# Continuing discussion of SpaceX launches \[edited title\] **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [December 22, 2015, 12:37am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048 "2015-12-22T00:37:01Z")\ **Posts on this page:** 20\ **Page:** 23
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [February 1, 2018, 4:17am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/441 "2018-02-01T04:17:46Z")
3-engine landings save propellant. The dominant effect on landings are gravity losses–the fact that every second the rocket spends in flight, it accelerates by 9.8 m/s downward. The way to reduce this is to land as quickly as possible, which means waiting until the very last moment possible to enable the engines, and burn at the highest reasonable thrust. Of course this is difficult because the timeframes are shorter, and the control systems can only move so quickly. So there are limits as to how quickly you can decelerate–all 9 engines would be too much. But it seems that 3 is doable. There’s a smaller effect from air drag–terminal velocity is lower as you go deeper in the atmosphere, the longer you wait the more time the atmosphere has to slow things down. A three-engine burn means you get a few more seconds of free velocity shedding. You are correct that even a single engine has more thrust (at minimum) than the weight of a stage, so in any case they have to do what Musk calls a “hoverslam” (also called a suicide burn by Kerbal Space Program players). You get one shot at the landing and you time it so that you’re at zero velocity at zero altitude. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [February 1, 2018, 2:05pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/442 "2018-02-01T14:05:38Z")
Thanks, that makes sense, though I’m a bit surprised that the difference is non-negligible. I wonder if it’s at all feasible to reenter the atmosphere in a flat spin and then transition to a vertical landing? I know our sounding rockets do that, but they use parachutes, which I suppose makes it much easier to get out of a spin. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [February 1, 2018, 9:58pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/443 "2018-02-01T21:58:43Z")
It’s hard to give truly realistic numbers since it’s such a complicated system, but suppose the stage is at a terminal velocity of 250 m/s and one engine gives a thrust of 20 m/s^2 (two gees). Since you have to subtract gravity, that means the engine has to fire for 25 seconds, which means it’s expended 500 m/s of delta-V. With three engines the thrust is 60 m/s^2, which after gravity means they only fire for 5 seconds. That’s 300 m/s of delta V expended. That’s a difference of 200 m/s–not insignificant. Of course these numbers only bear a passing resemblance to the real ones, but I’m fairly confident the answer is in the ballpark (i.e., low hundreds of m/s). That’s enough to be the difference between crashing vs. not for plenty of missions on the edge. A flat spin sounds bad–that’s a stall, by definition. SpaceX does use the rocket as a crude lifting body and does get some benefit from that. But increasing the angle of attack by too much would cause a stall and they’d end up worse off, I think. ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [February 1, 2018, 10:33pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/444 "2018-02-01T22:33:44Z")
Calculating fuel used based on Dr. Strangelove’s delta V estimates (25600 kg empty stage mass, 282 s Isp), the 3-engine burn uses 3000 kg propellant, while the 1-engine burn uses 5100 kg propellant. Two tons of propellant savings is pretty substantial. Put another way, I’m pretty sure any rocket scientist/aerospace engineer would happily sell their soul to shave off 7% of the inert mass from a stage… ---
**Author:** ![Snowboarder\_Bo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snowboarder_bo/32/229_2.png) [@Snowboarder\_Bo](https://boards.straightdope.com/u/Snowboarder_Bo)\ **Post date:** [February 4, 2018, 3:24am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/445 "2018-02-04T03:24:40Z")
[Good news everyone!](https://futurism.com/spacex-falcon-heavy-launch-license/) > [@](#): > > READY TO LAUNCH > > SpaceX’s Falcon Heavy rocket is currently sitting at Cape Canaveral in Florida, with successful testing under its belt, ready to launch into orbit. The rocket passed static fire testing with flying (or rather, burning) colors last week. SpaceX then confirmed that if all goes according to plan, the Falcon Heavy would be ready for launch by February 6th. [And that IS the plan](https://twitter.com/elonmusk/status/957361443023695872?ref_src=twsrc%5Etfw): > [@Elon Musk](#): > > Aiming for first flight of Falcon Heavy on Feb 6 from Apollo launchpad 39A at Cape Kennedy. Easy viewing from the public causeway. I have family in the area still; I hope someone goes out to watch. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [February 4, 2018, 4:07am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/446 "2018-02-04T04:07:52Z")
