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The first F9 booster landing attempts failed because the booster broke up. After that, they began lighting the engines in the upper atmosphere to slow the booster.

It seems to me that their problem was created by the fact that they pointed the ends of the slender booster into the airflow, preventing it from slowing down in the upper atmosphere. Wouldn't belly flopping the F9 (and Super Heavy) like Starship solve this problem?

The answer to this question depends on a number of questions. What is the CG of the booster? For the F9, empty weight is 25.6t, of which on 4.23t us the engunes. This suggest a the CG us very close to halfway up. For the Super Heavy, it seems that the engines weigh 50 tons, the interstage weighs 30 tons, and the tanks weigh 80 tons. It would seem that the Super Heavy CG is also close to the middle.

EDIT

Many are suggesting that the boosters cannot handle the lateral forces. But the above data shows that both of them can withstand accelerations of 30gs axially when empty.

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  • $\begingroup$ Why don't SpaceX boosters aerobrake? $\endgroup$
    – Mazura
    Apr 14, 2023 at 0:59
  • $\begingroup$ @Mazura "aerobrake" is a term more applicable to orbiting / deorbiting than the regime that the boosters are in. Aerodynamic drag is already a substantial contribution to the F9 booster deceleration, though. $\endgroup$
    – Erin Anne
    Apr 14, 2023 at 1:48

4 Answers 4

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Think about heat and air resistance.

The falcon 9 has no heat shield on its side and Starship does, so making the falcon 9 booster reenter sideways could break it.

Also the amount of drag at such high altitudes is very low especially at low speeds. The reason for Starship belly flopping is because it is at an orbital velocity. To slow down from an orbit requires a lot more energy than a suborbit. The drag that Starship experiences is also much higher because of the higher velocity. 8km/s vs 2 km/s.

To make a falcon 9 booster belly flop would just take a lot extra money and time to develop, just for a small benefit.

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    $\begingroup$ And also the key word is "slender": Starship is kinda chubby, for a rocket stage, in comparison to Falcon 9 Boosters $\endgroup$ Apr 13, 2023 at 5:36
  • $\begingroup$ @RegenerativelyCooledAstronaut true, the slender shape makes it have a small surface area, that is also degreasing drag. $\endgroup$ Apr 13, 2023 at 5:43
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    $\begingroup$ Also the structure of the Falcon9 cannot withstand the drag laterally (relative to its axis), it would need some reinforcement, adding mass which is highly undesirable. $\endgroup$
    – JFL
    Apr 13, 2023 at 8:36
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    $\begingroup$ I'm not so sure how much money it would cost to "belly-flop" the booster; it has attitude control enough, so it's "just" a software change. It is rather that it would burn or outright break up. Perhaps the largely uninsulated fuel or oxygen would start to boil. Increasing mechanical stability and insulation is of course expensive because it requires a redesign, and would not lead to improved performance, probably all to the contrary. Cost is probably not the real reason, it's an engineering problem within the physics constraints that has no solution. $\endgroup$ Apr 13, 2023 at 15:09
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    $\begingroup$ To add some specifics, the booster structure is designed entirely to resist vertical forces, not horizontal. So not only would you have hypersonic drag heating an aluminum/carbon fiber structure not designed to deal with it, that heating weakens its structure further while also applying a significant horizontal force. $\endgroup$ Apr 13, 2023 at 17:29
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The boosters don't belly-flop because doing so would be counterproductive.

The boosters are already reinforced to deal with endwise loads -- they need to be, to keep the body of the rocket from being crushed between the engines and the second stage during launch. They would need additional reinforcement to deal with the stresses of a sideways re-entry, reducing the payload capacity.

There's also the issue of heat: the entry burn starts at an altitude of 65 kilometers and a speed of about 8100 km/h. At that speed and altitude, there's already substantial compression heating, but not much drag. The engine mounts already have some heat shielding to protect them from the heat of the engines, and additional shielding is provided by the entry burn itself (yes, rocket exhaust is cooler than compression-heated air). A belly-flop re-entry would require plating an entire side of the rocket with some form of heat shielding, further reducing the payload capacity.

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This is just speculation, but I suspect, this is mainly due to history:

When the Falcon 9 was developed, it was developed as a throw-away booster with minor modifications to allow for subsequent development of recovery. I.e. the primary goal was develop a rocket that could economically launch a payload even if not reused. When they developed the recovery process, they literally experimented with hardware that was space junk produced by successfully launching something into space for a paying customer. So, while reusability was kept in mind during the design, only minimal hardware was added (grid-fins, legs and restartable engines) to facilitate recovering the rocket. Before Falcon 9, the company did not have an established market and regular income to pay for their research. Adding the complexity of

  • large control surfaces (flaps) to control belly flopped flight

  • massive control authority for flipping back to vertical quickly

  • header tanks that facilitate starting engines in belly flopped configuration (plus the additional valves and plumbing)

would have severely raised the danger of complete economic failure.

With the Starship development, things have changed. Now, the company has an existing stable income, so its survival does not depend on getting the rocket to fly within a short budget. With that, they have much more freedom for radical design changes and figuring out how to make it work. Now, they can focus on developing a rocket, that will be cost efficient in the long run. I.e. they can focus on developing a maximally reusable rocket that maximizes useful payloads and provides interplanetary performance. And if that costs them some additional prototypes to develop, so be it.

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Why would belly-flopping be of any benefit? The original attempts failed because even falling tail-first the booster couldn't take the forces and heating of falling back into the atmosphere, not because it was falling too fast to land. Belly-flopping isn't going to lower those forces one bit, it's just going to transfer them from end-on (where it already has to be strong) to side-on (where there's not much need of strength.)

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  • $\begingroup$ Because it enables a more gradual deceleration profile. $\endgroup$ Apr 15, 2023 at 8:19
  • $\begingroup$ @Abdullah I would expect the reverse--harder deceleration. That's moot, though, the issue is the booster can't stand up to the reentry friction and heating even now. $\endgroup$ Apr 16, 2023 at 2:37

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