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.
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.
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.
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…
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:
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.
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.
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.
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.
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.”
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
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)
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.
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. Although you’ll probably want to turn the sound off. Trust me on that . . .
