Delta V.. What do I look at?

[LadyAthena]

I've decided Its time I really start learning whats what, down too the nitty gritty. I've asked a few questions in the past, and I've learned. I've made a ship capable of reaching all planets, but never one that's capable of landing on any and returning except the smaller moons.

I'm using MechJeb to check my ships Delta V as last I heard that's what I should be looking at. I've also learned its extremely hard, if not impossible to create a ship that could land on any planet and return, due to the differences of each planet and reaching them.

So my question is ultimately, as I'm building each stage of my rocket, and particularly the first stage (the lander), what do I look at to determine how much delta V and or lift my lander needs to land on any given planet and how do I check how much each planet is gonna need?

I'd really like to start building specialized ships for each planet, and or Moon.

Here is a picture just as an example, if someone could base their examples, and explanation off of this so I have a visual http://cloud-3.steampowered.com/ugc/615049894329104112/01A50EB206158E580C8BE8919EDC6A7C23FBBBFF/ Thanks ^.^

[Sirine]

This http://wiki.kerbalspaceprogram.com/wiki/Cheat_Sheet tell you all you need to know about how much Dv you need.

More over, It is recommended that you took that MechJeb "off" until you fully understand the NavBall, and Yeah the NavBall is more important than the Dv.

[xcorps]

I've decided Its time I really start learning whats what, down too the nitty gritty. I've asked a few questions in the past, and I've learned. I've made a ship capable of reaching all planets, but never one that's capable of landing on any and returning except the smaller moons.

I'm using MechJeb to check my ships Delta V as last I heard that's what I should be looking at. I've also learned its extremely hard, if not impossible to create a ship that could land on any planet and return, due to the differences of each planet and reaching them.

So my question is ultimately, as I'm building each stage of my rocket, and particularly the first stage (the lander), what do I look at to determine how much delta V and or lift my lander needs to land on any given planet and how do I check how much each planet is gonna need?

I'd really like to start building specialized ships for each planet, and or Moon.

On the Dv display in the VAB, you can adjust the display per body. For instance, you can have it show the TWR for the Mun, or Minmus or whatever.

The Dv is the Dv. It doesn't matter what planet you're going to, the Dv you have is what you have. So if you build a landing module for Minmus with 2800 Dv and enough TWR to liftoff, you can land, make a few hops around for more science, and return to Kerbin.

[Claw]

Well, start off by deciding what you want for a mission profile. Then have a look at a delta-V map.

The delta-V map will tell you how much dV you need for each phase of your trip. From this, many people will design their rocket stages to have enough dV for particular parts of the journey.

For example, if you want to take a trip to Mun and back. From the dV map...

Liftoff to LKO: 4500 m/s

Burn from LKO to Mun: 680 + 180 = 860 m/s

Capture in Munar orbit: 80 m/s

Down to Low Mun orbit: 230 m/s

Land on Mun: 580 m/s

So if you add all that up, you need 4500 + 860 + 80 + 230 + 580 = 6250 m/s

So your ship needs around 6250 m/s of total dV to land on the Mun. (Doesn't include the return journey.)

Now, some people will break that dV requirement into different stages. Here is one example of how to break it down.

Lifter portion (to get to LKO) will have ~4500 m/s, these parts get left behind in LKO or deorbited back into Kerbin.

Munar Injection portion will have 680 + 180 + 80 + 230 = 1170 m/s. This stage gets you from LKO into Munar low orbit.

Lander portion needs 580 m/s. This allows you to land on the Mun.

Generally speaking, you want to add a bit extra to all those numbers to allow for mistakes. An extra 10% is an okay rule of thumb. Add more if you need it, take some away if you have lots of practice. Once you have an idea of how you want to break apart your flight profile, you can use MJ to help sort out how much dV is in each section. It might take a little extra adding up on your part since MJ doesn't group pieces together.

Edited May 6, 2014 by Claw

[LadyAthena]

This http://wiki.kerbalspaceprogram.com/wiki/Cheat_Sheet tell you all you need to know about how much Dv you need.

More over, It is recommended that you took that MechJeb "off" until you fully understand the NavBall, and Yeah the NavBall is more important than the Dv.

I know how to do everything without mechjeb. I use Mechjeb just for ship stats while building, and fine tuning intercepts when trying to match up with another object to dock, sometimes the little buttons don't give me enough fine tuning I need.

[LadyAthena]

On the Dv display in the VAB, you can adjust the display per body. For instance, you can have it show the TWR for the Mun, or Minmus or whatever.

The Dv is the Dv. It doesn't matter what planet you're going to, the Dv you have is what you have. So if you build a landing module for Minmus with 2800 Dv and enough TWR to liftoff, you can land, make a few hops around for more science, and return to Kerbin.

So I guess I need to redefine my question too, how do I know how much TWR I need?

[EdFred]

So I guess I need to redefine my question too, how do I know how much TWR I need?

I try to have at least 1.3 TWR for ascent, and 2.0 for descent.

[Yasmy]

A TWR of about 1.8-2.2 is about what you want for takeoff. That TWR is relative to the local gravity.

Options:

1) Set MJ or KER or whatever mod you use for info to display TWR for your vehicle relative to some other planet or moon.

2) Find the TWR relative to Kerbin and multiply it by the surface gravity from the wiki and divide by 10 (Kerbin's approx. surface gravity.)

Once in a stable orbit, TWR is less important. Low TWR in space just means longer burn times.

[xcorps]

So I guess I need to redefine my question too, how do I know how much TWR I need?

Launching from Kerbin I like to have around 2 until the middle atmosphere, then I like to have around 4. But all you NEED is about 1.2. If I'm landing somewhere, I like to have a really high Dv, like 5 or 6 whenever possible.

Too much TWR in an atmosphere and you start to run afoul of Terminal Velocity. Any speed over terminal velocity costs you additional Dv in drag/gravity loss. You want your vertical speed to be as close to terminal velocity as possible. I make a custom ascent window for Mechjeb that shows stage Dv, TWR, vertical speed, AP, heading, roll, and pitch.

[EdFred]

A TWR of about 1.8-2.2 is about what you want for takeoff. That TWR is relative to the local gravity.

If your TWR is that high on an atmospheric body, you end up having to throttle back or you're wasting burn against terminal velocity - at least that's what happens when I go that high.

[cantab]

You need a TWR of 2 to hit terminal velocity in a vertical ascent, since that's the thrust that will result in a net upwards force equal to the rocket's weight. However, since the TWR rises as you burn fuel a liftoff TWR of 2 results in needing to throttle back. The "ideal" launch TWR is going to depend on a lot of factors - the size of the first stage, the engines available, and the staging used.

[Yasmy]

Sure, sure. Perfect efficiency is not what I was going for. A little wastage can be just fine.

We're not talking about optimal ascent here. Just making sure we can return from some other surface.

Edit: If you want to talk about how bad a non-ideal TWR is, come join me.

Edited May 6, 2014 by Yasmy

[LadyAthena]

Thank you so much for the responses guys ^.^ This has helped me greatly!