsitter. But the writers wanted it to have cool twin VTOL engines anyway. So they avoid this problem by postulating that the engine really only works in a vacuum. When fired in space, the engine produces a pretty, ethereal glow: But firing it in the atmosphere? Well, that's a different story. Specifically, a story with a very big KABOOM. In the show, firing the main powerplant in-atmo produces a catalytic reaction with atmospheric oxygen which results in a tremendous, sky-wrenching, city-flattening firestorm-like explosion: And of course this also seriously damages the engine compressor coil, so you wouldn't ordinarily do this unless you were trying to escape a bunch of cannibalistic space pirates (which is what they were doing in this situation). What is a compressor coil? No one knows. What is a catalytic reaction with oxygen? Again, no one knows. It's just handwavium, just like the engine itself. But that's fine. The authors wanted it to look a certain way, and so that's how they made it look. Then by all means, combine rocket engines and a pusher plate. The writers of Firefly wanted to make a spaceship that looked like a lightning bug, and so they did. You can do the same. Just make it work the way you want it to, but don't expect hard science to fill in the gaps. That is not remotely true. Any vehicle which uses pulsed blasts for propulsion (let alone pulsed blasts with a ridiculously heavy and inefficient pusher plate) will have inferior dV and inferior TW in comparison to a vehicle which uses a continuously-burning engine with the same fuel. Let me say that again just so there is no confusion. Given any energy source, a controlled continuous-thrust engine will have far greater thrust and impulse than a pulsed-blast engine. This is a fundamental fact of reaction physics. It's just how things works. A nuclear saltwater rocket harnesses thermal-neutron criticality. Orion harnesses prompt-neutron criticality. The two have virtually nothing in common. It is like comparing a fuel-air explosive with a coal-burning stove and saying "well they're both the same because they're both burning hydrocarbons with air." (Technically a NSWR harnesses water as a reaction mass, boiled by thermal-neutron criticality, while Orion harnesses tungsten as a reaction mass, vaporized by a thermonuclear explosions that is initiated by prompt-neutron criticality. But that's beside the point.) This is a perfect example of why you are incorrect. An antimatter orion would be horribly wasteful and useless in comparison to an antimatter rocket. Given any energy source, a pusher-plate arrangement will always have less thrust and less impulse. Now if you want a dual-thrust-axis spacecraft with VTOL rockets and a pusher plate, then by all means create one, and handwave the reason why. But don't expect science to back you up. The "orion system" is a massive accident waiting to happen. It is a horrible, horrible idea. It is incredibly hazardous and incredibly wasteful. There is absolutely no reason to use a system like orion unless you are stuck with current-tech thermonuclear weapons as your only propellant choice.
  23. This has to do with power cycles, which are probably beyond the scope of what you are trying to figure out here. The predominant challenge for any orbital-class rocket engine is not the combustion or the choice of propellant or the cooling system, but the question of how to force propellant out of the tanks and into the thrust chamber at a high enough pressure that the pressure in the thrust chamber doesn't push back out into the tanks. Unless you have immensely high pressure in your tanks, you will need a turbopump of some kind. For expander-cycle engines, where the heat generated by the engine is used to directly drive the turbopump, the straightforward approach is to run a little cryogenic propellant into the coolant loop and allow it to emerge in an auxiliary chamber where the heated propellant can expand into a gas. That escaping gas can then turn a turbine, which turns the turbopump, which pumps the propellant. You can do regenerative cooling with any propellant, of course, but an expander-cycle engine will require a cryogenic propellant, which you probably don't want for your SSTO.
  24. No, that is not true. No orbital-class engine can "survive the heat" of active combustion, even with low-energy chemical combustion like kerolox or hypergols. That's why all orbital-class liquid-fueled engines are regeneratively cooled. A metallic hydrogen engine would require auxiliary propellant, not to "water it down", but to provide remass. The release of energy does not provide impulse without a means of transferring momentum. The same is true for pulsed-charge Orion designs; they require reaction mass in the form of powdered tungsten. A metallic hydrogen Orion would require the same, but there would be no reason to make a metallic hydrogen bomb Orion, both because (a) metallic hydrogen is not nearly as energetic as a thermonuclear warhead, and (b) if you have access to metallic hydrogen, you simply use it in an ordinary engine with a liquid remass like anhydrous ammonia. The pusher-plate design has nothing to do with surviving heat. It has to do with an inability to control nuclear explosions. That is nothing at all like what a pulse jet is. A pulse jet is simply a jet engine which uses pulsed combustion rather than continuous combustion. It was first used in combat in World War II with the V-1 flying bomb and it has never been used since because turbojets are more efficient. All of this worrying over launching "an orion" and nozzle size is stuff and nonsense. The nozzle can be as big or as small as you want it to be. No, that is not true. The heat is carried away by the escaping propellant, and any heat that is transferred to the nozzle is carried back into the propellant via regenerative cooling. Making a nozzle thicker won't keep it from melting. Making it thicker will, in fact, make the heat management problem worse. Nope, you do not need to do that. You simply use regenerative cooling, like every modern liquid-fueled orbital-class rocket engine in existence today. A rocket engine operates in a steady-state mode with a constant chamber temperature. If you have a super-material with a melting point higher than the temperature of combustion, then it doesn't matter how long you operate it for; it won't melt and you won't need to stop and let it cool off. However, you will still need regenerative cooling somewhere in the cycle or the rocket engine will transfer that heat to other parts of your ship and melt them, too. There is no limit to the heat management capabilities of regenerative cooling, and there is no reason to think that future technology with the capacity to build SSTOs would somehow forget what regenerative cooling is. If you want to broil everyone in the ship, sure.
×
×
  • Create New...