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Very excited about the upcoming test to land the first stage of the next Falcon 9 on an ocean platform. Elon Musk & Co. make it sound incredibly difficult though.

I'm assuming that the first stage of the Falcon 9 could potentially be split into 2 parts:

  • A very large and simple section containing most of the fuel
  • A relatively smaller and more complex section containing all of the expensive stuff like the engines etc.

Wouldn't it be much easier to just dump part containing the fuel (Assuming this part would be relatively cheap), and only try to land the smaller (And presumably more expensive) section?

See my sketch below:

Split first stage into 2 parts

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  • $\begingroup$ in 2015, two companies showed concepts along these lines: ULA with the Vulcan and Arianespace with the Adeline. Whether or not these will be used in practice remains to be seen. $\endgroup$
    – Hobbes
    Commented Jan 3, 2016 at 12:07

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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: the first stage has most of its weight at the bottom, which is the most stable configuration. The engine section would have its center of gravity halfway up, which makes it more likely the stage will start to tumble.

The fuel tanks are mostly empty space, but they are made from spacecraft-grade alloys and are welded on expensive machines. This is not a coke can you're throwing away. If you're going to reuse the first stage, it makes sense to reuse as much of it as possible.

If you remove the tanks, you reduce the weight by at most half, so removing them doesn't make the landing much easier. They have plenty of power available to land the weight. The complexity is in controlling the descent to land in exactly the right spot. By making the stage more stable, the fuel tanks actually help make the landing simpler.

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    $\begingroup$ Interesting point: "plenty of power to land the weight"... seems the problem is that they have too much power to land the weight - having to resort to tricky "slam dunk" landings. Perhaps the extra mass of the tankage is a good thing by raising the final burn impulse requirement to something that can be more easily achieved with one of the existing engines rather than having to install a different, smaller touchdown thruster. $\endgroup$
    – Anthony X
    Commented Jun 3, 2018 at 18:43
  • $\begingroup$ @AnthonyX well, those slam dunk landings make sense anyway because the shorter you make the landing burn, the less fuel is needed. In particular, hovering just wastes fuel without contributing any Δv. That's why the landing rocket on Soyuz is just an “explosive cushion”. What's indeed tricky about this is that firing the engine too early would be disastrous – the more mass you bring down, the higher your tolerances on timing can be. $\endgroup$
    – leftaroundabout
    Commented Aug 16, 2018 at 9:43
  • $\begingroup$ @leftaroundabout but then, the "explosive cushion" comes at the end of a parachute descent (the parachutes are keeping it right side up as long as there's tension in the lines) and the landing isn't "precision" - the vehicle is going to drift where the winds will carry it... the Falcon 9 is trying to land on a designated spot, actively steering itself as needed. If the burn is too short, there's no time to compensate. Also, while it may be almost empty and is obviously unoccupied, there are still likely limits on g loads, so a shorter more intense terminal burst isn't necessarily better. $\endgroup$
    – Anthony X
    Commented Aug 18, 2018 at 1:55
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Let me add a few details that tears apart the justification a little more. But I'll have to introduce some speculation about the motives of reusing only part of the first stage, then I can go about tearing them down:

  1. Perhaps the OP thinks it will be easier to maneuver without the top tank, or at least less likely to break
  2. Perhaps only recovering the bottom part will require either less fuel or less thrust, making it more viable

I will argue that the real effect of both of these are actually opposite of the intention.

One analogy is that that balancing the first stage is like balancing a baseball bat on the top of your hand. But I think this ignores several aerodynamic factors. The center of gravity is higher with the top of the tank attached, but the center of pressure also moves in a stabilizing mode. If you drop a pipe with a weighted end, then the natural forces will tend to keep it upright. This doesn't change the need for active control once you fire the engines, but you need that anyway.

Also, regarding the point #2, consider the following:

http://cosmiclog.nbcnews.com/_news/2013/03/09/17251376-spacexs-elon-musk-shows-off-grasshopper-test-rockets-latest-hop?lite

"Grasshopper touched down with its most accurate thus far on the centermost part of the launch pad," SpaceX said. "At touchdown, the thrust-to-weight ratio of the vehicle was greater than one, proving a key landing algorithm for Falcon 9."

When the first stage lands, its thrust to weight ratio is greater than 1. If that sounds crazy to you, you read it right. As such, it would actually be more convenient to have the stage weigh more when it lands. Saving mass at this point isn't a benefit, it's a detriment. You can't throttle the engine down enough to get a perfect hovering type of landing. If you shed more mass, then you'll have to hit the ground harder, and with less margin of error.

TANGENT: This scheme doesn't make any sense for the first stage, true. But maybe it makes sense for the 2nd, or final, stage? If this rocket reaches orbit, then you have a big problem for making it back in through the atmosphere and propulsive reentry is going to be decimated by propellent budget. As such, I think there is growing acceptance that final stage re-usability can not be anything like first stage.

Speaking extremely speculatively, a fix for this might possibly be to reenter some of the final stage's hardware inside of a capsule and heat shield sent up separately. The idea is that you tear down the rocket in an orbital workshop and prioritize the most valuable and easy to reuse parts. If you can pack those parts from several launches into a single capsule, then it could possibly make economic sense. This would be a form of re-usability for the engines, but not the tanks.

I don't know if it's a very good idea, but it could be coherent.

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