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Everything posted by sevenperforce

  1. One other cool possibility: SpaceX could use a Dragon XL spacecraft with an APAS docking adapter on the tail end and a hatch on the side. The XL could perform all the reboosts using its main forward thrusters and it could also allow Polaris II to dock with it and use it as an airlock to test Hubble servicing.
  2. I don't believe so. Modifications to allow them to be restarted would invalidate human-rating. Also they are less efficient than the Dracos, both in terms of vacuum specific impulse and cosine losses. I think the gee-loading could potentially be within limits, although it would be iffy. The SuperDracos are not only canted out but are also canted at an angle to allow roll control by differential throttling, so you'd have to fire a minimum of four. Even at 20% (minimum throttle), that's 58.4 kN, reduced by cosine losses to 56.4 kN. Crew Dragon masses about 12.5 tonnes and Hubble is 11 tonnes and so that's about 0.25 gees which is probably just on the edge of what Hubble and the docking system can handle. I'm sure that adapting the docking software to allow "back-in parking" is a shorter pole than major hardware redesigns.
  3. Keep in mind that everything in the galaxy is orbiting the galaxy (except the pressure wave arms themselves) and so there's no meaningful astronomical alignment that would happen once per orbit.
  4. Someone over on NSF pointed out that these are probably ellipsoidal caps, not spherical caps, so I recalculated. While the lower (assumed CH4) booster tank volume somewhat intuitively remains the same, the upper (assumed LOX) booster tank volume goes up from 238.3 cubic meters to 255.4 cubic meters, bringing the apparent mixture ratio up from 2.99 to to 3.2 which is still surprisingly fuel-rich but not as severely as before. Booster propellant mass goes up, from 363 tonnes to 382 tonnes, and upper stage prop mass goes up from 84 to 90 tonnes. If the 1.27:1 TWR holds then we are looking at a total vehicle dry mass percentage of 11.1% which makes sense.
  5. So this is a flyback booster with a side-slung expendable upper stage? Kinda giving me Rockwell 1969 vibes but that's cool. Not sure I would want to launch humans on anything side-slung after STS, though.
  6. Yes, but then you have no way to reboost Hubble, because the main thrusters are under the nosecone. Cosine losses on the aft translational thrusters would be prohibitive.
  7. They confirmed that they contemplate a direct docking with the existing soft-capture mechanism, without the need for a grapple arm. Hubble's current velocity at apogee is 7.593 km/s and its perigee is 333.7 miles. To flip the perigee and apogee and bring the new apogee up to the 375 miles mentioned on the call, Hubble's velocity would have to increase to 7.611 km/s. So it only needs a change in velocity of about 18 m/s. Crew Dragon carries 1,388 kg of propellant and its main thrusters get 300 s of specific impulse in vacuum. Crew Dragon has a mass of approximately 12.5 tonnes on orbit and Hubble has a mass of approximately 11 tonnes. It would need 143 kg of propellants to develop 18 m/s of dV, which would be a burn of around 77.7 seconds. My guess, then, is that Crew Dragon (assuming a good insertion by Falcon 9) has enough capability to fully circularize Hubble's orbit at ~375 miles and return to LEO and then deorbit. They just said they are looking at pulling off up to 70 km of reboost which would bring it to almost 380 miles. I'm guessing that using the onboard propellant is the limiting factor, then. If they were docking nose-first and putting new thrusters in the trunk then they could put much more propellant back there. That mode would make it impossible to EVA unless they used the ground access hatch.
  8. Not really, no. You'd have to have a complete redesign. And there's no way to pass through the heat shield so getting back to the trunk would require an EVA.
  9. Something like this? Crewed flights can absolutely carry stuff in the trunk, but it just can't be very large. See, here's where the top of the F9US sits: AND YES they are doing a feasibility study for reboosting Hubble with SpaceX!
  10. Another possibility -- place an extensible adapter in the trunk of Crew Dragon and have it rendezvous tail-first, with crew on EVA available to troubleshoot any problems attaching properly to Hubble. Remember that the main thrusters on Crew Dragon are under the nosecone so if you want to reboost Hubble efficiently you'd want to dock tail-first.