Should be exciting. I was tempted to fly out, but it’s not worth the risk unless I can give it a week or so of delay margin. Can’t quite justify that. Musk has tried to keep expectations at reasonable levels: > [@](#): > > I hope it makes it far enough away from the pad that it does not cause pad damage. I would even consider that a win, to be honest. Not blowing up another pad would indeed be nice. I think the main uncertainties are the aerodynamics and other flight dynamics, which should really only be a problem well into flight. The Falcon Heavy is sometimes compared to the ill-fated Soviet N1 “moon rocket” due to the large number of first-stage engines (27 and 30, respectively), which failed four out of four times, but even then only once destroyed the pad. ---
**Author:** ![Alex\_from\_CB](https://avatars.discourse-cdn.com/v4/letter/a/278dde/32.png) [@Alex\_from\_CB](https://boards.straightdope.com/u/Alex_from_CB)\ **Post date:** [February 5, 2018, 1:17pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/447 "2018-02-05T13:17:40Z")
You rocket aficionados… what do you think the odds are that this launch goes off tomorrow (2/6) during it’s 1:30 - 4 PM window? I’ll be boating north tomorrow morning about 10 miles up the Banana River as far north as one can get to view the launch. I’m currently on the water almost downtown Cocoa Beach. Expecting quite the boat parade north tomorrow by mid morning. Coast Guard stops boat traffic just north of the 528 (“Beach Line”) Causeway. Here’s a good map of the area. [File:Canaveral.svg - Wikipedia](https://en.wikipedia.org/wiki/File:Canaveral.svg) Think we’ll be stopped right about where the word … **Port** … is on this map. Launch is almost due north from LC39A. LZ1, where the 2 rockets are scheduled to land 8 minutes after the launch is right about where LC13 is on this map. So what’s your guess? Go or no-go tomorrow? ---
**Author:** ![Ravenman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ravenman/32/2929_2.png) [@Ravenman](https://boards.straightdope.com/u/Ravenman)\ **Post date:** [February 5, 2018, 1:25pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/448 "2018-02-05T13:25:27Z")
> [@Alex\_from\_CB](#): > > So what’s your guess? Go or no-go tomorrow? Odds are looking pretty good. [http://www.patrick.af.mil/Portals/14/documents/Weather/L-2%20Forecast%206%20Feb%20Launch.pdf?ver=2018-02-04-101812-937](http://www.patrick.af.mil/Portals/14/documents/Weather/L-2%20Forecast%206%20Feb%20Launch.pdf?ver=2018-02-04-101812-937) ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [February 5, 2018, 11:59pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/449 "2018-02-05T23:59:27Z")
[Official video](https://www.youtube.com/watch?v=Tk338VXcb24) > [@Elon Musk](#): > > Falcon Heavy launches to Mars orbit tomorrow. If it doesn’t explode into tiny pieces, it will carry Starman in Roadster over 400 million km from Earth at 11 km/sec on a billion year journey through deep space. > > Should it work, Falcon Heavy will be the most powerful rocket in the world by a factor of two and the highest payload launch vehicle to reach orbit after the Saturn V moon rocket. Could do crewed missions to the moon and Mars with orbital refilling, but better to leave that to the BFR program. ---
**Author:** ![Snowboarder\_Bo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snowboarder_bo/32/229_2.png) [@Snowboarder\_Bo](https://boards.straightdope.com/u/Snowboarder_Bo)\ **Post date:** [February 6, 2018, 12:03am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/450 "2018-02-06T00:03:13Z")
I just spoke with my aunt; she says the weather is going to be perfect tomorrow for the launch (she lives in Cape Canaveral). ---
**Author:** ![HurricaneDitka](https://avatars.discourse-cdn.com/v4/letter/h/96bed5/32.png) [@HurricaneDitka](https://boards.straightdope.com/u/HurricaneDitka)\ **Post date:** [February 6, 2018, 1:09am UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/451 "2018-02-06T01:09:15Z")
I’d give it a 67% chance of actually happening tomorrow. ---
**Author:** ![Alex\_from\_CB](https://avatars.discourse-cdn.com/v4/letter/a/278dde/32.png) [@Alex\_from\_CB](https://boards.straightdope.com/u/Alex_from_CB)\ **Post date:** [February 6, 2018, 12:26pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/452 "2018-02-06T12:26:49Z")
Weather looks fine from here in CB. Wait. Let me rephrase that. **The weather is A-OK.** 1-2 PM temp will be 73 degrees with ESE winds of 8 mph. Weather is… Go. Boat is cleaned and gassed and ready head 10 miles north up The Banana River. Plenty of food, drink, sunscreen, podcasts and music packed… (Kinks, Byrds etc.) My question re chances for launch today was less about weather but mostly about the chance of a scrub due to a glitch. New rocket configuration here today, you know. Took a lot of duct tape to stick those 27 motors together, I’ve heard. ---
