Momentum Seems to Stick to the Decoupled Part?

[Kerbal2023]

So I built a little probe head on top of a booster, that was supposed to decouple and continue upwards from the momentum of the booster after the rocket was spent, without being dragged down by the weight of the rocket. But when I decoupled I noticed that the burned out booster shot ahead over the probe head. It seems that the momentum did not transfer to the part that got deoupled. Is that the case? I figured the whole vessel has upwards momentum and if I decouple the heavy part the head will fly ahead.

Reference: We found out in a previous experiment that a smaller rocket can reach a higher altitude than a big one, because its heavier weight pulls it down far longer than it has extra thrust. This would imply that one wants to get rid of the spent rocket as soon as possible.

[GluttonyReaper]

It's a bit hard to tell from that picture (I'm not sure exactly when you separated / the design of the probe & rocket), but I think this might a drag issue? A light, non-aerodynamic probe would experience a lot a drag as soon as it's released into the atmosphere, slowing it down quite a lot. While your rocket (even empty) is probably heavier and more aerodynamic, and is able to "punch" through the atmosphere a lot easier. I can't find any examples now, but I remember that Scott Manley used to keep his empty boosters attached a little longer while in atmosphere if he wasn't thrusting, for this exact reason.

As for that experiment you've linked - I don't know the specifics because I'm not familiar with Tweakscale, but @Lisias might be able to clarify - it looks like the result there was that the higher cross-sectional area was what was slowing down the larger booster enough to counteract the additional momentum from the higher weight, which isn't always necessarily true for every design. Downwards acceleration from gravity is the same no matter your weight is.

[Kerbart]

Gravity works the same on both with 9.8 m/s², but air resistance doesn't. It's a function of frontal area and aerodynamics. Both the booster and the probe aren't particularly aerodynamic, so they both receive drag, and the booster probably more, since it's got a larger front area.

But... the effect of drag is of course force/mass and while drag force on the booster is more, its mass likely is a lot more. So it slows down less. The probe benefits from the separation force of the decoupler at first, but that wears off after a few seconds, and then the booster goes faster (more correctly: is less slowed down by drag ) than the probe.

[Bej Kerman]

23 hours ago, Kerbal2023 said:

Reference: We found out in a previous experiment that a smaller rocket can reach a higher altitude than a big one, because its heavier weight pulls it down far longer than it has extra thrust.

I don't think you understood Lisias' explanation. The larger flea slows down because more air hits it. Its dry mass is not as big a factor here as its drag.

Also, a feather will fall at the same rate as steel in a vacuum.

Edited November 24, 2023 by Bej Kerman

[Kerbal2023]

On 11/23/2023 at 2:03 PM, GluttonyReaper said:

Downwards acceleration from gravity is the same no matter your weight is.

I seem to forget that!