Calculating delta V

[Kar98]

I'm currently trying to build a rocket that will take me to Eve and back and according to the Wiki I need 11,500 delta V to take off from Eve. I currently have mechjeb installed (not the latest one) but I find the delta V calculations don't match between the Ascent stats and Vessel information. Is there a tool or some way to calculate delta V easily on my current rocket?

[iDan122]

Apparently you are someone who doesn't read things carefully because it says "Delta-V for takeoff" and MechJeb says"total delta-V of the vehicle.

You need 11500 delta-V to takeoff, but you craft's delta-v is bigger than that obviously

MechJeb doesn't say the delta-v for takeoff in particular, he's not that smart yet.

Edited April 23, 2013 by iDan122

[tomf]

The delta-v to take of from sea level on eve is indeed about 11,500. Mechjeb tries to show the delta-v of you rocket while you are building it in the VAB, but for rockets over a certain complexity it isn't perfect.

Do you have a ship that has the required dv? My eve sea level ascent vehicle has 1.7 tonnes of capsule etc sitting on top of 456 tonnes of fuel and engines. I have spent ages fine tuning it in excel and I want to know if anyone can do significantly better.

[FluffyBob]

Engineer Redux offers a little more detailed delta-V display, including atmospheric vs space. Comes up with slightly different values.

iDan22 - you are probably just a kid and so are still learning whats appropriate, calling someone an idiot in this situation is not appropriate.

[iDan122]

Engineer Redux offers a little more detailed delta-V display, including atmospheric vs space. Comes up with slightly different values.

iDan22 - you are probably just a kid and so are still learning whats appropriate, calling someone an idiot in this situation is not appropriate.

Who are you calling a kid kind sir? The problem isn't MechJeb, the problem is the impatient newbie who cannot tell the difference between takeoff delta-v and total delta-v

Edited April 24, 2013 by iDan122

[Vanamonde]

the problem is the inpatient newbie who cannot tell the difference between takeoff delta-v and total delta-v

That's the thing with newbies: they don't know stuff. Like how to spell iMpatient.

[EndlessWaves]

I find the delta V calculations don't match between the Ascent stats and Vessel information. Is there a tool or some way to calculate delta V easily on my current rocket?

How much of a disparity are you seeing? They should generally move in sync so you may have hit a bug.

11.5km/s is a little high, 10.5km/s is probably closer to a minimum requirement.

[flightmaster]

Who are you calling a kid kind sir? The problem isn't MechJeb, the problem is the inpatient newbie who cannot tell the difference between takeoff delta-v and total delta-v

It's people like you that make others hesitant about asking questions and being curious, we should be trying as hard as possible to help people with questions, not calling them impatient newbie's.

[drakesdoom]

More importantly than the above bickering land on a mountain and save yourself several thousand delta v.

[iDan122]

It's people like you that make others hesitant about asking questions and being curious, we should be trying as hard as possible to help people with questions, not calling them impatient newbie's.

They need to at least know how to ask their question! I mean he already had the answer and he asked us again? I don't see a reason in that. When people do that I just want to do a mega facepalm

[CalculusWarrior]

I prefer the Kerbal Engineer Redux, it provides a lot more information than Mechjeb.

[Specialist290]

They need to at least know how to ask their question! I mean he already had the answer and he asked us again? I don't see a reason in that. When people do that I just want to do a mega facepalm

His question also seemed to imply that he was getting different results between MechJeb's initial calculations and the actual amount of delta-v the rocket actually took to get into orbit. Perhaps it's your own reading comprehension that's at fault here, and not his phrasing.

But to answer that concern in particular: MechJeb's autopilot isn't perfect, and the minimum targets on delta-v charts generally assume a perfect performance. You're probably seeing a discrepancy in the stats as a result of things like gravity, drag, and steering losses. Depending on how your ascent profile is set up, those losses can get pretty heavy as you pass through the atmosphere.

[chainedlupine]

But to answer that concern in particular: MechJeb's autopilot isn't perfect, and the minimum targets on delta-v charts generally assume a perfect performance. You're probably seeing a discrepancy in the stats as a result of things like gravity, drag, and steering losses. Depending on how your ascent profile is set up, those losses can get pretty heavy as you pass through the atmosphere.

Heck, I find a lot of the time that I over-estimate. I can make a stack that has exactly 4500 m/s dV (atmo) to get into orbit, but of course that value changes as I get higher up or depending on what sort of cluster stack I use. So I end up with booster stage in orbit that still has, say, 900-ish dV still kicking around.

It's like Christmas. Woo, free dV to apply towards my ejection burn! But still, means I am not very good at estimating my dV requirements. But that's just it. I'm estimating, based on what Kerbal Engineer Redux tells me. What I get for not mathing it.

[dragoncannon]

You can also calculate DeltaV manually... dV= exhasut velocity x ln (mass full/mass empty). The exhaust velocity is just the Isp multiplied by acceleration due to gravity... However, be careful for an Eve mission... I haven't tested this, but I believe that the Isp (atmosphere) given in the game is at Sea level on Kerbin. If that is the case, you can expect significantly worse performance on Eve. As a matter of fact, that may be the cause of your discrepency.

