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AckSed

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    SpaceX Discussion Thread

    in Science & Spaceflight

    Posted

    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.

    SpaceX Discussion Thread

    in Science & Spaceflight

    Posted · Edited by AckSed

    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.

    SpaceX Discussion Thread

    in Science & Spaceflight

    Posted · Edited by AckSed

    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'.

    SpaceX Discussion Thread

    in Science & Spaceflight

    Posted

    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."

    SpaceX Discussion Thread

    in Science & Spaceflight

    Posted

    8 hours ago, DAL59 said:

    Why has there been little discussion of using Starship for planetary probes? Wouldn't access to a cheap, super heavy launch vehicle allow missions like Dragonfly and the Uranus Orbiter to be much quicker by allowing direct routes instead of needing multiple gravity assists? I know Starship isn't an operational vehicle yet, but if NASA is willing to use it for as its plan for Artemis in just 2.5 years, why don't any missions slated for 5+ years in the future, when even pessimistically Starship will be available, plan to use it? A faster mission profile also reduces the plutonium requirements. 

    I think it's institutional inertia. Any mission using channels that aren't 'established' must wriggle around the bureaucratic blockages in the arteries of the whole organisation. They need champions on the inside, like Ingenuity, which was almost killed-off.

    If you'll allow me a little rant here, no-one is prepared for what's coming when both StarShip reuse and in-orbit refuelling is proven out. No-one. Not. A. Soul.

    NASA? As the monks in the cathedral apply the gold leaf to their metaphorical hand-drawn parchment, they're trying very hard not to think about the printing press, even as a few juniors in the back rooms marvel at the process.

    The cubesat and smallsat builders? Not ready.

    Universities and research organisations? Yeah, I think I heard about Starship...

    Only a few in the commercial sector are thinking about it (e.g Varda and Sierra Space), and it's more about heavy-lift than the ability to send Starship off as a monster probe from LEO with a kick stage.

    I suspect SpaceX isn't fully ready for it either. The hardware for the Mars mission before the colony will still need specialised equipment, and that is still but a dream.

    For Questions That Don't Merit Their Own Thread

    in Science & Spaceflight

    Posted · Edited by AckSed

    It should, but there's no free lunch in that case. Sure, you're pushing further, but you now have to get the beaming station back into place, and even with a small, efficient chemical rocket that's a lot of fuel. Allowing it to be pushed back into position by the laser (because both sides of the cavity are bouncing photons back and forth) halves the force.

    This is what solar sails would excel at. Either acting as a statite at L1/L2/L5, providing opposing force and maybe solar power for the beam, or used for the return trip. Diffractive sails are a fascinating variation that provide "a component of force perpendicular to the sun line, thereby allowing navigation without sacrificing the amount of solar power on the sail."

    You could use boring solar-powered Hall-Effect Thrusters too. If it's staying in one place, there's no drop in power.

    For Questions That Don't Merit Their Own Thread

    in Science & Spaceflight

    Posted

    Could someone check my assumptions here? I'm trying to work out Photonic Laser Thrusters (PDF) (really quick explanation: form an optical cavity in between source and mirror with gain medium to bounce laser back and forth thousands of times, creating useful thrust at mirror) and how they respond to added mass.

    The 1 gigawatt laser is turned off when it is 30,000 km away from the beaming station, due to attenuation. I am assuming the beam is firing from the Moon and the receiving craft is in orbit around the Moon, so it is travelling at ~1600 m/s.

    The PDF gives the example of a future craft with 1-ton total mass, 50-metre mirror and 500kg payload, has an acceleration of 32.27 m/s2 and is within range of the beaming station for 1.2 hours, or 4320 seconds. It exerts a force of 3227N, and final velocity is 141km/s.

    With a 10-ton total mass, the same mirror, and 9.5 tons of payload, that acceleration is reduced by a factor of ten to 3.227 m/s2, because a = F / m.

    Here's the assumption: I can apply the same force for longer on a greater mass, to gain the same final velocity once it passes beyond range of the laser applying motive force, correct? Obviously forces when leaving orbit are more complex as they have to take a vector, but to a naive approximation, is this correct?

    How livable is Titan?

    in Science & Spaceflight

    Posted

    22 hours ago, farmerben said:

    I'm curious if you could withstand crushing inside the gas giants.

    Neptune is surprisingly not an instant death - if you're inside a sufficiently strong aerostat/submarine. And you avoid the fastest storms in the Solar System.

    Radiation is sorta benign this far out from the sun. The gravity at the 'surface' is 1.14 g, and the temperature at 1 bar is a brisk -201 deg. C. Go deeper and the temperature rises. Bob about in the relative calm of the North Pole and about *waggles hand* 30 bar pressure, and you'll almost be warm enough to not freeze.

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