Why not just splashdown it like they did with the Shuttle’s SRB’s to reuse it?
It is cool and all to land the thing upright, and if we can master that it should advance our knowledge, but is it really needed and cheaper considering the extra fuel required for this + the chance of non-reusability from anything but a soft landing?
As I understand it, the shuttle SRBs suffered a lot of damage and corrosion. Each had to be completely taken apart and refurbished before any parts could be reused. There really wasn’t an economic benefit from refurbishing them [political snark]beyond giving tax dollars and jobs in exchange for votes[/political snark].
Plus, parachutes are actually a lot heavier than the fuel and equipment needed for landing. In many cases I believe the first stage is carrying some extra reserve fuel anyway, or it can easily enough because the Falcon 9 was designed with more than enough payload capacity for most missions.
You got several answers when you asked about this last time. 
The Shuttle’s boosters were essentially just giant metal tubes that had solid propellant crammed into them, not too much different than fireworks or the rocket motors hobbyists use to launch model rockets. The SRBs had none of the engines, pumps or plumbing that a liquid fueled rocket has. Even then, the SRBs had to be dismantled and refurbished after each use.
You might as well ask why they go to all the trouble of making pinpoint landings on runways with commercial jets when it’s be a lot easier to drop the plane in the ocean and then tow it to shore for re-use.
SpaceX’s goal is to essentially land the stage, refuel it and go again. Dropping it in the ocean would (at best) mean having to do extensive refurbing before it could be used again.
Some much higher quality video
So close! The little nitrogen thruster tries its best to keep it upright, but it wasn’t enough. I think a leg might have been broken at that point.
I love the explosions at the end. It reminds me of how cars in movies tend to explode if they bump into a tree or something. Cars don’t actually do that, but rockets certainly do! Of course, despite looking like a pretty gentle tipover, it was nothing of the sort–there’s nothing gentle about a 200 foot rocket falling over.
Another angle show how close the landing was. It comes to a nearly complete stop, just tilted a few degrees too much. Can’t tell whether the legs were damaged by the slide into home base, or when one or two legs were supporting almost the whole rocket.
They’re getting damn close to being successful aren’t they? I’m confident they’ll nail it in two or three more goes.
Very close. I’ll be surprised if they don’t nail it next time, but I also thought that last time, and there’s always a chance of some unknown issue causing a failure.
They’ve demonstrated several times now the ability to make a hypersonic insertion into the atmosphere and hit a spot with perhaps 10 meter accuracy. This is already unbelievably impressive, and they’ve done it enough times that they aren’t depending on luck.
Obviously they’re still having issues with the last few meters. It’s tough because they get exactly one shot at it. Despite having only one engine on and reasonably deep throttle capability on the Merlins, the stage has a thrust-to-weight ratio of well over 1. So it can’t hover and get to a “comfortable” place before the final descent. It has to do the final deceleration, any last second positioning, and horizontal velocity cancellation all in a single well-timed burn.
They’ve obviously done a ton of simulations and the basic control system is probably working pretty well. It seems that their model of the hardware isn’t perfect, though, and in this case the final rotation didn’t quite work out. I think there’s a very good shot that they’ll iron out the remaining issues by the next attempt.
They’ll have to have two or three good barge landings before they’re given permission for the land attempt. It helps that their positioning already has a great record; neither of the past two crashes would really have caused much damage to a land-based pad (nothing like an exploding, fully-fueled rocket). Still, they want to be careful.
It’ll also probably take a few landings before they re-fly anything to orbit. I’m sure the first one will be dissected completely, and later ones will be used as suborbital testbeds.
Still, they have enough flights this year that we may see successful (if not economic) reuse by next year.
Nitpick mode ON
You mean a weight-to-thrust ratio well over 1. Or more traditionally, a thrust-to-weight ratio well under 1.
Thrust-to-weight over 1 would mean they could not only hover, but abort a landing, climb, and try again to make multiple approach attempts.
Nitpick mode OFF
Or maybe they do have over t-to-w over 1 and you really meant something else. Certainly fuel supply is very limited, so even with lots of thrust they don’t have lots of time.
Nope–I meant thrust-to-weight (eh, let’s use TWR). What I failed to say is that the minimum TWR is well over 1. Rocket engines don’t like to be throttled; the Merlin 1D does better than most but can still only go down to ~60% of full thrust.
You’re right that it could (if it had enough fuel) do a full “go around” to make a second attempt. What it can’t do is hover at a constant altitude, since that would require a TWR of exactly 1.
Doing a bit of math (since I wasn’t sure of the exact numbers):
A single Merlin 1D engine (remember that the stage has 9 of them, but only the center one is firing here) has a thrust of 654 kN. Public information says it throttles to 70%; the speculation I’ve seen is that it’s a bit closer to 60%. So the minimum thrust is 392 kN.
The stage weighs 18 t empty, or 176 kN. We don’t know how much propellant is left, but it’s probably not much. To cancel the remaining velocity it needs under a ton of remaining propellant. Let’s suppose it has 2 t for safety; that’s 196 kN.
As it happens, that ratio is exactly 2. Taking gravity into account, it means the vertical acceleration is 1 G at a minimum.
Couldn’t they inject inert gas into the combustion chamber to reduce the TWR to a more manageable level?
They could, but it would be more mass, and alter the design of the engine. There’s no reason why the current technique can’t work, and it’s what they’d want anyway in the long run (a short, high thrust, last-second burn uses the minimum fuel). It’s just tricky.
I wonder if they could get things working with two engines. It would be even more efficient, and also more resilient to engine failure earlier in the flight (since they would have four pairs to work with). However, I think the gimbaling is not up to scratch on the outside engines.
Thanks for the clarification. I’d completely forgotten about the relatively limited throttleability of rockets vice turbines. :smack:
That was impressive!
How far off the coast of Florida is that barge?
Roughly 200 miles. Also, here’s a graphic that some might find helpful explaining the process. The first stage separation happens at about 2 km/s. That’s fast, but it’s nothing compared to the 8 km/s that the second stage reaches. This is the main reason why they can only do this for the first stage (for now).
