Calculating available DV?

[strider3]

Here's a scenario (these are made up numbers for an example)...I'm in orbit around Duna and trying to get home to Kerbin. I created a maneuver node and it's calling for 3,200 M/S for the burn. I have 1,353 liquid fuel and an RE-L10 "Poodle" engine. Is there some calculation I can make which tells me if I have enough fuel, with this engine at full thrust, to complete the burn?

Thanks,

Vic the Newbal

[Empiro]

There's not enough information to compute that unless you give the mass of the rocket. Consider the rocket equation:

https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

The exhaust velocity is the specific impulse of the Poodle (350) * 9.8 m/s/s.

You'll need to use the initial mass and final mass. So far, you've given the amount of fuel (e.g. initial mass - final mass). 1 unit of liquid fuel or oxidizer is 5kg, or .005 tons. However, without the final (or initial) mass, we don't have enough variables defined.

As a final note, 3200 m/s is extremely high for a Duna -> Kerbin transfer. It should be much lower than that. In addition, the thrust of the engine makes very little difference unless it's so low that the burn takes 10+ minutes.

[GoSlash27]

1 hour ago, Victor3 said:

Here's a scenario (these are made up numbers for an example)...I'm in orbit around Duna and trying to get home to Kerbin. I created a maneuver node and it's calling for 3,200 M/S for the burn. I have 1,353 liquid fuel and an RE-L10 "Poodle" engine. Is there some calculation I can make which tells me if I have enough fuel, with this engine at full thrust, to complete the burn?

Thanks,

Vic the Newbal

As Empirio said, you need a little more info than that. If you have the units of fuel, oxidizer, ship's mass, and Isp of the engine, calculating the DV available is pretty simple. Thankfully, all of that info is available in a vanilla game.

First step is to make sure that you won't run out of oxidizer before fuel or vice- versa. Fuel comprises 45% of your propellant mass while oxidizer is 55%. If you don't have them in proper proportion, you must calculate your total propellant mass based on whichever you have less of.

Assuming that's in order, add your fuel units to your propellant units. Divide that total by 200. That's your propellant mass in tonnes. Save that value in memory in your calculator.
 Example: You have 45 units of fuel and 55 units of oxidizer. 45+55= 100. 100/200= .5 tonnes of propellant.

Now go to your map view and click the info icon. That will show you your current ship mass. Subtract the propellant mass from ship mass and save the result in memory. That's your dry mass.

Now divide your current ship mass by the value in memory; wet mass over dry mass. That gives you your wet-dry ratio.

Take the natural log of your wet-dry ratio, multiply it by 9.81, and save it.

Now go to your ship view and right click on your engine. It will show the Isp of your engine. Take that Isp and multiply it by the value in memory.

That gives you your total DV remaining.

It takes longer to explain it than it does to actually calculate it. If you're like me and dead- set against using add- ons, I recommend creating a spreadsheet to do this. It's a calculation you use pretty often during a mission.

Best,
-Slashy

 

 

Edited October 1, 2016 by GoSlash27

[strider3]

(Posted this while you were replying, GoSlash.)

You should have seen the original...6,348 M/S! LOL. I was not in the perfect phase angle by any means. As I said, all hypothetical. I'm just trying to figure out, in any situation, if I have enough fuel to make a burn or not. I realize there is much rocket science for a "true" answer. I'd like some way of deciding "yes, I can make this burn, as is" or "Nope, your screwed on fuel".

You had me at "natural log". Hopefully Excel has that formula, somewhere ;-). I knew there wouldn't be an easy answer because...well...it IS rocket science!

Thanks gang.

Edited October 1, 2016 by strider3

[Snark]

14 hours ago, Victor3 said:

You had me at "natural log". Hopefully Excel has that formula, somewhere

It does.  Also, any decent calculator has it-- you want the key labeled "ln".

Previous posters are right that you shouldn't need anywhere near 3200 ms.

Basically:  You figure out how much dV you have, based on the Isp of your engine, and on your mass ratio (your total mass including fuel, divided by your "dry mass" after fuel is expended).  Then you just wait for a launch window that gets you home within that dV amount.

There are various launch window planners out there that can help you.  My personal favorite is http://ksp.olex.biz, just because it's so simple and uses effective graphics to make things clear.

