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Musk has stated that they added the delta wing to the BFS design because; from one flight to the next the ship may have differing payloads and differing fuel loads. These changes shift the center of gravity of the overall ship. So the wings were added to compensate for this.

It seems to me that this is only a problem as the ship renters in a long side orientation like the shuttle, rather than an engine first orientation like the Falcon 9. An engine first orientation would probably not have this center of gravity problem, and need less heat shield.

Does anyone know why they have chosen to reinter this way rather than engine first?

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    $\begingroup$ Despite what Kerbal Space Program tells you, in the real world, engines make lousy heat shields. $\endgroup$ – Mark Apr 11 '18 at 1:27
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F9 can enter engine first because it isn't returning from orbital speeds. While fast, it's a fraction of the speeds something returning from orbit (or further) comes in at.

So the engines are out as an entry surface, you need more protection. One way to achieve this is Dragon-style: put a heat shield on the bottom, and engines in the sidewalls. Great for Dragon, not so good for something the size of BFS - The engines are quite a bit larger and more complex, side mounted engines are inefficient (energy lost to the angled exhaust), and cylinders aren't ideal shapes for hiding behind a heat shield - the hot gas flows will reattach themselves to the long straight walls, causing unwanted heating.

So BFS sticks with the tradition of bottom engines, and covers one side with heat shield material instead. They shield the engines behind an interstage which stays with the upper stage rather than being discarded with the first stage like typical rockets (the sleeve surrounding the engines at the aft of the ship, in the image below), and it provides a very favorably large surface area to mass ratio, reducing the peak heating during entry.

enter image description here

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    $\begingroup$ It's also worth mentioning that entering belly-first creates aerodynamic lift like a reentry capsule which helps to manage descent rate. Ballistic reentry sucks because velocity is shed too rapidly in the lower atmosphere which would likely cause the BFS to break up, or at least be very bad for any humans on board due to g-forces. The lift might also help in the tenuous atmosphere of Mars to maintain altitude for longer and thus bleed off more velocity and it might give some meaningful cross-range on Earth, though I'm not sure that'd be useful (maybe for very rapid return and relaunch). $\endgroup$ – Blake Walsh Apr 11 '18 at 10:31
  • $\begingroup$ @Saiboogu can you clarify "lets them shield the engines behind some attached interstage"? Are you talking about the first stage (booster) or second stage (ship) of the BFS? Or will both reenter with a similar heatshield and attitude? $\endgroup$ – mb21 Apr 11 '18 at 11:49
  • $\begingroup$ @mb21 I'll try and dig up some images to clarify that description. This is all about the upper stage ship, or BFS. BFR will function much like Falcon 9 stage one because of the lower speeds, just adding launch-pad landing to the existing routine. $\endgroup$ – Saiboogu Apr 11 '18 at 13:46
  • $\begingroup$ @mb21 Added an image and rephrased that bit. $\endgroup$ – Saiboogu Apr 11 '18 at 14:24
  • $\begingroup$ @Saiboogu thanks a lot! Now I understand what you meant... $\endgroup$ – mb21 Apr 11 '18 at 15:06
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Saiboogu's answer about protecting the engines is great. Just want to add that Falcon 9's supersonic reentry burn is a delicate issue already (*). Starting the engines and keep them burning would become even more an issue if reentry speed was orbital.

Another downside for engines first reentry from orbit is efficiency: aerodynamic drag would be lower, i.e. more kinetic energy would need to be removed propulsively. So you'd need to carry extra fuel. With a sideways reentry with heat shield, much of the energy is removed by aerobraking. So, I guess, the weight of the heat shield is probably less than the otherwise additionally needed propellant.

(*) Heard Elon Musk say it somewhere, but even if I don't have sources at hand atm, it makes sense. Just imagine there'd be a couple of 100's mph wind blowing into a car's exhaust)

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    $\begingroup$ "Aerobraking" is an established term referring to the use of an atmosphere to shed unwanted speed. And yes, it's spelled "braking". "Breaking" is something you do not want your rocket to do. $\endgroup$ – anaximander Apr 11 '18 at 10:14
  • $\begingroup$ @anaximander good one ^^ $\endgroup$ – Everyday Astronaut Apr 11 '18 at 10:50

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