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AckSed

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Posts posted by AckSed

  1. On a tangent: is anyone else pursuing the mass-production of satellites and, more importantly, satellite sensors suitable for using on probes?

    Starlink and its sister project Starshield seems like it's not only a turnkey solution for communication and surveillance, but a full satellite bus for any customer to place their satellite/probe on.

    Project Kuiper is 'just' a Starlink competitor, but it could be turned to the same ends.

    OneWeb is... lagging behind.

    If you can figure out how to mass-produce a generic suite of planetary science (like narrow and wide-field cameras with multiple filters, spectrographs and radar) that fits onto a cheap satellite, you'll have a product.

    Now I think about it, the real thing stopping a swarm of sats being placed around Mars, Venus or the outer planets is now not hardware, it's the lack of ground-support: receiving signals, upkeep, debugging, interpreting data. SpaceX is already doing that with over 6,000 sats and rising.

    If someone could figure out how to run a planetary science sat swarm (P3S for short) on minimal personnel, such that it could be sustained with throughput over 5 years for half the current cost, then you have an attractive product.

     

  2. The first Z-pinch machine was ZETA, in 1957. It did not end up working, but the British government thought it had: https://www.iter.org/newsline/-/2905

    The Centauri Dreams blog has multiple articles on fusion drive: https://www.centauri-dreams.org/?s=fusion

    That led me to this Popular Mechanics article from 2012 on pulsed fusion drive (which is what a Z-pinch essentially does - pulses of fusion explosions): https://www.popularmechanics.com/space/rockets/a7715/the-big-machine-that-could-lead-to-fusion-powered-spaceships-9450996/

    With regards to an alternate history, fusion is a dense subject that is filled with failures and lessons learned. You would need steady funding, a willingness to take risks with almost certain knowledge you aren't going to get it right the first or fifth time, perhaps a nuclear reactor for power and, ideally, a reasonably-affordable heavy-lift launch system to test it in space.

  3. As I've heard, in a two-stage reusable rocket, going up on a flatter trajectory is a lofted trajectory, and it's indeed an advantage for reusable boosters if you can push the work of getting to orbit on to the second stage. Because if the fuel burn rate is constant, the booster uses less energy to punch through the thick lower atmosphere, and to boost back to cancel its sideways momentum so it can return to the launch site.

    In the first instance, though, boosting till you leave the SOI is a tad trickier to do from Earth.

    If this cheat-sheet is correct, the amount of delta-V to escape Kerbin and into an elliptical orbit (3400 + 930 = 4330 m/s) is slightly less than that of escaping the SOI of Mars (3800 + 1440 = 5240 m/s):

    Spoiler

    21.png

    An equivalent manoeuvre to leave Earth and reach its escape velocity requires 9400 + 3210 =  12,610 m/s. So the bar is set nearly three times lower for Kerbin.

  4. Propellant density is important with SSTO, yes, but I've seen other proposals say that it can work with normal propellants.

    And I would reluctantly decline developing horizontal-takeoff SSTO spaceplanes, no matter how cool they are. Skylon's air-breathing rocket engines currently aren't getting the funding to turn them into reality.

    Perhaps you heard that hydrogen doesn't have the density for SSTO. It doesn't, not until you make it very big like the ROOST proposal. Second/third stage rocket engines working in vacuum need lower pressures, and hydrolox's high specific impulse is an advantage for for orbital manoeuvring, so it's more common there.

    Check this table of bulk densities for different fuels. That's the combination of LOX and a fuel in the optimal ratio. Any air-breathing rocket proposes not taking the LOX along for most of the trip to save mass.

    Methalox is middle-of-the-road in performance, and was passed over for the longest time, except when talking about colonising Mars. Even now, it's the propellant of choice for Raptor because of SpaceX's Mars ambitions AND engineering talents AND the amount of money coming in from launch services and Starlink AND their tolerance of risk are pushing it to its limit.

    Its ratio of 3 parts oxygen to one part methane means that the majority of propellant is dense LOX, increasing its bulk density to 8/10ths kerelox. Methane's handling is manageable and it is cheap.

  5. If metallic hydrogen could be a thing in multi-ton lots, it wouldn't need it. MH changing phase and decomposing back to regular hydrogen would release incredible amounts of energy (216 megajoules/kg) that makes hydrolox (10 MJ/kg) and TNT (4.2 MJ/kg) seem wimpy.

    Melting carbon and such to make methalox is gilding the lily, so to speak.

