Delta V question

[Foxster]

I have two ships. Both are lifters to get a load from the surface of kerbal to somewhere like Duna.

Both have a 16 ton load on top before test launching. The first is a multi-stage asparagus rocket. The second is a simpler, lighter three-stage rocket.

The first has, according to MJ, a delta V of 12,516 and the second has a delta V of 13,046.

Just looking at it it feels like the first will get farther because of all the extra fuel it is hauling. And, sure enough, it does. It gets to Duna with a load of fuel to spare. It could probably make it back to Kerbal.

The second though, despite it higher initial delta V, I can't even manage to get into circular orbit around Duna before its out of fuel.

So there is obviously more to this than just the delta V numbers. I just dunno what. So, what am I missing here?

[diomedea]

Without having a look at a detailed picture, or the craft file of your lifters, this is an exercise at guessing what you may be missing.

Sure there are circumstances where Delta-V alone isn't showing the true performance of a vessel. Quite easy to point the case when you know the vacuum Delta-V, but the true Delta-V you have is curtailed by the vessel flying in atmosphere. A slow ascent, then requiring longer to leave the atmospehre, would certainly impact on Delta-V more than a fast one.

Then, a slow ascent also means more gravity losses. So, if the ascent stages have a lower TWR, your craft is going to lose more of its initial Delta-V than if those stages have higher TWR.

High TWR (or, better, acceleration) is also required to properly perform some orbital maneuvers. Like ejection or injection burns. Some engines have very high ISP but conversely very low thrust, and they take a long time to provide the Delta-V required in those burns. But to be effective, such burns are to be performed close to periapsis, in a limited amount of time. If it takes too long, some of the energy gain from burning at periapsis (read: Oberth effect) are lost.

[Mr Shifty]

Hard to know: how much dV do you have left on both crafts once they reach Kerbin orbit? What engines are you using for transfer?

Thrust is a possible culprit. If your thrust is too low, you can lose a bunch to gravity during your ascent from Kerbin.

With 12-13 km/s though, you can almost go back and forth between Duna and Kerbin four times. If you're having trouble circularizing after a trip out the Duna, something is way off.

[weezl]

As shift and diomedea have said.. look to your TWR. You must have a good TWR at liftoff.. no amount of delta-V will compensate for that. Most posters here aim for at least a TWR of 2.0 at liftoff from Kerbin. This gives you good acceleration out of the gate.

[cantab]

Another factor to consider is how the engine Isp changes with atmosphere. All engines lose efficiency in atmosphere, but some are worse than others. Generally speaking the most efficient engines in vacuum, like the LV-N nuclear engine, LV-909, or KR-2L suffer the biggest performance hit at sea level.

And another is your flying. Inefficient ascents can cost you a fair bit of delta-V. Inefficient transfers can too; did your two missions use about the same size burn to leave Kerbin orbit?

[Zzabur]

I would say same as guys before :

-Ascent speed > terminal velocity : lose a lot of energy because of aerodynamics, atmosphere brake your rocket, i think your second rocket got perhaps more engines on at the same time, so higher TWR, perhaps too high.

-Ascent speed << terminal velocity : lose a greater part of the energy to gravity

Both cases should be seen when you're LKO, if your DV is already much different on LKO on both ships, you found your DV sink.

Second loss is, like Diomedea said, during ejection burst, low TWR and you have to burn far before and after your maneuver node, the further you are from ideal node, the more energy is lost. I sometimes use a small trick, don't know if it's worth it though : when i need a long burn (more than 10min) because of low TWR, i split this burn in 2 or 3 maneuvers, first time, i activate engines 2 or 3 minutes before the node, and stop at the same amount of time after, then i wait a complete revolution around the planet, and i do another burn to complete the maneuver, 2 maneuvers next to the periapsis seem to save extra energy loss than 1 long. However, be careful about not reaching moons SOI between your 1st and second burn. As i don't understand completely the Oberth effect on the paper, i am not sure this is efficient, but it seems so from my own experience.

[Foxster]

Thanks for all the info, guys.

I tried to make both journeys as similar as I could. Used MJ for the launch with terminal velocity limiting on. Went to the same orbit. Then transferred in the same ways, again using MJ. Repeated it a couple of times to be sure.

It seems pretty obvious just from looking at the craft that one is going to get further, it has tons more fuel and lots of that left by orbit.

Its just that I always see DV seeming to be fairly fixed for particularly journeys, with the DV trees showing what you need to get places. So I have been struggling with why its not the whole picture after all.

I think its the TWR I need to be looking at as well as DV. Its just that I was hoping for some simple thing (like I thought DV was) that will let me see that one rocket is going to get further than another. Seems its not simple though.

[Pecan]

...Seems its not simple though.

It's rocket science!

Actually, it is more or less that simple, except when you're dealing with an atmosphere. Launches/landings make it a bit more complicated but without an atmosphere they're still fairly easy to deal with, it's the atmosphere that's such a drag.

[Mr Shifty]

I think its the TWR I need to be looking at as well as DV. Its just that I was hoping for some simple thing (like I thought DV was) that will let me see that one rocket is going to get further than another. Seems its not simple though.

Do you have screenshots of the two craft in the VAB, with the MechJeb ship info panel open?

[problemecium]

If your ship has deployable landers/rovers/probes/missiles/sub-ships, etc., MechJeb might be counting those as part of its total deltaV and thus giving you bad data. I find the only way to really know a ship's real, actual, official deltaV is to HyperEdit it out to deep space (not too deep, but a little past Eeloo is good), write down your orbital speed, and then throttle up and burn until the fuel runs out. Your deltaV is how much faster you're going after that.

[Superfluous J]

Its just that I always see DV seeming to be fairly fixed for particularly journeys, with the DV trees showing what you need to get places. So I have been struggling with why its not the whole picture after all.

I think its the TWR I need to be looking at as well as DV. Its just that I was hoping for some simple thing (like I thought DV was) that will let me see that one rocket is going to get further than another. Seems its not simple though.

It's a little simple. TWR at launch from Kerbin in a stock game should be around 1.6 or so. Less, and you'll probably never reach terminal velocity so will lose some dV to gravity. More, and you'll hit terminal velocity and either lose some to excessive air drag, or have to throttle down which means you're carrying a heavy engine that you're not using to its fullest and pay the fuel price for that.

Once you're in space, TWR doesn't mean as much. All it means is that your burns will take a long time. I frequently have >10 minute burns to reach Jool and Moho.

Spaceflight is usually a tradeoff between TWR and dV (and soon, cost). For the same weight, you can have more TWR or more dV. Or you can use the 48-7s and have both

[GoSlash27]

The thing that jumps out at me is that the small 3 stager is generating more DV on paper than the larger asparagus setup. Unless they are lifting radically different payloads that should not be. A 3 stager can get close to the mass efficiency of an asparagus but it can't quite match it, let alone exceed it by a wide margin.

I'm thinkin' MJ is misleading you about how much DV you're actually generating.

Best,

-Slashy

[Foxster]

The thing that jumps out at me is that the small 3 stager is generating more DV on paper than the larger asparagus setup. Unless they are lifting radically different payloads that should not be. A 3 stager can get close to the mass efficiency of an asparagus but it can't quite match it, let alone exceed it by a wide margin.

I'm thinkin' MJ is misleading you about how much DV you're actually generating.

Best,

-Slashy

Yes. I think you are right.

I have fairly complicated plumbing on the bigger ship and I don't think MJ is taking this into account accurately.