  11. Remember that humans are space orcs. Compared to virtually all other terrestrial life, our resilience, endurance, healing factor, and capacity for accepting diverse sources of energy is utterly unmatched (in the aggregate, not on every individual level).
  12. A quick check shows flights from LAX to Sydney run between 14 and 23 hours (with stops or nonstop). The flight with Starship would be what, under 30 minutes? That's a lot of flight opportunities during the 14+ hour flight the old-fashioned way. Such a system would be hub-based (again, I find the idea incredibly unlikely, I'm just steel-manning the concept). Get people to a distant hub, then they can take a plane, or possibly another rocket I suppose (to aggregate travelers). Dubai likely has ~0 weather scrubs. Most cities would have more weather issues, obviously, but if you wait a few hours to leave, then fly to the opposite side of the world in 30 min, it's still a short trip. I, for one, can't stand long flights. I agree that the market for it is extremely speculative. But we'll see. They're building Starship anyway for orbital purposes, so if the market exists, there's no barrier to access, just to execution.
  13. It’s based on an image stack which I believe was already linked upthread.
  14. Someone on Twitter overlaid all the images to get the maximum possible resolution, and so I used that to put together an animation of the last five seconds prior to impact in real-time at the blazing speed of ten frames per second (which is at least enough to get a sense of motion). I think I'll do an animation of the full impact but double the speed every five seconds, so it's 1x speed from T-5 to T-0, 2x speed from T-15 to T-5, 4x speed from T-35 to T-15, 8x speed from T-75 to T-35, and so forth. That should be nice for viewing.
  15. It's accurate in the sense of a head-on collision. However, it is flipped relative to ecliptic north. This should be readily apparent because Dimorphos orbits Didymos retrograde to the solar orbit. I'm not qualified to duel numbers with you - but this strikes me as wrong. I googled a bit and I wonder if your number is classic newtonian? "If the impactor has pushed a mass equal to its own mass at this speed, its whole momentum has been transferred to the mass in front of it and the impactor will be stopped. For a cylindrical impactor, by the time it stops, it will have penetrated to a depth that is equal to its own length times its relative density with respect to the target material. This approach is only valid for a narrow range of velocities less than the speed of sound within the target or impactor material. If the impact velocity is greater than the speed of sound within the target or impactor material, impact shock causes the material fracture, and a higher velocities to behave like a gas, causing rapid ejection of target and impactor material and the formation of a crater. The depth of the crater depends on the material properties of impactor and target, as well as the velocity of impact. Typically, greater impact velocity means greater crater depth. You're absolutely correct, but that's why I characterized it as a point impactor. For a hypervelocity impact like this one, the crater depth is not a function of penetration depth, but a function of the energy delivered to the substrate via the impact shock. Basically you can ignore the size and physical characteristics of the impactor and just treat it as an energy source emanating from a single point. To your ballistic examples from earlier...imagine firing a plastic BB at ballistic gel at such terrific speeds that the BB completely disintegrates on impact. Ordinarily, temporary cavity formation is approximately cylindrical because it is formed by the conical shockwave coming off the bullet as it penetrates the gel. Here, however, there is no penetration, and so the cavity formation is hemispheric from the point of impact. It makes sense, though. Sure, there's a massive amount of energy compared to the gravitational binding energy of the moon, but there's no way to transfer that energy uniformly throughout the moon. You could almost liken it to the Liedenfrost effect.
  16. But the system is near periapsis of a more elliptical orbit, so it could still be going faster than Earth. I took exception to this earlier, but having reviewed it in more detail, I was completely wrong. The velocity of Didymos at periapsis is 34.8 km/s, significantly faster than Earth, and DART was not "catching up" to it; rather, it was catching up to DART. In other words, DART's solar-orbital velocity at impact was lower than Didymos's, not higher. This also explains why the view on approach looked the way it did. Per NASA, the images shown are mirrored on the x-axis (due to the design of DRACO's camera) and show the ecliptic north toward the bottom. The actual approach image, if corrected for how we would intuit it should be viewed, would look like this: Dimorphos has a retrograde orbit, so since DART was coming in "against" the orbital direction, it needed to impact Dimorphos while it was on the sunward side of its orbit. This also explains why the right-hand side is illuminated.
