Equation help?

[W. Kerman]

Hello, I am planning a Duna mission, but I have encountered a problem. The ∆V equation works fine in an orbit, but I don't know how to calculate ∆V per second.

Could someone help?

[DD_bwest]

by Delta V per second do you mean your max accel?

[Aegolius13]

1 hour ago, DD_bwest said:

by Delta V per second do you mean your max accel?

You can see your max acceleration on Kerbal Engineer Redux.  Or, if something shows you TWR scaled to Kerbin surface, you can divide by ten to get an approximate number (since gravity is same as Earth, 9.81 m/s²).  

But I'm not sure why you'd really need to... maneuver nodes incorporate this into the burn times.  

[Guest]

∆V is ∆V, in orbit or out of it. It just means how many metres/second the craft is able to accelerate in total.

Things do get complicated when launching and landing, because you're losing ∆V to gravity, atmosphere, and manoeuvring. There is no formula to calculate that, because it depends on how you're flying. The faster you accelerate/decelerate, the less time you spend in the atmosphere and the slipperier your ship is, and the less you manoeuvre, the less ∆V you lose. 

I.e. the way to go is (1) have sufficient thrust to weight ratio to be able to accelerate/decelerate reasonably fast -- say, between 1.2 and 2.0, depending on where you're launching/landing -- (2) allow for reasonable ∆V losses, where "reasonable" depends on your skill as a pilot and your TTW ratio, and (2) learn to do launches and landings efficiently.

Furthermore, in the case of Duna, you have the atmosphere. This means that you'll need to burn much less landing -- or even none at all, after the deorbit burn, depending on how you've designed your craft -- but you will lose ∆V to atmospheric resistance when taking off, and you won't be able to launch in the most efficient angle (about 30 degrees if you have TTW 2.0 or thereabouts).

So if I've understood your question correctly, there is no answer to it. Knowing your ∆V is the starting point; figuring out how much you're going to lose to gravity, the atmosphere, and manoeuvring, balancing the cost of hauling parachutes vs. fuel all the way there, and learning to pilot is up to you.

(And me. I'm also planning a Duna mission.  )

Edited September 26, 2016 by Guest
Clarification on atmospheric losses.

[DD_bwest]

1 hour ago, Aegolius13 said:

You can see your max acceleration on Kerbal Engineer Redux.  Or, if something shows you TWR scaled to Kerbin surface, you can divide by ten to get an approximate number (since gravity is same as Earth, 9.81 m/s²).  

But I'm not sure why you'd really need to... maneuver nodes incorporate this into the burn times.  

yup, i just wasnt exactly sure what he meant. im a math noob, but i believe a TWR of 1 should be a max accell of 9.81m/s^2 as that would match it exactly.   its good to know i was on the right track. 

 

[DrLicor]

If you have Kerbal Engineer Redux installed, you can indeed see you delta-v and twr.

As staded above:

• delta-v = the max acceleration your craft can have, for instance: when in vacuum with a craft with 100 delta-v left at a speed of 2215m/s, you can get the maximum speed of 2315m/s. keep in mind this only applies to when your in vacuum, like @Brikoleur said.  
• twr = is different on every planet. It has to do with your drag on the planet. You can see it by sliding the altidude bar (when set to atmospheric) in the Kerbal Engineer Redux menu that your delta-v will change by altitude. To keep it short. If you have a rocket of 1kN. and the rocket can produce 1Kn of thrust. Your TWR will be 1. If it can produce 2Kn: your TWR will be 2. In atmosphere, your TWR has to be above 1 to get your rocket straight in the air. Most people prefer a TWR of 1.2 - 1.4 for the first stage, so you don't burn to much fuel and delta-v away in the atmosphere.

So now you know the basics, you can design a rocket. A very useful tool is a delta-v map.

Spoiler

According to this map, you shoud need 3200+950+130+250+360= 4890  delta-v on you craft to get a low duna orbit. This is calculated without doing a gravity assist from mun or other planets. 

Hope this helped you out    

[Guest]

The map also doesn't account for those pesky losses. I'd figure at least another 1000 m/s to get into LKO. Kerbin has a deep gravity well and thick atmosphere, you will lose less taking off from Duna.

Corrected, it does.

Edited September 26, 2016 by Guest

[DrLicor]

The Low orbits are quite accurate, although I don't play stock ksp anymore (only RSS/RO), I could get in a LKO with  3200 delta-v by doing the right gravity turn.

 

Edit: the orbital speed is 2300m/s in LKO. The maps says you need 3200 delta-V. So it does take account for another 900 delta-v loss in the atmosphere, same for duna. only less loss indeed. you need a speed of at least 900m/s. Therefor you need 1200delta-v according to the map

Edited September 26, 2016 by DrLicor

[Guest]

You're right, I had the version 1.0 numbers in mind. I stand corrected, thanks.

[DrLicor]

Sorry, took an old map, here's the new one for ksp 1.1.3