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On calculating atmospheric delta-v


Rosco P. Coltrane
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So, with all the discussions about needed dV to orbit now with the new aero... Does anyone knows or can offer a pointer as to how mathematically calculate this? I know this will depend on the shape of the rocket, whereas in pre-1.0 pretty much all parts had a drag coefficient of 0.2. But in any case, is there a method that would give as a ball-park figure?

I tried launching multiple times by hand but I got very different values, even with the same rocket... because my skills are lacking I guess. :P

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At some point, I think you jsut have to plan for the worst, or use mechjeb.

It's not just shape of the rocket, it's your speed, relative to terminal velocity (which changes with altitude), and angle of attack: which are impossible to flawlessly reproduce.

in reality, 90% of your design and skill are going to save you up to 500 m/s. The last 10% of perfection is only going to save you ~15-20 m/s

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There's unlikely to be a nice closed form formula even for reasonably simple ascent profiles, you'd most likely need to use numerical methods. Even in the old model with fixed 0.2 drag, optimal delta v speeds could only be calculated in closed form for vertical ascent, and I'm not sure if the amount needed could be computed algebraically.

Spitballing some ideas, you could start by using energy to calculate a minimum, and then try to add in Gravity, drag, and steering losses, but, as I said, you'd likely be using numerical methods to find the latter three.

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If you are looking to calculate delta-V, you can install the mod Kerbal Engineer Redux (KER) and I know its 1.0.2. Each engine you add to the rocket will add a value to a table and give you detailed information on not only your rocket but your orbit as well such as inclination, ETA's and more. I use, I love it.

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If you're getting vastly different amounts of fuel (thus dV) in orbit with the same vessel on multiple launches, all I can say is keep launching until you've got a consistent profile. No amount of math will get you past that.

And once you have a consistent (and moderately efficient) profile, screw the math. Just note your dV on launch and in orbit (and your TWR at launch, and also how your TWR changes during launch), and you can expect roughly the same for vessels with similar dV and TWR.

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Problem is, as the author of a dV map, I'd like to have something to back me up. So when somebody comes saying "your numbers sux, dude" I have something to answer that would make look smart. :D But no seriously, that's pretty much, I'd like to have something to reference when asked.

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I'd think the problem only arises on Kerbin, Eve, and Laythe ... and on Eve, you have enough of a problem anyway! It seems like there's a fairly good consensus on Kerbin ascent by this time. About all you can do, in the end, is say "this is what people who know what they're doing are getting".

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Even on airless bodies, ascent dV is highly sensitive to TWR (or rather, gravity losses are sensitive to ascent time). So your numbers were already ballpark. Indeed, due to drag, things even out a bit at the high-TWR range for aired bodies. :)

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On every ascent, put a little * next to the number.

At the bottom of the chart, put *YdVMV. Numbers are based on community consensus and optimal ascent trajectories. It's not my fault if you lack the skillz, bro.

...or something slightly less cheeky.

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Even on airless bodies, ascent dV is highly sensitive to TWR (or rather, gravity losses are sensitive to ascent time).

Oh yes, but that ballpark figure was backed up by a solid set of equations that worked in giving you said ballpark figure. So I had the excuse! Listen buddy, don't blame me, blame the math if you like but I have nothing to do with it. My method for getting those numbers is objective and you can calculate them too if you like... and a long list of etceteras where I don't take any responsibility for the newbie's misunderstanding of what a dV map is about. :D

I think I'm gonna wait until MechJeb is updated and they punch the numbers for different ascent into the autopilot and see how they work. Last time I checked in 1.0 MJ was having one hell of a time piloting your rockets on the way up... so I don't know. Might as well try with kOS or KSPRC scripts.

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If you're making a dV map, keep the estimates conservative.

Fewer people complain about having more gas in the tank then planned.

Community consensus is the best approach anyway. That way when people ask for help to hit those numbers...

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Yeah, I'm pretty sure this is one of those problems that can't be solved analytically. Rather you could play with numerical models to assist, but there's still going to be variations based on differing grav losses from TWR, and drag losses, especially in the turns, or when they're done...or what shape the rocket is in. (Tall vs wide)

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I do not think the old dV maps were calculated. I think they were done experimentally. That's why they varied a bit.

Yep. For ascent from atmospheric bodies, I just tried some ascents experimentally to come up with delta-v values. It depends a lot on the size and shape of your rocket (and on the heat tolerance of your parts), even assuming that you fly the perfect gravity turn.

Orbital maneuvers in vacuum can be calculated, atmospheric maneuvers not so much.

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I would think that the DV would depend more on engine selection than on drag. Drag will affect all launches fairly evenly, with exceptions only being for funky shaped designs. But your DV needed will vary by a large amount depending on what engines you use at the various altitudes of your launch because of the changes to engine ISP and thrust.

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I would think that the DV would depend more on engine selection than on drag. Drag will affect all launches fairly evenly, with exceptions only being for funky shaped designs. But your DV needed will vary by a large amount depending on what engines you use at the various altitudes of your launch because of the changes to engine ISP and thrust.

Drag losses can be very different between large, thin rockets and small, fat rockets. Drag force is proportional to the surface area that goes into the windstream. The acceleration due to drag (i.e. delta-v losses) is drag force divided by mass. For example, a 1.25m ship will have twice as much drag losses as a 2.5m ship that has the same shape but scaled up.

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