**Author:** ![Alex\_from\_CB](https://avatars.discourse-cdn.com/v4/letter/a/278dde/32.png) [@Alex\_from\_CB](https://boards.straightdope.com/u/Alex_from_CB)\ **Post date:** [February 6, 2018, 12:39pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/453 "2018-02-06T12:39:56Z")
Nice article on today’s launch attempt here… > **[What to expect: SpaceX's Falcon Heavy launch from KSC](https://www.floridatoday.com/story/tech/science/space/2018/02/05/what-expect-spacex-falcon-heavy-launch-tesla-roadster-ksc-florida/307016002/)** > > SpaceX's Falcon Heavy rocket will light up the sky over KSC Tuesday, capping off months of excitement for the launch vehicle's demonstration flight. Musk comments… Musk, however, still emphasized the mission’s challenges on Monday, noting that plenty of things can’t be tested on the ground: interactions between the boosters while in flight; the effect of supersonic shockwaves; the possibility of ice falling off the upper stage and impacting one of the boosters; and the correct operation of the separation system, to name a few. **“Guaranteed to be exciting, one way or another,” he said. “I say tune in – it’s going to be worth your time.”** ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [February 6, 2018, 2:05pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/454 "2018-02-06T14:05:38Z")
> [@Cell\_Guy](#): > > On [instagram](https://www.instagram.com/p/BdA94kVgQhU/) Elon has posted pictures of his original Tesla Roadster being loaded as payload for the Falcon Heavy test flight. The goal is to put it in an elliptic Mars orbit… It is not a elliptical orbit of mars but a hyperbolic orbit of mars, which just means a flyby, it will be elliptical around the sun > [@](#): > > …If he’s just throwing Teslas away like this he should feel free to send one my way. It’s also a test of a new rocket system and all, and his rocket to do what he wants with, but it does seem something more useful can be sent. He may be hoping that one day it will be recovered and worth lots of money, and will still hold the title, a unusual way to pass money from his company to his personal account (or descendants) ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [February 6, 2018, 4:06pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/455 "2018-02-06T16:06:10Z")
> [@kanicbird](#): > > It is not a elliptical orbit of mars but a hyperbolic orbit of mars, which just means a flyby, it will be elliptical around the sun I believe it isn’t any kind of orbit of Mars, as Mars won’t be anywhere nearby when it crosses Mars’ orbital path. My understanding is that it’s essentially a Hohmann transfer orbit outside of the actual transfer window. ---
**Author:** ![Kiber](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kiber/32/317_2.png) [@Kiber](https://boards.straightdope.com/u/Kiber)\ **Post date:** [February 6, 2018, 8:20pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/456 "2018-02-06T20:20:07Z")
So - after delays due to upper level winds, it looks like we’re on for launch around 3:45 Eastern. But the window is closing fast - if they can’t launch by 4, then it’s scrubbed for the day. Streaming coverage of the launch will be [here](http://www.spacex.com/webcast). Although you’ll probably want to turn the sound off. Trust me on that . . . ---
**Author:** ![Scoobysnax](https://avatars.discourse-cdn.com/v4/letter/s/c4cdca/32.png) [@Scoobysnax](https://boards.straightdope.com/u/Scoobysnax)\ **Post date:** [February 6, 2018, 8:29pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/457 "2018-02-06T20:29:58Z")
The webcast is now live. ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [February 6, 2018, 8:51pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/458 "2018-02-06T20:51:56Z")
No asplosions yet. Not sure if I should be disappointed or not. Three booster cores currently in re-entry burns. ---
**Author:** ![Ravenman](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ravenman/32/2929_2.png) [@Ravenman](https://boards.straightdope.com/u/Ravenman)\ **Post date:** [February 6, 2018, 8:54pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/459 "2018-02-06T20:54:42Z")
Watching the two boosters land simultaneously was pretty damn cool. ---
**Author:** ![Snowboarder\_Bo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snowboarder_bo/32/229_2.png) [@Snowboarder\_Bo](https://boards.straightdope.com/u/Snowboarder_Bo)\ **Post date:** [February 6, 2018, 8:56pm UTC](https://boards.straightdope.com/t/continuing-discussion-of-spacex-launches-edited-title/741048/460 "2018-02-06T20:56:11Z")
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