[RoboRay]

A little off-topic, but if you're looking for a good landing site on Eve with minimal delta-v requirements for relaunching back into orbit, I don't think you can do any better than this mountain. The relatively flat top is 5km above sea-level and just 5° south of the equator. This results in less than half the aerodynamic drag from launches near sea-level while preserving nearly all of the benefit from easterly launches near the equator.

More pictures.

I lost the LAT/LON coordinates to the forum meltdown, but if you enter a low equatorial orbit you really can't miss it.

Edited April 25, 2013 by RoboRay

[Stealth2668]

Who are you calling a kid kind sir? The problem isn't MechJeb, the problem is the impatient newbie who cannot tell the difference between takeoff delta-v and total delta-v

How about you stop being a douche? He came here to ask a question. Noone forced you to come and read it.

[Kar98]

How much of a disparity are you seeing? They should generally move in sync so you may have hit a bug.

11.5km/s is a little high, 10.5km/s is probably closer to a minimum requirement.

I did a test with a small rocket and this is what I got:

Start stage

End Stage:

At the start it looks like I only have 1800 delta V available but the actual is at 2200 delta V. Now I'm aware that the 1800 m/s reading is for sea level, but I didn't expect such a large change (~20%) in delta V when only 10000m up and how this will effect rockets on Eve

So to measure Delta V I will use the ascent stats to find out the actual delta V, but there is only so much I can burn before I go outside Kerban's gravity and the ascent module turns off (around 8000m/s Delta V)

Edited April 26, 2013 by Kar98

[tavert]

Eve's atmosphere is 5 times thicker than Kerbin's, comparing sea level to sea level. The scale height, or change in altitude such that pressure changes by a factor of 2.71828, is 7000 meters on Eve compared to 5000 on Kerbin. Here's a handy chart: http://wiki.kerbalspaceprogram.com/wiki/File:Atmosphere_kerbin_eve.png

Luckily engines never get /worse/ Isp than they do at sea level on Kerbin, so you will get that same Kerbin-sea-level-equivalent Isp up to about 11 km on Eve. For this reason Eve is the best occasion in the game to use aerospikes with their high atmospheric Isp (and lower drag coefficient), at least for your first few stages. For non-aerospike engines, right-click on them during your test ascents from Kerbin and watch how quickly the Isp rises towards the vacuum number. What I usually do with MechJeb's vessel info numbers is add the atmospheric value only for the first stage, and the vacuum number for the rest, and it's often fairly close to reality.

Instead of just escaping Kerbin with all that delta-V you're building, a better test is to take off from Kerbin once into a low orbit, then deorbit, land on Kerbin, and takeoff again. If you can make orbit a second time you should be in the right ballpark for Eve.

RoboRay, look for the Google cache of the Smallest Eve Ascent Vehicle challenge. There are some mountains of around 6400 meters altitude, where most people were landing for that challenge. I don't recall how close they were to the equator though...

[CalculusWarrior]

I believe your problem is that the engines' efficiency changes depending on altitude. Note in the parts description how there are two Isp values: one at sea level, and the other in vacuum.

Isp is related to how efficient your rocket engines are (essentially the exhaust velocity, which increases with height as there is less air to get in the way), by the Tsiolkovsky rocket equation.

I assume Mechjeb is calculating your delta-v at each instant. This will change the most with NERVA nuclear engines: around 200s at sea level, but then skyrockets to 800s in space!

I hope this solves your question, and it was just a simple misunderstanding, pay no heed to iDan.

Happy flying!

[RoboRay]

RoboRay, look for the Google cache of the Smallest Eve Ascent Vehicle challenge. There are some mountains of around 6400 meters altitude, where most people were landing for that challenge. I don't recall how close they were to the equator though...

Depending on the age of that thread, the surface features of Eve may be different now.

Anyway, barometric pressure on top of that mountain was 2.4 bars. Another kilometer or so would help get it down close to 2.0, probably, but if you're far off the equator it may not be much of an over all gain.

[tavert]

Good point, if you look at the cache be sure to check the dates. I think that thread started after 0.18, but I could be wrong. At least that's when it started getting interesting and people were getting below 60 tons for the mountaintop ascents.

Edit: Eve's sidereal period is almost 4 times longer than Kerbin's, the surface velocity at the equator is under 55 m/s. A few hundred meters higher starting altitude and that much less drag will easily get that back.

Edited April 26, 2013 by tavert

[Temstar]

There are 4 mountain ranges on or near Eve's equator all with peaks greater than 6km. They come in two closely coupled pairs with one pair on the "ocean" side of Eve and another pair on the "mountain" side of Eve. Those are prime landing spots. A 6.4km launch reduces delta-V requirement from 12,000m/s required from sea level to about 7500m/s. A saving of 5000m/s makes an enormous difference in the size of the rocket required to return. I think the smallest stock SERV could return two Kerbals (hanging onto ladders on the side) back to LEO in under 20 tons.

Before 0.18, Eve had one mountain top that was almost 11km high. Then 0.18 gave Eve's surface a complete do over so now the highest point is only 6.5km.

Edited April 26, 2013 by Temstar

[Goosexi]

I must finish Delta V without Tears!