RCgothic

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Posts posted by RCgothic


  1. Rockets are constant thrust devices. It doesn't how fast you're going, they push the same regardless.

    Work is force times distance.

    If you apply 10N to a stationary block and it doesn't move, you've done no work.

    If you apply 10N to a trolley going 1m/s, you've done 10J of work.

    If you apply 10N to a rocket going 10,000m/s, you've done 100kJ of work.

    Rockets are therefore more efficient when they're going fast. The fastest part of an orbit is deep in a gravity well. Only velocity parallel to the direction of motion counts, so burning sideways is equivalent to a standing start.


  2. A huge expendable Saturn V style booster would have been fine; if it were much, much cheaper, probably as a result of not being man-rated.

    There's no reason for crew to go up on a big booster and make both functions more expensive by association.


  3. SLS in current version (Block 1) is too underpowered (and Orion too overweight) to recreate Apollo.

    Block 1B can't do so either, and its future looks shaky.

    Block 2 could, but it's so far over the horizon as to be effectively never.

    SLS is effectively what you'd get it they hadn't decided to all-up test Apollo/SaturnV and the lightened Apollo Block 2 had been cancelled along with SII. Not enough rocket, too much capsule.

    Also I note that Falcon 9 will probably be man-rated before SLS will.


  4. 28 minutes ago, tater said:

    That's the problem with designing a really, really expensive rocket and spacecraft without any particular goal in mind.

    This was the original complaint many of us had about SLS, years ago. Not that it was overly expensive. Not that it had a low launch cadence. Not that it was primarily about pork. Not that it was past due and way over budget—though it is all of those things. Our complaint used to be summed up as "rocket to nowhere."

    Too big for LEO, to small for anything else. That remains the primary problem. The other issues like cost—those only matter because if it was much cheaper, if it had a much higher cadence on top of that, then we could work around the fact it's simply not the right size via distributed launch.

     

    This exactly.

    Block 1B can't do Orion and a useful payload, so it's dead. Block 2 could, but it's so far over the horizon as to be effectively never.

    So we're stuck with Block1.

    Block 1 requires commercial support. Many launches of commercial support, with rendezvous somewhere around the moon. And if you're requiring commercial support then commercial rockets are more than capable of both putting the CSM in orbit *and* delivering separately a naked second stage with residuals to dock to and serve as an earth departure stage, and of doing it again for a lander. It's more moving parts, sure. More moving parts than rendezvousing at a gateway in LLO? Nope.

    So what exactly is the point of Block1?

    How many flights does it take to man-rate Falcon Heavy? 7? Guaranteed to cost less than EM1.


  5. Of course the danger to SLS is that if you can decide "Orion and not-a-lander to TLI" is not meaningfully better than "Orion Only to TLI", then what's to stop "Not-Orion on not-SLS to TLI".

    There are a number of commercial options nearing completion. And nothing of value would be lost./s


  6. The only way to hold a balloon in position is to tether it in multiple directions with the tethers at a reasonable angle to resist the wind and excess buoyancy such that the balloon can resist the tendency of the wind to cause it to dip.

    But as has been mentioned the weight of the the cables themselves cause the tethers to hang almost vertical at the balloon, so the restraint is much reduced.

    Plus now your tethers are more like 6km long, the excess buoyancy is additional load (e.g. Weight) they have to bear, your balloon needs to support at least 3 of them, and your tether base stations are on a circle 12km in diameter.

    Distinctly non trivial.

    It would probably be easier to design a drone that actively resists the weather with propulsion, even if you have to land to refuel. If you need constant coverage have more.


  7. 1 hour ago, tater said:

    This is an interesting idea. It requires more mass (like the plans to stabilize regolith with resins), but doesn't require the complex equipment for the spraying process. Wonder if they could then tile the actual landing area in the center with metal plates?

    My team had a little bit of experience with rocket exhausts interacting with warship coatings. It's very hard to get away from "just go out and repaint whatever the exhaust just burned off".

    Not saying it's impossible at a spaceflight budget level, but we certainly struggled with a frigate.

    Metal plates are sturdier than tarps/coatings. But hard to ship to the moon though.


  8. You can get around atmosphere pushing the medium down by having the inside be lower than the outside. The pressure head of the liquid on the exterior side will resist the push of the atmosphere on the inside.

    If ethanol is almost as suitable as mercury it would be a far better choice from a hull pressure point of view. A 5m tall vehicle would have approx 10atm of pressure at the base of the hull in a pool of mercury, but only about 1.5atm in a pool of ethanol.

    Add 1atm internal pressure and that becomes 9barG Vs 0.5barG.


  9. A vacuum airship *might* be possible on Mars. The atmosphere there has a higher density to pressure ratio than here. This is important because density is what gives buoyancy but pressure is what crushes. This is because of the higher average molecular weight of the atmosphere there - CO2 is denser than N2. Also the gravity that needs to be overcome is less.

    All other things being equal simply being in a thinner atmosphere doesn't work. As pressure reduces so does buoyancy.


  10. The only thing SLS is any good for is enormous C3 for unmanned probes, which kind of obviates the expense of man-rating the thing.

    SLS cannot by itself land a manned mission on the moon. It can't comanifest enough payload.

    Orion by itself is never going beyond earth's sphere of influence. It's too small. Orbital construction is required.

    I would rather scrap the pair of them and delay the manned space flight programme in exchange for a design that can either put man back on the moon in a single launch, or preferably a big dumb cheap booster that can put massive payloads into leo frequently and inexpensively and rendezvousing with a capsule put into orbit on a far smaller man-rated booster, usable for many purposes. Ares really did have the right idea.