My thoughts about designing spacecraft and delta v.

[steamypete]

Hello kerbonauts.

Love the game!

But I have a Delta-v question when designing spacecraft. Actually it's not really a question. I'm just

looking for someone to confirm if I'm using the right approach when designing my spacevessels.

I always start al my designs by defining a mission or goal. For example:

1. Launch from Kerbin.

2. Fly to the Mun.

3. Deliver payload to Mun surface.

4. Return to Kerbal orbit and await new payload.

Basicly I need to design 3 craft to complete this mission

- the lifter (Kerbin to orbit)

- the transporter (Kerbin-Mun-Kerbin)

- the payload (which is a simple lander in this case)(Mun orbit to Mun surface)

So now it's all about the delta v when designing the spacecraft, right?

- Lifter needs 4500dv; I assume thats right.

- Transporter needs 2140dv. (860 to reach the Mun + 210 to establish orbit. and then I double this to

make the return trip but I don't know if this is correct.)

- The payload needs 640dv to land on the Mun. (one way trip)

I know that to build the lifter with the neccesary dv, I first need the weight of the transporter and

the payload. So I start building backwards. Payload first, then Transporter and finally the lifter.

I'm using the Kerbal engineer redux mod show me the dv when building. But that's not going too well. It's showing me the dv of the transporter without its payload. So thats' nowhere near correct. Is there a mod where you can enter payload weight and then recalculate? Or do I need to keep doing that by calculating myself?

I love to hear your thoughts about my approach to this.

Thanks.

[Karriz]

Mechjeb shows the delta-v for each stage separately, and takes the mass into account when calculating. I suppose that does what you're looking for?

[Gniuz]

MechJeb FTW

even if you want to fly the mission manually it's worth using for the Dv information your after.

[cxg2827]

If your starting module is the lander going to the moon, put the engineer chip on that. Then add the transporter module and you'll see the delta v change. Add the lifter and you will get the lifters delta v. Since your want the transporter to make the return trip, save it using sub assembly loader, then create a new ship, pick the smaller probe, and then pull in the transport from the sub assembly loader.

[steamypete]

Thanks for the help. I'm actually using mechjeb to help me calculate some stuff but it only seems to work when staging using decouplers/separators. If you attach your payload using a docking port Mechjeb cannot calculate its delta v.

I'm starting to think this is why I'm having all these problems figuring out delta v for my space craft. I guess I'll have to keep calculating it myself.

[capi3101]

If you also use Subloader Assembly module, what you could do is put together a dummy payload that has a similar mass to your desired payload (say...an probe chassis with an orange tank, BUT NO ROCKETS, if your payload is somewhere around 36/37 tonnes); Mechjeb oughta tell you that you have no delta-V, which would be right since you haven't got a rocket on it yet. You can then build a lifter with 4500 delta-V off of that. When you get there, save the lifter portion of the rocket (the part without the dummy payload) with Subloader, switch to your actual payload and simply attach the lifter to it.

[Chocki]

I just toss a small decoupler between docking ports to get delta v. Eng redux will also display delta v by stage, and in relation to what ever cestial body you choose. You just need to not keep it in compact mode. Mechjeb has a nicer layout and look (along with tons of customization), but lacks the delta z in relation to another body. Helps a ton when wanting to know the delta v and twr of a lander while it is still being designed.

[Fractal_UK]

You can estimate your ÃŽâ€V pretty simply, just keep an eye on the exhaust velocity of your ship (i.e. specific impulse*9.81). Once you know the exhaust velocity, then you know you need about 1.7*the dry mass of your ship in fuel tanks in order to get your exhaust velocity in ÃŽâ€V. So, if you're using nuclear rockets you can get about 8,000ms^-1 of ÃŽâ€V if about 63% of your ship's mass is fuel.

Be careful with landing ÃŽâ€V though, the optimal way to land is to maximum burn at the altitude that will result in your hitting 0ms^-1 precisely at the surface (which you may not know the exact altitude of). It's very tough to actually do that in practice so be sure to pack plenty of safety margin.

[steamypete]

If you also use Subloader Assembly module, what you could do is put together a dummy payload that has a similar mass to your desired payload (say...an probe chassis with an orange tank, BUT NO ROCKETS, if your payload is somewhere around 36/37 tonnes); Mechjeb oughta tell you that you have no delta-V, which would be right since you haven't got a rocket on it yet. You can then build a lifter with 4500 delta-V off of that. When you get there, save the lifter portion of the rocket (the part without the dummy payload) with Subloader, switch to your actual payload and simply attach the lifter to it.