  17. I don't think over-penetration is even remotely feasible. The density of rock/rubble is several times greater than the density of the spacecraft so DART wouldn't have been able to punch more than a meter or so deep. But it seems that the energy clearly wasn't transferred. Perhaps the answer is that >99.99% of the kinetic energy was converted into thermal energy at impact due to the high collision speed. DART is about 1.3 meters wide and would have penetrated less than a meter, so we can take it as a point impactor. You can approximate the energy required to completely crush a volume of rock to powder if you know the compressive strength of the rock, since compressive strength is given in units of pressure (pressure units, force per unit area, are equivalent to energy per unit volume). Brittle material silicate rock usually has a compressive strength on the order of 140 MPa. Do a bit of math and you find that the entire kinetic energy of DART would be sufficient to obliterate about 89 cubic meters of rock . . . a crater about 3.5 meters deep. Of course, it's a rubble pile, not a solid homogenous silicate rock, so that approximation will only get you so far. But it's potentially a good indicator of how quickly kinetic energy can be dissipated.
  18. But with three orders of magnitude at play, even if only 0.11% of the kinetic energy actually ended up pointed into the rubble pile, you've still got full delivery of the gravitational binding energy. My guess is that it would penetrate roughly nine times its length, since it's about 9 times as dense as a rubble pile asteroid.
  19. I feel like that's more likely when you are using a projectile which is many times more dense than the target, like lead. Per Newton's approximately for kinetic impactor penetration depth, the penetration of an impactor is independent of velocity and is a function of the impactor length and the relative densities of the impactor and the target. DART's physical dimensions (box only, not including solar panels or other protrusions) are 1.2m * 1.3m * 1.3 m, giving it a total volume of 1.87 cubic meters and thus a density of approximately 300 kg/m3. Using the numbers from NASA's tweet above, Dimorphos has an approximate/average density of 4.8 billion kg / 2.14e6 cubic meters or 2,243 kg/m3. That's about what we would expect for the density of a gravel pile. But this means we would not expect DART to penetrate significantly more than its own body length. Is it under-penetration, rather than over-penetration, that would prevent Dimorphos from simply exploding? Does all of the impact energy get concentrated into a tiny region, blowing all that to atoms and flinging it out at terrific velocity but failing to deliver the energy to the object itself?
  20. Ah mite nawt be a maths guy, but one of those thangs looks bigger than t'other. There's some variation in the impact mass of DART (I've seen a range from 500 kg to 570 kg) and the stated impact speed (I've seen 5.95 km/s to 6.6 km/s) but still, we are dealing with three orders of magnitude here. Either my numbers are wrong, or Dimorphos got obliterated. Here's a NASA tweet which seems to confirm these numbers. To a first-order approximation, GBE = (3*G*M2)/(5*R). 3 * 6.67e-11 N*m2/kg2 * (4.8 billion kg)2 / (5 * 80 m) = 1.15e7 Joules. Surely I'm missing something here.
  21. And images from Shanghai: And a video from an observatory in the Indian Ocean:
  22. The grid fins are much closer to the center of mass on the way up so their moment effect on the ascent is negligible. There's a little drag of course but probably less than if they were folded. Drag on a big vehicle like this is pretty low regardless. You're already out of most of the atmosphere by the time you hit supersonic speeds, at least on ascent. Descent is another issue altogether.
  23. To a first-order approximation, the gravitational binding energy of Dimorphos is 1.18e7 J. At impact, DART had a mass of 570 kg and a relative impact speed of 6.6 km/s, giving it a total kinetic energy of 1.24e10 J. Wait, what? Here's another view from an Earth-based telescope:
  24. Another video of the collision, this one picked up from a telescope in South Africa:
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