As for your ship:  You say you have 1353 units of liquid fuel.  I'll assume that you have the proportionate amount of oxidizer to go with it.  That's a total of just over 15 tons of propellant.  You didn't say how high your orbit over Duna is, but let's say it's 100 km.  If you check out the orbit calculator at http://ksp.olex.biz, you'd need about 605 m/s of dV to go home to Kerbin from there.  That's the absolute best optimal case-- let's double that to give you some wiggle room, the ability to do a mid-course correction, etc.  So, let's say 1200 m/s to be really safe.  We divide that by (Kerbin gravity times your Isp), and this gives us 0.35 as the natural log of your mass ratio, so take e^x of that, and the needed mass ratio is 1.419.  With 15 tons of propellant, this means your total ship mass right now (including your 15 tons of propellant) can be up to 50.8 tons.

If we ran through that same exercise, without the factor-of-two safety margin, the upper limit on total-ship-mass-including-fuel would have been 92.7 tons.

So, basically, it boils down to this (assuming that you're in a roughly 100 km orbit over Duna):  What's your current total ship mass?

• If it's over 92.7 tons, then no, you can't get home, even in the most ideal case.
• If it's under 50.8 tons, then yes, you have scads of dV, more than double what you need as long as you pick a good launch window.
• If it's somewhere between those two, then you have enough, but with a narrower safety margin and you need to be careful about setting up your return path.

 

 

 

[strider3]

Houston...we have a problem. I am not in a "standard" orbit around Duna...I'm orbiting East to West and I don't know if any calculator can help me here. My orbit is average 365 km.

I'm going to work on an Excel spreadsheet tomorrow and see if I cant get the calculations needed into it. For now...I'm going to warp until I get a decent chance of getting my Kerbals home (sniff).

[Snark]

50 minutes ago, Victor3 said:

Houston...we have a problem. I am not in a "standard" orbit around Duna...I'm orbiting East to West and I don't know if any calculator can help me here. My orbit is average 365 km.

That's fine.  For purposes of interplanetary ejection, it doesn't matter which direction you're orbiting-- the dV will be the same.

The ejection angle will be somewhat different from what a tool like http://ksp.olex.biz will tell you (since they assume prograde rather than retrograde orbits), but that just means you need to spend a little time fiddling with your maneuver node.  It'll be somewhat close to 180 degrees away from what a tool would tell you.  (Not exactly 180, because the planet's gravity will be bending your trajectory in the opposite direction that the tool assumes).

But the size of the burn will be identical regardless of which way you're going.

When you say your orbit is "average" 365... is it fairly close to circular?  Or is it really elliptical?  That'll make a slight difference to the dV requirement.  Not all that much, though.  A 100 km orbit needs 605 m/s; a 365 km circular orbit needs 583 m/s.  So, not a lot of difference.

Also, what's your total ship mass?  How much mass does that 15 tons of propellant need to shove?

[strider3]

Could one of you take a look at this initial "Remaining DV" Excel spreadsheet and tell me if I'm on track?

http://users.frii.com/viciii3/Total DV calc.xlsx

Or, if you have security concerns DL'ing this, here's a picture:

Just need to make sure my Excel math skills are up to the task ;-)

As always...thank you all.

 

[strider3]

46 minutes ago, Snark said:

That's fine.  For purposes of interplanetary ejection, it doesn't matter which direction you're orbiting-- the dV will be the same.

The ejection angle will be somewhat different from what a tool like http://ksp.olex.biz will tell you (since they assume prograde rather than retrograde orbits), but that just means you need to spend a little time fiddling with your maneuver node.  It'll be somewhat close to 180 degrees away from what a tool would tell you.  (Not exactly 180, because the planet's gravity will be bending your trajectory in the opposite direction that the tool assumes).

But the size of the burn will be identical regardless of which way you're going.

When you say your orbit is "average" 365... is it fairly close to circular?  Or is it really elliptical?  That'll make a slight difference to the dV requirement.  Not all that much, though.  A 100 km orbit needs 605 m/s; a 365 km circular orbit needs 583 m/s.  So, not a lot of difference.

Also, what's your total ship mass?  How much mass does that 15 tons of propellant need to shove?

My Apo/Peri is 388 and 343. Ship mass is 28.5.

Edited October 2, 2016 by strider3

[Snark]

14 minutes ago, Victor3 said:

My Apo/Peri is 388 and 343. Ship mass is 28.5.

So you've got 15 tons of propellant on a 28.5 ton ship with a Poodle?  That's the total mass, including the 15 tons of propellant?

In that case, you're good to go.  You've got over 2500 m/s of dV available on your ship, for a trip that only needs 600 m/s if you catch a good window.  That's a safety margin of better than 4 to 1, you're golden. 

With that big of a margin, you don't even need to hit a perfect window.  It just needs to not be egregiously bad.

