Work with dV Stats?

[T3E]

Sawp, me (once again) with a question I cannot find an answer for with search/google.

There are a bunch of Stats out there in the internetz where is written how much dV I need to get to planet/moon X.

I have some good constellation to Duna and started to shoot up a few probes and rovers in the orbit and on intercept with Duna.

My problem occurs with the manned lander now.

On the wiki I read that I need appr. 7800dV.

But is it ATMOSPHERIC or VACUUM dV?

So I build a rocket that would have strong skipper/Rv-30 for the first two stages and and has about 5000dV (atmospheric).

The last stage + lander has the high ISP engines LV-N which result in another 5000 dV ( vacuum). It has enough TWR for the gravity on Duna, so I guess it would work out.

The problem that occurs is that I dont know on which stats all the Orbital Maps/Wiki guides are based on.

Thanks in advance (I know someone smart knows the answer )

Edited May 30, 2013 by T3E

[Crush]

Delta-V (change of speed) is Delta-V, no matter if it is in vacuum or in atmosphere.

It's just that the Delta-V capacity of a craft is reduced while being in the atmosphere because engines have a lower Isp.

Also, any speed you lose due to atmospheric drag or overcomming gravity drag is wasted Delta-V. You can't generalize this loss in Delta-V, because it largely depends on the ascension speed and how well you execute the gravity turn.

The actual Delta-V you need to get to Kerbin orbit is thus hard to estimate. Delta-V budgets become a lot more useful as a measure when you travel from orbit to orbit.

Edited May 30, 2013 by Crush

[willow]

T3E, I presume you have this map in mind?

It's based on delta-v, wherever you are. If you were to put a delta-v counter in your ship (For example mechjeb) and get to orbit your counter would indicate the number as indicated on the map. With most engines the difference between vacuum and atmospheric delta-v isn't so big. Especially if you're on solids for the first 10km. I usually bring 4500 delta-v for takeoff from Kerbin.

So to answer your question: "Safely ignore atmospheric delta-v, but bring the proper engine for the job."

[Uberick]

To get to LKO takes about 4500 d/v, on average. To get to Duna costs about 1050 dv in addition to that. Throw in another 100 or so for perfecting your orbit so you end up aerobraking, and you can land for free, and get yourself into orbit for a little bit more than that.

[T3E]

Yes exactly willow.

Aswell as

Recommended Delta-V: ~7760 m/s

from the wiki regarding Duna.

But okay, I slowly understand the dV-displa mechanics. So to come back back to my OP, to plan roughly, the the way to LKO i use the atmospheric dV and when I plan my voyage I calculate with the vacuum one? (regarding your "its based on delta-v, wherever you are" .

Good thing with the

Safely ignore atmospheric delta-v, but bring the proper engine for the job

I will keep that in mind.

@Crush ok sorry for my dumb assumption that there are two different dVs .But since several plugins indicate the 2 dVs, I was confused regarding the other dVStats on the web. But good to know theres is actually only one

[tomf]

For the complete mission you shouldn't sum the delta-vs like that, you should instead be looking at 4500ish of in atmosphere delta-v for you launch (so idealy you might use aerospikes) and then a thousand or so in vacuum (lv-n s would be perfect) for you transfer to duna, some more in atmosphere delta'v to lift of from duna (although the presure is low enough that each engine will be towards it vacuum isp) and then more vacuum delta v to get home

[Rune]

Actually, you can pretty much ignore atmospheric delta-v much sooner. From about 15 kms you already get almost vacuum performance (the reason aerospikes are not good for spaceplanes with turbojets).

And in Duna LV-N's still get upwards of 700s. Still, a good practice in general is to have some delta-v margin (20% on the first flights?) so you can screw up and/or have effect like gravity drag taken into account.

Rune. Godspeed on your journey!

[willow]

Actually, you can pretty much ignore atmospheric delta-v much sooner. From about 15 kms you already get almost vacuum performance (the reason aerospikes are not good for spaceplanes with turbojets).

This is my observation as well, when taking off from Kerbin I don't mind switching to nuclear engines at 20km and I guess 15km would do too.

Thats why I say you can ignore atmospheric delta-v if you bring the proper engine. You won't likely be off more than +/- 200 deltav..

So if you plan your ascent from kerbin, calculate roughly 4500 deltaV, then add about 950 deltaV for leaving kerbin, add 110 deltaV for your orbital insertion at Duna. 1380 for landing. That's 6940 deltaV for a complete one way duna mission.

Of course your landing is going to much cheaper than 1380 because of the atmosphere. You can land on kerbin with 250 delta V no matter what speed or angle. Duna is more difficult because of the less dense atmosphere. With a fairly flat descent and the right parachutes you could still land for free.

However, this is a perfect world calculation. Bring 10% more for random errors and losses, this brings us to 7634 which is only a bit lower than the recommendation you found.

The return trip isn't on the chart, but in my experience it's a lot cheaper than the trip to duna, first of all, takeoff is only 1380, escape from duna to kerbol orbit should be about 300 (I'm just guessing here) return to kerbin 110 and landing on kerbin is free with the parachutes you didn't forget to pack. So add 2000 deltaV for the return trip. 10k for a round trip is more than enough and doable.