KSP Weekly: The Falcon

[Terwin]

On 10/11/2018 at 9:01 AM, Thygrrr said:

Basic DeltaV is a mod.

Also, DeltaV is generally dependent on drag, but AFAIK no DeltaV Mods calculate this, and that makes perfect sense.

One could calculate a "perfect ascent" DeltaV, but that differs from craft to craft. However, if you know that "4000 m/s dV are enough to get into a decent orbit", and you know your craft has that much, you will be fine.

In space, where maneuver precision matters more and more, DeltaV calculations are easier and easier because the crafts get simpler and there is no drag.

dV is only a measure of total available acceleration.

The major causes of dV being different in an atmosphere vs in space are 1) Relative combustion chamber pressure compared the the local environment(which is why dV is always higher in vacuum) and 2) Atmospheric engines generally need a high TWR, which limits how much you can optimize them for efficiency(which is why high-efficiency/low-thrust engines like the terrier and poodle are generally vacuum optimized)

When launching, the greatest losses are due to gravity because you need to need to get off the ground and stay off the ground until you reach orbital velocities, and this means you must waste fuel on accelerating 'up' instead of just accelerating to orbital velocity.

If you were on an airless body with infinite TWR, you could get to orbit with barely more dV than your final orbital velocity(And possibly less depending on the rotational speed of the body and if there were any obstacles preventing a horizontal launch.(you would need a circulation burn to avoid eventually hitting the surface again however)).

For most rockets, Atmospheric drag only has a trivial effect on the amount of dV expended to get to orbit because having a lot of drag needs both high speed and high pressure, while rockets get higher(where there is less air to cause drag) as they accelerate.   (Maximum dynamic pressure is related to drag and is a significant design constraint, but the actual losses to drag are trivial compared to gravity losses on all but the smallest of rockets)

A 'perfect ascent' is primarily an issue of TWR and minimum altitude for a stable orbit(often with a nod to maximum survivable dynamic pressure in the real world, but any Max-Q down-throttle is usually pretty brief).   

The 'perfect ascent' for an airless Kerbin with mountains known to reach 70km along part of your orbital path would be virtually identical to a 'perfect ascent' on Kerbin, assuming the same TWR throughout the launch.(assuming both are 'rocket shaped')

 

 

[Thygrrr]

That's exactly what I was implying.

However, with air pressure, ISP will change, meaning your DeltaV will change; also, if you decide to hover on the Mun you're wasting dV without going anywhere.

So there is a perfect and a worst case ascent.

[Adsixnine]

On 9/29/2018 at 11:45 PM, Noname115 said:

 I eagerly await the news!

yeppp