    The decomposition is at a ferocious 6000+ deg. C, so you're stuck taking hydrogen or some other fluid anyway to cool the reaction chamber. Water might work, though the heavier exhaust products would halve the specific impulse, but increase the thrust.

  6. 29 minutes ago, farmerben said:

    I wonder what happens when a tokomak quenches all at once.  The magnetic field in the inductor cannot change instantly so the activity of the plasma must spike.

    The plasma will hit the sides and burn some shielding. The inductor's field would drop rapidly and be expressed as heat in the coil; it'd probably burn out. The plasma's temperature might be impossibly high, but the total thermal energy is relatively low. Part of the reason they are so damn difficult to build is that you have a very slippery snake of plasma and you're containing it with, essentially, magnetic rubber bands. Soon as you lose containment, it squirts out the gaps.

    If you're thinking Z-pinch = short in coils = fault will crunch/heat tokomak plasma so much it will explode, no. You have to design this ability to compress to fusion temps/pressures in. It's also not trivial.

    We are learning that we can make coils that can survive a quench with minimal violence, too. I'll repost what I posted in Science News: https://news.mit.edu/2024/tests-show-high-temperature-superconducting-magnets-fusion-ready-0304

    Quote

    Whyte says, “Basically we did the worst thing possible to a coil [a quench], on purpose, after we had tested all other aspects of the coil performance. And we found that most of the coil survived with no damage,” while one isolated area sustained some melting. “It’s like a few percent of the volume of the coil that got damaged.” And that led to revisions in the design that are expected to prevent such damage in the actual fusion device magnets, even under the most extreme conditions.

     

  7. Helion is developing its reactor that uses field-reversed confinement with magneto-hydrodynamic pickup, and iterating on engineered prototypes with realistic expectations, which is a huge win. I think it's the closest to true fusion right now. But it and its banks of capacitors are the size of a building.

    Here's the thing. A superconducting magnetic coil is an energy storage device, or SMES. It's not great on volume, it's kind of heavy, but the energy it stores and the current is amazing. Further, it's being used today, mainly in smoothing out power delivery in chip fab plants.

    Hand-in-hand with most developed fusion reactors are... superconducting magnetic coils. We already have comparatively lighter and smaller coils from MIT's SPARC project. I say 'comparatively', but the test magnet is 9.6 metric tons.

    Even if Helion or the other reactors coming out don't work out as a reaction drive, what if they were combined in some near-future ship? FRC reactor charges the coils, coils discharge into a Z-pinch drive.

    It'd be an absolute pig, but we have a reusable heavy-lift vehicle coming online in a few years.

    There'd be advantages to a charged superconducting magnetic coil as well, like maintaining an active storm shelter.

  8. 4 hours ago, sevenperforce said:

    The name of it isn't coming to me at the moment, but I believe there is a version of Z-Pinch that would use a fission reactor to charge a capacitor that would then dump into a metal ring to create the collapsing magnetic field. The capacitor and metal ring would all be sacrificial and would be composed of a high amount of lithium to seed the fusion reaction. You'd have a magnetic nozzle, also powered by the fission reactor, to point the escaping plasma in the correct direction. Basically a really tiny pure fusion Orion.

    Not useful for power generation on Earth because it's technically net-negative in terms of the ability to generate usable power, but very suitable for spacecraft propulsion. Good for going to Mars and back.

    That'd be the Pulsed Magneto-Inertial Fusion: https://www.scientia.global/dr-john-slough-fuelling-the-next-generation-of-rockets-with-nuclear-fusion/

    I remembered this because the D-D Fusion Magneto-Inertial Reactor is a key card in High Frontier 4 All, and a favoured way to build a rocket. (If you're a rocket nerd, you need to play this boardgame.) The Appendix helpfully listed the specs (0.1 Hz firing rate, Q of 200, 510MWth and the person responsible, John Slough, though its version uses a "350kW solar-powered initiator".

  9. Tests show high-temperature superconducting magnets are ready for fusion

    tl;dr

    New superconducting REBCO magnets tested that run at 20 Kelvin and made a steady field strength of 20 Tesla. Unique, no-insulation design that allows you to lower the voltage, which leaves more room for more cooling or stronger structures. Deliberately quenched a full-scale coil to see what broke and where the models failed, so they could update their models: one corner melted, most of the rest of the coil survived with no damage. Led to revisions in design that are supposed to prevent the failure mode.

  10. A read of xkcd's What if? on Fire By Moonlight gives a reasonable explanation of how optics and light-gathering by a lens works in explaining why you can't set fire to anything with moonlight, no matter how large a lens: for one, you can't exceed the surface temperature of a heat/light-emitting black body, and the Moon's is ~100 deg. C; for another:

    Quote

    Except lenses don't concentrate light down onto a point—not unless the light source is also a point. They concentrate light down onto an area—a tiny image of the Sun [or Moon].

