3 ms·
Come on LOL. Falcon 9 is an orbital booster that's landed dozens of times on US soil. Suggesting this spacecraft has achieved some kind of world first is total
by davedx 3y ago
Come on LOL.
Falcon 9 is an orbital booster that's landed dozens of times on US soil. Suggesting this spacecraft has achieved some kind of world first is totally ridiculous and diminishes the actual achievement.
- peeters 3y agoI mean sure, there's ambiguity around the word "spacecraft". But Falcon 9's booster isn't orbital, its payload is. So it might still be a first for an orbital spacecraft. The gravitational potential energy of a 100kg payload in 100km orbit is 98 MJ. The kinetic energy of a 100kg payload in 100km orbit is 3042 MJ, over 30x more. So surviving reentry from orbit is orders of magnitude more difficult than surviving a straight up/down shot. Now F9 boosters obviously have a horizontal velocity as well that they need to cancel out, but suborbital and orbital spacecrafts are very different beasts and it's absolutely noteworthy. It's the difference between surviving a car crash at 20 mph vs 95 mph. One is trivial, the other requires a heck of a lot of engineering.
- davedx 3y agoF9 landing is not trivial, and despite all the comments on this thread trying to back up the claim this is the “first commercial spacecraft to land on US soil” ludicrous.
- peeters 3y agoI wasn't trying to extend the analogy to saying landing F9 is trivial. But F9 surviving re-entry is fairly trivial, which is the important difference between the two. Once you've survived re-entry and slowed down to terminal velocity, there's not a huge difference between landing from orbital and sub-orbital trajectories. I agree that the way it's phrased is fairly meaningless and reeks of baseball commentary cherry-picking, but surviving from orbit is still a very different milestone than surviving from sub-orbit.
- aj7 3y agoHuh? Did you flunk freshman physics? E=K+U=GmM/2r−GmM/r=−GmME/r. We can see that the total energy is negative, with the same magnitude as the kinetic energy. For circular orbits, the magnitude of the kinetic energy is exactly one-half the magnitude of the potential energy. Do the calculation over.
- versteegen 3y agoYou are correct (aside from the s/ME/M/ typo), but the snark isn't needed.
- peeters 3y agoNo, they are not correct either.
- peeters 3y agoYou're quoting this article, but you're misunderstanding it: https://phys.libretexts.org/Bookshelves/University_Physics/University_Physics_(OpenStax)/Book%3A_University_Physics_I_-_Mechanics_Sound_Oscillations_and_Waves_(OpenStax)/13%3A_Gravitation/13.05%3A_Satellite_Orbits_and_Energy#:~:text=E%3DK%2BU%3DG,magnitude%20of%20the%20potential%20energy https://phys.libretexts.org/Bookshelves/University_Physics/U.... This is calculating the gravitational potential between two point masses. In other words, the potential energy if both masses were singular points at a distance from each other. I was quoting the potential energy differential between being 100km over the ground, and on the ground. This is 98 MJ for a 100kg object, and that's what you have to cancel out to land back on Earth. The total gravitational energy of that object to the Earth's centre is more like 6300 MJ, but that's a meaningless number.
- Tuna-Fish 3y agoI disagree. Falcon 9 is an orbital booster, but only the second stage goes to orbit. The hard part is surviving re-entry, the first stage only barely requires a heat shield. Dragon would qualify, except it lands on the ocean, so not "US soil".
- kragen 3y agoapparently the hard part is getting the control systems and landing legs simultaneously lightweight enough and reliable enough, if i correctly understand spacex's long history of failing to land rockets (which i may not, since all i know is a small sample of what they've published) agreed about dragon. those fuckers do orbit!
- Tuna-Fish 3y agoThe hard part for landing a suborbital rocket vertically is that. The hard part of landing anything from orbit is still definitely the re-entry.
- kragen 3y agooh, sorry, you're right. a few rocks do survive re-entry every day without the benefit of any engineering at all, but they have a very, very low success rate
- ianburrell 3y agoFalcon 9 boosters are not orbital. The point of first stage booster is to get rocket part of the way to orbit and the second stage gets it into orbit. The boosters don't have the heat shield to return from orbit.
- myself248 3y agoUmm, no. It's a booster of an orbital rocket, the booster itself is not orbital. Falcon 9 stages super early even compared to other rockets, specifically so the booster can come back before it gains anywhere near a significant fraction of orbital energy. This is great because it does make it practical to recover the booster, but it's only possible by making the second stage significantly larger and more powerful than comparable rockets that stage later. If the booster got up to orbital velocity, it would burn up just like the Rocket Lab "mothership" host satellite that accompanied the Varda capsule. They were coupled together, the mothership performed the braking burn to adjust their orbit into a reentry trajectory before the two separated. Only a minuscule amount of drift separated the two as they hit the atmosphere, but while the mothership burned up, the capsule survived. If Falcon's booster achieved orbit, it would burn up just the same. The only reason it doesn't, is that it's only going a tiny fraction of orbital velocity when it comes home.