32
$\begingroup$

The term suicide burn occasionally comes up in answers and comments, usually without any real explanation or definition.

What is meant by the term "suicide burn"?

$\endgroup$
51
$\begingroup$

For a powered descent to the surface of a massive body like the Moon, it turns out to be most fuel efficient to do all your deceleration at the very end of the trajectory, right before impact.

(This is because if you decelerate sooner than that, you will be in flight longer; the longer you're up, the more fuel you need to spend counteracting gravity accelerating you toward the surface.)

So the fuel-optimum landing involves falling faster and faster toward the surface before doing your deceleration burn, which looks suicidal (and will be, if you don't leave sufficient margin for error).

Hence: suicide burn.

SpaceX's first-stage recovery is another example. In that case, suicide burn is a requirement, because even at minimum stable throttle, the engine produces more thrust than the weight of the stage, so it can't hold a constant low downward speed for any length of time; it has to time the burn so that it hits altitude zero just before it reaches velocity zero. SpaceX calls this maneuver "hover-slam".

I'm not sure of the origins of the term "suicide burn"; it's extremely popular in the Kerbal Space Program community. It doesn't appear in the Google books nGram viewer at all. It comes up in some SpaceX discussions but a lot of that looks like bleed from KSP.

$\endgroup$
  • $\begingroup$ If you've seen Interstellar, I'm pretty sure they do a dramatic version of this when they land on Miller's planet. $\endgroup$ – Ricky Jul 28 '15 at 23:54
  • $\begingroup$ The proper stackexchange term is suicide by-design. $\endgroup$ – uhoh May 2 '17 at 20:02
  • 2
    $\begingroup$ Tidbit: Your most fuel efficient landing isn't falling like that, but rather you set your periapsis very, very low (the more thrust your engine puts out the lower your desired periapsis) and do your landing burn from orbit, a gravity turn in reverse. It's still a suicide burn, though. $\endgroup$ – Loren Pechtel Jun 24 '17 at 20:45
  • $\begingroup$ @LorenPechtel It's the same thing: because you can't do the entire deceleration instantaneously, your near-horizontal initial approach turns into a near-vertical final. $\endgroup$ – Russell Borogove Nov 10 '18 at 1:27
  • $\begingroup$ @RussellBorogove No--as soon as you drop below orbital velocity you start accumulating gravity losses. Thus you want to minimize the time from your deorbit burn to touchdown, thus do your deorbit as low as you safely can. It's the same as going up--you turn horizontal as soon as you can, even incurring some increased drag to do so. $\endgroup$ – Loren Pechtel Nov 11 '18 at 2:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.