    So the diagrams are simplifying for illustration because you can't show every beam of light entering, but also lying in the process. Lenses do gather light from as many directions as possible to project an image. The "point of sharp focus" is where the smallest image gathered by the main lens is. Focal length is how far behind the point of sharp focus the lens (or group of lenses) projects the now-inverted image. More on focal length, field of view.

    The double lenses are for reducing chromatic aberration (the rainbow at the edge of images), as different wavelengths of light are refracted differently to a greater or lesser degree by the same material, so (at least in the earliest achromatic doublet lenses) two different types of glass that form a single lens were used, the second type with a different refractive index to bend the light back.

     

  11. 4 hours ago, Piscator said:

    If you're building this many satellites you probably get a few rejects you can use to test the dispensing mechanism. I agree though that it wouldn't be very sensible to release operational satellites on a suborbital trajectory and that trajectories will likely remain suborbital until a deorbit burn is demonstrated.

    They could go Aperture Science and test this test by asking 'are the argon thrusters enough to raise their orbit from a near-orbit parabola?' These will not be small satellites, and perhaps they have space to add extra argon or thrusters.

  12. There's an interesting comment about how Starlink could/should have transmitted all through the descent:

    Quote

     @holyknight51: Hey Scott, A note on the hypersonic communication blackout problem. The frequencies that are cutoff is a function of the density of the plasma, so the more dense the plasma, the higher the cutoff frequency, for reentry vehicles this can go as high as 40 GHz depending on several other factors. However, about a month or two ago, SpaceX placed a starlink terminal on a dragon capsule in order to experiment with using starlink as a bent pipe similar to how the space shuttle handled the problem. So with starlink using higher frequencies to go above the cutoff frequencies and being placed on the backside of starship where the plasma is less dense and thus a lower cutoff frequency, I would have expected them to be able to maintain communications through the descentl My credentials are a masters in engineering physics, having studied Ionospheric scintillation in college and currently work as an RF test engineer.

    So 1: when it cut out would have been when the airframe couldn't take it any more; 2: next test we should see it stream its reentry all the way to the ground. Ocean.

  13. 2 minutes ago, JoeSchmuckatelli said:

    Very late to the party - just watched with my students. 

    Why was Booster so much above terminal so deep into the atmosphere? 

    It was supersonic at 5km.

    I think it was going for the hoverslam, but the roll oscillation starved the engines and removed the 'hover'.

  14. All right, let's say Exoscientist's suspicions are justified, and the Raptors, even now, are prone to exploding. Presume they are also a pain to relight and require ullage and settling of the propellant before they'll behave.

    Can you make a staged combustion engine more reliable? Peter Beck of Rocket Lab says you can: by building it to withstand and run at extremes, then under-driving it, you end up in the same level of reliability as a gas-generator rocket engine. Neutron's Archimedes is ox-rich, not full-flow, but building to run at max, then under-running at a more comfortable level is a solid path to good reliability.

    Can they program Starship and/or Superheavy to under-run the engines at the cost of payload? I say yes.

    Will they, or is Elon cowboying ahead with demands for, "More pressure! MORE thrust! More payload! Stuff blowing up! Boom!" and laughing maniacally? I don't have that insight, but they are complying with FAA regulations and NASA requirements, and I do believe Musky-boy knows when not to push his engineers and the physics to make things blow up, now the basic stack mostly works.

    On relight: How many flight tests would it take to uncover all the quirks in handling the stack, and implement the necessary hardware and software changes to make relight reliable? We have at least four more planned, because that's how many boosters are being built right now. There are more on the way.

    SpaceX has been using Falcon 9 Starlink launches to hone reuse parameters, learn more about the airframe and where its margins are e.g. jettisoning the fairings half a second earlier each flight. It's clear they intend to do the same with Starship/Superheavy. They will have at least four more attempts to relight engines in orbit and on landing, and test heatshields further.

    Starships and Superheavies can and have been modified, or scrapped entirely, thanks to their stainless steel construction. The engines are the most expensive part and apparently problematic, but I have outlined a path to making them more reliable. If the engine design is fundamentally flawed and cannot relight at all without a forest of proper ullage thrusters they may have to add them. And that's fine. They will do that.

    When I look at SpaceX and what they've achieved, I'm reminded of Parson Gotti, in the webcomic Erfworld: "We try things. Occasionally they even work."

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