92

Ironically, the answer is in his own (or rather SpaceX's) video. Still from 0:49 of the video showing cold gas thruster firing The first stage of the Falcon 9 uses a set of nitrogen cold gas thrusters to perform its flip after separation, and you can see them repeatedly firing in the video. As the compressed gas leaves the thruster its pressure drops very ...


59

There are many key points to this, probably none on their own sufficient to ditch the parachutes approach (except economics, those are good enough on their own), but together they make for a compelling case against it; Descent control: As already mentioned, there's a significant guidance uncertainty with the use of a parachute system. Some of it comes from ...


44

SpaceX initially was looking to buy an engine but could not find one on the market that would allow them to meet their goals. Once they decided to develop their own, they had to consider their goals: Reliability, and reduced costs. Nine engines means at most any time in the flight profile, you can handle an engine out event. The Saturn V with 5 F-1 ...


43

There are some major challenges with this. For starters, the engines of the first stage produce far too much thrust for the last stage, which would require extra structural mass to allow the rocket to hold together, and would not allow manned rockets at all, as humans have fairly tight g-force limits. If you reduce thrust to manage these problems, you're ...


31

That is my estimation of CoG of nearly empty Falcon 9 booster. On the left is GoG with legs opened, on the right with legs retracted. The landed vehicle is pretty stable. It could withstand the winds of 50 m/sec (97 knots!), if sliding is prevented. There are relatively narrow limits on horizontal component of landing speed as well as on rotational velocity ...


30

Besides limiting aerodynamic stress and drag losses as you and Antzi mention, using the core engine only at high altitude means the engine can be optimized for low-pressure use by putting a larger nozzle on it. This optimizes expansion of the exhaust, and in the case of GSLV MkIII contributes to a ~6% increase in specific impulse over the sea-level version ...


28

It's actually a combination of several systems: Nitrogen gas thrusters for attitude control in vacuum (one can see them firing in the video) 4 grid fins (two of them visible in the video) for attitude control during the atmospheric descent Gimbaled engines for attitude control during powered flight Reference: Falcon 9 User's Guide


24

The reason for the SRBs is that the stack weighs something 7.4 million lbs. The two SRBs provide about 2.8 million lbs of thrust each. The three SSMEs provide about 600Klbs each. If you subtract 5.6 million lbs of thrust, and the SRBs weigh 1.3 million lbs each, then the remaining stack, about 4.7 million lbs only has 1.8 million lbs of thrust. So it ...


23

Nathan's answer is good and cover almost everything but let me add a last bit: An engine nozzle can be optimized for only one given altitude ambient pressure. This has a great impact on the rocket performance. You probably wouldn't want to drag that heavy, inefficient engine all the way up, even if you could. Some designs attempt to circumvent this issue ...


20

The Falcon 9 first stage uses 3 engine burns in its landing trajectory: Boostback, to kill its forward velocity and return in the general direction of the landing pad. This decelerates the stage from 5000 km/h to 0, plus a bit of return speed. Reentry, to reduce its speed as it enters the atmosphere. Starts at 45 km up, the stage is supersonic at the ...


20

SpaceX was testing a more efficient, faster landing technique. The test produced useful telemetry data. See Musk's tweet and photo. Normally, the terminal phase of Falcon 9 first-stage landing finishes with a single engine firing. This attempt used three engines for the landing. By firing three engines instead of one, the final deceleration of the stage ...


19

I can answer this part "Has this same (or similar) design always been used for such tests?" In the early days of the program Shuttle would occasionally do a pad test where the main engines were fired briefly. During this test (and at all other times) the Shuttle/External Tank/Solid Rocket Booster stack was held down to ...


18

Your assumption that we need max thrust at takeoff is partially wrong. Although right at takeoff you do want max thrust, it might be counterproductive short after: Your rocket and payload are Max G rated. You can't have an arbitrary high acceleration Atmospheric drag is higher at lower altitude and increase with the square of speed. If you go too fast too ...


17

Edit 2: added a paragraph about what happens after the landing (see at the bottom) So I've got a pile of arguments against this scheme, but this is the big one: You've made the legs twice as long (the real ones are about 7 m), which means they're at least twice as heavy. Probably more, since the load will be applied further out so the legs need to be ...


16

Because the F9 rocket's first stage already has nine rocket engines. Why isn't a helicopter landed with parachutes? Because it has an engine and rotating wings. It can use them to start, fly and land. Same thing with rocket engines.