[Reactordrone]

On 01/10/2016 at 0:44 PM, Victor3 said:

You had me at "natural log". Hopefully Excel has that formula, somewhere ;-). I knew there wouldn't be an easy answer because...well...it IS rocket science!

Thanks gang.

I usually punch mine up on a handheld calculator but there is always the built in windows calculator. The more recent versions don't give you a huge amount of visible function keys but the hotkey for natural logs is "n".

[Plusck]

6 hours ago, Victor3 said:

Could one of you take a look at this initial "Remaining DV" Excel spreadsheet and tell me if I'm on track?

http://users.frii.com/viciii3/Total DV calc.xlsx

Or, if you have security concerns DL'ing this, here's a picture:

Just need to make sure my Excel math skills are up to the task ;-)

As always...thank you all.

 

Not quite - you do not have a wet/dry ratio of 13.465. That number is your dry mass. So your wet/dry ratio is 28.5/13.465 = 2.116.

Here is my spreadsheet (first sheet useful, second sheet data, third sheet old messy calculations) - KSP delta V.xlsx Yellow cells are modifiable. The columns to the right try to work out the rest of the necessary data based on what you put in the yellow cells. It isn't as clearly labelled as your spreadsheet, but should be mostly understandable.

I get a delta v of 2574 m/s. If you correct your cell C10 to read "=D7/(D7-B7)" then it gives the same result.

Edited October 2, 2016 by Plusck

[strider3]

Plusck, corrected my spreadsheet and my numbers now match yours exactly.

Thanks, as always, to all of you! I could list names...but it would go on and on...and you know who you are.

[Chiron0224]

You could use KER btw.  It does the dV calculation for you.  Also as someone else pointed out your return dV is really high.  Did you wait for a return transfer window?  Again, there's a mod for that

 

[strider3]

2 hours ago, Chiron0224 said:

You could use KER btw.  It does the dV calculation for you.  Also as someone else pointed out your return dV is really high.  Did you wait for a return transfer window?  Again, there's a mod for that

 

Hey Chiron0224. I have KER but I don't know how it will tell me how much DeltaV is remaining on my ship? I use the web based Launch Window Planner by AlexMun.

Edited October 4, 2016 by strider3

[Chiron0224]

15 minutes ago, Victor3 said:

Hey Chiron0224. I have KER but I don't know how it will tell me how much DeltaV is remaining on my ship? I use the web based Launch Window Planner by AlexMun.

So if you have Ker there's a hud type thing when you are flying your mission.  I'm not sure who's screenshot this is (I googled it since I'm at work right now and can't generate my own) but do you see how his in mission HUD on the right hand side has a button labeled "vessel"?  And with that selected you should see an entry for "delta V current".  Although, I want to say in the current version it will instead say the stage like "S7     3025/6000 dv" which shows the total and current for that stage.  If I remember I'll try to get a screen shot of mine when I get home.  I use it all the time.

[Chiron0224]

Ah, here we go.  Here's one from my recent Duna and return mission (feel free to check out the whole thing under my mission report post titled "Project Odin" if you like) http://imgur.com/saKHgjG

[Plusck]

3 hours ago, Chiron0224 said:

Ah, here we go.  Here's one from my recent Duna and return mission (feel free to check out the whole thing under my mission report post titled "Project Odin" if you like) http://imgur.com/saKHgjG

You need to enable the "partless" option to get that to work without adding the KER engineer part to the ship. That may be why @Victor3 isn't seeing it.

Although adding the part does add some immersion to it, I've always been worried about the possibility of it breaking saves between different versions of KSP, or complicating the business of sharing craft or sfs files.

Edited October 5, 2016 by Plusck

[Chiron0224]

11 hours ago, Plusck said:

You need to enable the "partless" option to get that to work without adding the KER engineer part to the ship. That may be why @Victor3 isn't seeing it.

Although adding the part does add some immersion to it, I've always been worried about the possibility of it breaking saves between different versions of KSP, or complicating the business of sharing craft or sfs files.

Thanks for pointing that out.  I completely forgot about partless

[strider3]

Got it, just had to "install" the DV stuff in the KER UI. Good to go. Still like my little spreadsheet though...kinda proud of that . But I'm easily amused.

Thanks gang!

[Plusck]

4 hours ago, Victor3 said:

Got it, just had to "install" the DV stuff in the KER UI. Good to go. Still like my little spreadsheet though...kinda proud of that . But I'm easily amused.

Thanks gang!

You'll find your spreadsheet useful still - there are cases where KER fails utterly to get the calculations right (such as for Rapiers, if you connect ships using docking ports, sometimes with droptanks). So don't lose it