Calculating DV on a Spacecraft in orbit

[gilflo]

How do you calculate DV remaining on a SSTO in orbit, once RAPIER and Jet engines have been shut down, and you are remaining on LV-N atomic engines?

KER and Mechjeb see 865 DV remaining on my SSTO and I perform a 827DV run to Mun on my Atomic engines that costed me only 145DV out of my 865 remaining.

In fact it remains around 5 times the calculated DV.

[A_name]

I've also had problems with MJ and KER Dv calculations on spaceplanes with both rockets and jets. It's as if they can't figure out which engines you will be using, even if the jets are powered off.

You can use the rocket equation to find it manually.

Edit: for the 'Ve' variable of the equation, use the engine's specific impulse.

Edited August 5, 2015 by A_name

[brettuzzi]

You need 3 things:

ISP - Specific Impulse - how efficient your engine turns propellant into thrust

M0 - Current or fueled mass (kg)

M1 - Dry mass

dV = 9.80665 * ISP * log(m0/m1)

LF/OX both weigh 4kg/L.

If your ship has 90 LF, 110 OX, weighs 2,000kgs, and is powered by a LV909 Terrier...

90 + 110 = 200 units

200 * 4 = 800 kg

2000 - 800 = 1200kg

M0 = 2000, M1 = 1200, ISP = 345

9.80665 * 345 * log(2000/1200) = 1728dV

[Dante80]

Have not tried this, but here is a question. Since KER has an option to show stage Dv separately, wouldn't putting your atomics to a separate stage show the correct value?

[gilflo]

Thank's for the formulas.

[Nich]

I thought they were 5 kg/L Oscar-B Fuel Tank 0.225 full .025 empty 40 L

200kg/40L = 5kg/L

[GoSlash27]

Exactly the way Brutezzi said.

You have the units of fuel in the resource screen and each 200 units is a tonne. Count only the fuel when dealing with the LV-N.

You have the ship's mass in the info screen in tonnes.

You have the LV-N's Isp as 800s (give it a quick tap of throttle while looking at the engine's stats if you don't have it handy).

Step1 find the fuel's mass. fuel units/200 is the mass in tonnes.

Step2 find the ship/s mass when you're out of fuel. current mass minus fuel mass.

Step3 find the wet/dry ratio. Current mass/mass when out of fuel

Step4 find the natural log of the wet/dry ratio. ln(Mw/Md)

Step5 find the DV remaining. Multiply the result by 9.81 and the Isp (which is 800)

Best,

-Slashy

[A_name]

Have not tried this, but here is a question. Since KER has an option to show stage Dv separately, wouldn't putting your atomics to a separate stage show the correct value?

You'd think so, but it doesn't seem to work. At least for me, whenever I use a a SSTO spaceplane for orbit my Dv stats are ruined as soon as I switch on the rockets, regardless of staging, which engine is actually on or off, or where my fuel is sitting.

[gilflo]

Maybe it's a question of fuel line. I don't use any, I use TAC fuel balancer.

[brettuzzi]

You're correct, it's 5kg/unit for LF/OX (it's 4 for monoprop). I was working from memory, my bad!

[Red Iron Crown]

Show us a pic with the "Vessel" window of KER and the Resource Panel open and we can double check your numbers.

[OhioBob]

One thing that can get tricky when calculating ÃŽâ€v remaining is when you have some unusual stagging. If you have a SSTO, then that's probably not an issue. However I bring the point up in case it is ever an issue in the future.

A perfect example of what I'm talking about is the Apollo missions to the moon. Let's say you are on your way to the moon and your vehicle includes the CSM and LM. You want to calculate the ÃŽâ€v remaining in your service module. Well, the CSM is 30 t with propellant and 12 t empty, the LM is 15 t, and the ISP is 314 s. So your calculation is easy, right?

ÃŽâ€v = 314*9.81*LN(45/27) = 1573 m/s

But hold on a minute. You perform orbit insertion with the LM docked to the CSM, but all your other manuevers are performed without the LM. Your dry mass is not constant, it changes after you ditch the LM. The above calculation is rather meaningless because it doesn't provide the data you really want to know. But the above it what KER will tell you.

The first thing you have to do is calculate the amount of propellant burned during lunar orbit insertion. Let's say orbit insertion in 900 m/s. Therefore,

900 = 314*9.81*LN(45/(45-m_p))

m_p = 11.4 t

Therefore the amount of ÃŽâ€v remaining in the CSM after the LM has been detached is

ÃŽâ€v = 314*9.81*LN(18.6/12) = 1350 m/s

So the total ÃŽâ€v that you're going to get out of the service propulsion system when you consider all the stagging is, 900+1350 = 2250 m/s.

You have to be careful not to always take what KER tells you at face value. There are some calculations that the various tools in the game just can't give you because they can't anticipate what you plan to do. There are times when you're going to have to carefully think through each step of the mission and perform the calculations by hand.

Edited August 6, 2015 by OhioBob
spelling