16

After the Falcon 1, SpaceX initially planned a 5-engined rocket (around 2003). This Falcon 5 would be the minimum configuration with engine-out capability based on the Merlin engine. Around 2005, plans changed. Due to customer requirements SpaceX wanted a version with 9 engines. They extended the 1+3+1 configuration to a 3x3 grid, giving maximal ...


16

I think this is pretty self-explanatory: (Thanks to @TildalWave for finding an updated graphic) It's important to note that the landing pad was a different site to the launch pad (6 miles away), so it didn't return *exactly* to its' original point of take-off, as can be seen here:


16

It seems to me very unlikely that this would actually occur. One would have to get the fuel out to the barge, fill the rocket, monitor the mission to return, and run the risk of losing the rocket, etc. The cost to have the barge return is minimal in comparison, and in fact, the cost is probably about the same regardless of if there is a booster on top or not....


15

SpaceX has leased LC-13 on the Florida coast, and renamed it Landing Complex 1. Then renamed to Landing Zone 1. They are building 5 landing pads there, but the Environmental assessment (the document you have to get to be able to start building and construction) says they will only land one at a time. (So Heavy's three cores not quite yet). Gwynne Shotwell ...


15

Parachute descents cannot be guided with any accuracy, if you pop the chutes at high altitude the stage would get carried miles away from the landing pad and the stage would not have enough residual fuel for that much lateral movement.


15

TL;DR Without SRBs, by maintaining the real stack's thrust to weight ratio at ignition, you run out of fuel at about 167 seconds with (if we maintain the flight profile of the real stack) a velocity around 1.4 km/s and an altitude of around 70 km. Without SRBs, with an initial thrust to weight ratio of 1, you run out of fuel at about 289 seconds with (if ...


15

Issue #1: the rocket body is really thin (like a soda can) and the smallest force in the direction the net would apply would probably dent it. Issue #2: there was never a problem with stability which would need solving with some extra contraptions. It was: not enough hydraulic fluids for steering stuck valve and algorithms not accounting for the latency ...


14

There are several problems with your proposal: You need fuel to land the engine section, so in your proposal the stage would need two sets of fuel tanks: one disposable set and a second, smaller set that stays with the engine. This (plus the separation mechanism) makes the first stage much more complex. I suspect the empty tank also helps the stability:...


14

I thought I had heard Musk say that a rocket landing was for another very good reason, over and above those already listed: Practice/development for landing large mass on the moon, Mars, asteroids, etc. No atmosphere (or little) means parachutes are simply not an option, especially as the mass being delivered goes up. And the mass will go up a lot with ...


13

There's not much reliable data out there to estimate this ourselves, and SpaceX isn't exactly forthcoming with details. Or perhaps they would be, but media aren't asking the right questions during press conferences. So far, the only somewhat reliable data point is the canceled recovery attempt during the DSCOVR launch on February 11, 2015; According to ...


13

The "return to launch site" flight profile flown on December 21, 2015, requires that the first stage re-light three of its engines after separation, turn around, cancel out all its speed, and accelerate back towards the launch site. To do this, it has to keep a lot of fuel in reserve after the separation -- less than you might think, though, because the ...


11

Rocket manufacturers are optimizing for cost, not for total mass at launch. It's cheaper and easier to modify one thing to improve overall performance, even if the result isn't mass-optimal, than to modify two things. In the Delta IV medium, Atlas V, and Ariane 6 cases in particular, changing just the number of installed SRBs is both cheap and flexible; ...


10

According to Defense News, SpaceX plans to make the next attempt on land: While not providing details of when or where that attempt would occur, Gwynne Shotwell, SpaceX President and COO, told Defense News on Wednesday that the company hopes its next attempted landing will take place on land, not at sea.


10

SpaceX's Landing Zone 1 is just a few miles down the road from their launch sites at Cape Canaveral, so they use a crane to put it on a truck horizontally and drive it back to a hangar near the launch site (at LC-39A) for cleanup, inspection, and refurbishment. Eventually, presumably, they'll be able to prep a landed stage for reflight there and mate it with ...


10

Lets assume 5*10^6 lbs of thrust: Then 4 steel wires of approximately 60mm diameter should be enough. The special thing about a rocket is not its maximum force at zero speed but the fact that the force is generated in a way which is independent from the velocity difference to a medium in which the vehicle flies, and stays constant. Source for the ...


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