Sub-orbital Delta V

[CaptainKipard]

I'd like to be able to circularise orbits without interruption by leaving behind my final lifter stage on a sub-orbital trajectory while circularising with my ship. How can I calculate the dv required for my lifter reliably for all bodies?

If that's too difficult, at least then what is the required dv for a kerbin sub-orbital flight

Edited February 13, 2014 by Cpt. Kipard
spelling

[Poynting]

The amount of delta-v will be dependant on the path you take to get to orbit. For kerbin an optimum trajectory to orbit has a delta-v of approximately 4500m/s (ksp wiki - http://wiki.kerbalspaceprogram.com/wiki/Kerbin). Just then design your lifter stage to have this amount of dv (maybe slightly more to give a little margin for error and then ditch the spare fuel whilst still on a suborbital trajectory). For bodies with no atmosphere you should need not much more dv than their orbital velocity.

[Jimbimbibble]

Because you have gravity and air friction losses there is no exact solution. It depends greatly on the ascent profile and the design of the ship. What I usually do is dump my stage while it still has a little fuel left because that's much easier than trying to adjust everything perfectly and it gives you a margin of error.

[Claw]

It takes rougly 4500 m/s to get into orbit in stock KSP. As posted above, the exact amount will vary based on ascent profile and other factors. If you build with less than 4500 m/s, you will likely (but not always) end up suborbital.

Usually I do like posted above. Build a lifter with around 4000-4500 dV. Then during launch, I get my AP where I want it and leave the PE somewhere in the atmosphere so that the lifter will be in a decaying or impact orbit before I cut it loose.

A PE below about 22km will cause it to disappear on its own. Above that, you will have to manually delete the object (in the tracking station) or "fly" it so the physics engine will decay the orbit. Otherwise the background simulation will not deorbit the object.

Edited February 13, 2014 by Claw

[CaptainKipard]

Oh well I guess I'll just have to determine it experimentally

[Shna_na]

To achieve 80KM Orbit around Kerbin requires 4550m/s Delta V. If you create a lifter stage that has 4000m/s and an orbital stage with ~2000m/s you should be able to reach about half of the bodies in KSP. The best way to play is to just deal with the fact that you will​ be left with some orbital debris.

[Claw]

How can I calculate the dv required for my lifter reliably for all bodies?

I'm sorry, I completely forgot to address this question in my post.

Perhaps this will help you if you haven't seen it. This will show you the dV required to get from the surface to orbit for each of the bodies. For example: if you look at Minmus, it takes about 240 dV to get from the surface of Minmus to a 10km orbit.For Kerbin, it takes roughly 4550 m/s to get from the surface to an 80km orbit. If you want a suborbital for your lifter stage, then aim for a little less dV than that.

Hope that helps.

Here's another one that includes orbital plane changes, but that doesn't really address your question about dV to orbit.

[CaptainKipard]

Yeah I've seen those, but that doesn't really help that much. Additionally I use FAR so they don't even apply to me. I realise it's a very difficult question.

I've now worked out that if I keep burning the last lifter stage while pointing just below the horizon, it allows me to keep the ApA fairly constant while increasing the PeA a little untill it runs out of fuel.

Thanks everyone for trying anyway.

[KerbMav]

With FAR you need about 3500m/s dV to get into orbit.

It still remains a question of your ascent profile - even on bodies without an atmosphere.

If you want to make sure your last stage falls back to the surface/lower atmosphere, you can also add some sepratrons and let them fire in reverse at staging.

[KSK]

Hmm, it's maybe not quite what you're looking for but I just keep an eye on my periapsis during the ascent. Once it hits 65-68km, kill the engines, separate the final lifter stage and do a quick burn with your ship engines to lift your periapsis above 70km. Your final stage won't be on a suborbital trajectory but it will be in a decaying orbit. Switch to it and fly it around Kerbin until it deorbits, or just abandon it, secure in the knowledge that it would have deorbited in time.

[helaeon]

or... you don't worry about it and use sepatrons pointed upwards (so the nozzles face against the main thrust vector) that activate when you stage. All you have to do is put enough on there that when the ship is facing prograde the separated stage burns retrograde enough to get it into the atmosphere. Sepatrons are great.

[Claw]

I'm sorry we were not able to address your question. Realize the dV charts still apply to FAR on non atmospheric bodies.

[Red Iron Crown]

I just put a probe core and a battery on my lifters and leave a little bit of fuel in it after circularization. After circularizing, I separate, switch to the lifter, point retrograde and burn to deorbit. It requires very little fuel to do so, and I jealously hoard the fuel in my orbital craft so I don't want to burn them to achieve orbit.

[EtherDragon]

OP: You have two phases to determine...

First Phase - Ascent to sub-orbital.

Unfortunately, this one can only be figured out experimentally based on the lifter you are using because of differences in drag profiles, TRW, etc. Once you have this stage all set you can calculate the second phase with high accuracy (some will be lost due to atmospheric drag, but not much.) But in order to calculate second phase accurately your designs should always have the same result on whatever body - For Kerbin, for example, you might always set your ascent to sub-orbital as 75km by 40km.

Second Phase - Circularization.

Now that you have first phase complete, you can easily calculate the dV required to circularize, you're just looking for the dV required to change potential energy from a semi-major axis of 657.5km to 675km. (Scott Manly has a video to easily calculate this...)

Usually I have more specifics, but calculating dV for semi-major axis changes is not yet in my top-of-the-head mathematical library.