I need to calculate the minimum time, t, it takes to travel a given distance, x.

Constraints and Known Variables

Distance x is broken up into two distinct, known segments:

  • xv = vacuum distance
  • xa = atmospheric distance
  • x = xv + xa

While traveling the length of distance xv, I am under constant, known acceleration or deceleration:

  • ava = vacuum acceleration rate
  • avd = vacuum deceleration rate

While traveling the length of distance xa, I am under constant, known acceleration or deceleration:

  • aaa = atmospheric acceleration rate
  • aad = atmospheric deceleration rate

I will have known velocity constraints:

  • v0 = starting velocity
  • vf = ending velocity
  • vmax = max velocity (cannot exceed this velocity)

Example 1

I'm in a spacecraft 60km above the surface of a moon. I need to travel to the surface of the moon as quickly as possible. Ignore rotations/orbits. Both my starting and ending velocities are/must be 0 m/s. The moon's atmosphere begins sharply at 30km (there is no gradient here). I have four distinct acceleration/deceleration rates: acceleration in a vacuum, deceleration in a vacuum, acceleration in atmosphere, and deceleration in atmosphere. My spacecraft is governed by a top speed. What is the shortest amount of time it takes me to reach the surface.

Example 2

I'm in a spacecraft on the surface of the same moon. I need to climb to an altitude of 45km as quickly as possible (remember, the atmosphere ends sharply at 30km). My starting velocity is 0 m/s, but I do not care what my ending velocity is, so long as it does not exceed the top speed of my craft.


I've solved the above problem for a single rate of acceleration and a single rate of deceleration. However, the addition of the notion of two distinct rates of acceleration and two distinct rates of deceleration has really thrown me through a loop. I'm pretty much lost after banging my head against a wall for a few days.

  • $\begingroup$ why not solve for the two situations separately? it seems they can be thought of as distinct problems. $\endgroup$
    – The Count
    Aug 29, 2018 at 2:16
  • $\begingroup$ I second the above comment. You have starting velocity. First, find velocity at the point were atmosphere ends and that's your initial velocity for the second part of the problem. $\endgroup$
    – Vasili
    Aug 29, 2018 at 2:26
  • $\begingroup$ They are not entirely distinct problems. If my ending velocity was 0 and my final velocity coming out of segment 1 and into segment 2 was too large to decelerate from, I've got a problem. I'd have to cap my segment 1 final velocity to the max allowable velocity entering segment 2, considering ending velocity requirements and rate of deceleration. They are inter-related insofar as segment 1 must respect the overall problem's constraints with respect to what would happen in segment 2. $\endgroup$ Aug 29, 2018 at 16:43
  • $\begingroup$ Well, it seems you've since gotten it sorted out. At a glance, i would solve this using letters in place of the relevant starting and ending velocities, and then use those results to bound the relevant things. $\endgroup$
    – The Count
    Aug 30, 2018 at 1:25

1 Answer 1


You need to figure out where the breakpoints are, then it just becomes solving constant acceleration between the breakpoints. For your first example, you probably accelerate up to maximum speed, stay at that speed until you need to decelerate, and decelerate fully. The only reason this would not be true is if you can't get up to maximum speed before you need to decelerate, so take that assumption. Now see if you can get to maximum speed before the atmosphere starts. If you can, you just accelerate in vacuum up to maximum speed. See if you can decelerate from maximum speed while within the atmosphere. If you can, do that, and you have the breakpoints where you stop accelerating and start decelerating. If you can't decelerate within the atmosphere, decelerate fully in the atmosphere and solve for the velocity you need at the top of the atmosphere. Start decelerating in vacuum to meet this speed.

If you can't get up to max speed and back down again, you need to locate the altitude of maximum velocity and the velocity at that altitude. You will accelerate fully until you hit that point, then decelerate fully. Guess whether it is in or out of the atmosphere. One of the regimes will just have one acceleration rate, the other will have two.

For the second, accelerate fully in the atmosphere until either you hit top speed or you hit the top of the atmosphere. In the first case, just keep going to $45$ km. In the second, keep accelerating in vacuum until maximum speed, then just hold it.

  • $\begingroup$ Yeah, this was pretty much where I left off. For the part of this problem where only one acceleration rate and one deceleration rate exists, I created a function to determine max velocity over a given distance, considering a starting and ending velocity. I think I just need to calc that for the 2nd segment up front, and then pass that off as ending velocity to the function for the 1st segment. Thanks for the above! $\endgroup$ Aug 29, 2018 at 16:48

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