Thanks. I like this solution. I'll give it a try.

[DMagic]

Another thing to keep in mind, at least with mechjeb, is that the delta-v values it gives are for atmospheric flight (I assume that means at sea level on Kerbin) and vacuum, but not in between. I can't remember the exact numbers off the top of my head, but I think it takes about 3000-3200 m/s delta-v to get your initial apoapsis up to about 70km (assuming a good ascent profile). A lot of that initial delta-v takes place in the atmosphere, so I design my initial stages to deliver a little bit less than 3000 m/s of atmospheric delta-v. The circularization stage takes another 1200-1400 m/s delta-v, but that's all at vacuum, so it's easier to calculate.

I have a few sat-map probes that work out almost perfectly for calculating this. The first stage is just enough to get up to 75km, the second stage is just enough to circularize, the 3rd stage is just enough to get to Duna, or Eve, and the all the fine tuning after that is done with an ion engine. I'll try to launch one of those later today and see just how much delta-v is needed for each part of the ascent. I assume there is an easy way to calculate this based on atmospheric density, ISP, and TWR, but it seems just as easy to monitor how much delta-v is expended at each step.

[Eric S]

Transporter needs 2140dv. (860 to reach the Mun + 210 to establish orbit. and then I double this to make the return trip but I don't know if this is correct.)

OK, I think everyone has covered the MechJeb/KER delta-V issues well enough. ON this step, you can reduce the amount of delta-V required if you're willing to aerobrake when returning to Kerbin. In theory, that would be the entire 860 that it took to go from kerbin orbit to the transfer orbit, but I don't like to cut my fuel margins that close.

[RoboRay]

The division between raising Ap and circularizing varies greatly with your ascent profile. A long coast to Ap followed be a long circularization burn is less efficient than building more horizontal velocity once you clear most of the atmosphere (50km or so).

I frequently expend just 200m/sec or so circularizing. I've also done single-burn to circular orbit a couple of times.

[EndlessWaves]

looking for someone to confirm if I'm using the right approach when designing my spacevessels.

It depends on what you're trying to achieve.

Personally I'd be wary of using fixed stages, but instead build to the limit of what the engine will carry and then add whichever engine is most appropriate for that point on the mission's DeltaV and repeat.

It's a lot easier to get back from the mun, you're essentially just pushing your orbit up to escape velocity from the mun (nine hundred and something m/s), if you burn at the right point then that's it, you'll intercept kerbin and can aerobrake in the atmosphere. Then it's just the 150-200m/s needed to stop on Kerbin if you're not using parachutes.

[Francesco]

to put those values in context, you also need to state the desired Mun orbit altitude: 210 m/s for a quite high Mun orbit insertion is ok, but for a 25km one, make it ~270 m/s.

640 to land is fine, but you're on a tight budget.

since the transporter remains in orbit, you'll only need some 300 m/s to escape the Mun: that 2k value would be for a descent, ascent and return stage.

so the transporter would only need 1.4-1.5k delta-v - maybe pack just a little bit more for when you aerobrake on Kerbin and fine-tune your parking orbit, if you're not using mechjeb.

Edited May 7, 2013 by Francesco

[HoY]

I know it's been said before but it can't hurt to have it said more clearly lol.

Throw a stack decoupler in between your docking ports while you are building the ship, then make sure it's in the proper place in your staging. Once your stages are all configured how you want them you can see your dV correctly in mechjeb. Before you launch simply remove the stack decouplers from between the docking ports. As soon as you get into the launch pad right click on all of your docking ports and disable cross feed, otherwise All of your fuel will get burned out of the core of your primary ascent stage as soon as all of its asparagus tanks are dropped.

[numerobis]

From Mun orbit it takes only about 300 m/s to get home: you need to double your outbound burns indeed, but since you're arriving at a planet with an atmosphere, you can aerobrake -- that saves you the 840 m/s burn you did outbound. I'm adding a buffer to let you aim for KSC.

I use 600 m/s to lift off from Mun to a 5km or 10km orbit fairly consistently.

Edited May 7, 2013 by numerobis