How to calculate Delta V?

[JEB'S DESTINY]

Could anybody help me calculate delta V?

 

Thanks!

[GoSlash27]

Jeb's destiny, We can help you with that, but we need more info. Can you explain what you're trying to do in more detail?

Best,
-Slashy

[Rocket In My Pocket]

Most people use KER or MechJeb for a Dv readout.

If you actually want to calculate it manually, that's an entirely different problem; which someone else will have to help you with. (Life's too short to waste time learning more math than I was already forced to waste time learning in school. Still waiting for that bill to come in the mail with a letter variable in it.  Remember when the teacher told you that you wouldn't always have a calculator? Now, no one leaves home without an entire computer in their pocket. Lol!)

Best of luck!

Edited January 18, 2018 by Rocket In My Pocket

[sebi.zzr]

@JEB'S DESTINY

calculator

 

[JEB'S DESTINY]

KER is for 1.3 though, unless it is updated.

[Geonovast]

1 minute ago, JEB'S DESTINY said:

KER is for 1.3 though, unless it is updated.

It's stated in the KER thread that it works fine with 1.3.1.  Many people, including myself, are running it in 1.3.1 without any problems.

[JEB'S DESTINY]

ok thanks!

[Human Person]

The technical answer is: determine the wet mass and the dry mass of the stage and plug them into the rocket equation.

 

delta v = exhaust velocity * ln(wet mass / dry mass)

 

for some reason I don‘t understand, everybody gives us the specific impulse instead of the exhaust velocity. To get the exhaust velocity, you have to multiply the specific impulse by 9.81 m/s^2

serioussly, what‘s the point of the specific impulse? It looks like such a nonsense unit to me, why not give us the exhaust velocity right away?

[steuben]

4 minutes ago, Physics Student said:

serioussly, what‘s the point of the specific impulse? It looks like such a nonsense unit to me, why not give us the exhaust velocity right away?

because it works if you are using mks, mgs, fps, mtd, or any other combination of distance/mass/time you can things of. The oddest I've seen has been smoots/apples/milli-torontotoottawa.

[Pand5461]

13 minutes ago, Physics Student said:

serioussly, what‘s the point of the specific impulse? It looks like such a nonsense unit to me, why not give us the exhaust velocity right away?

Presumably, a workaround invented in the US while working with the Von Braun's team. Expressed in seconds×g₀, exhaust velocity was the same in SI and imperial units.

EDIT: Also, a useful characteristics when you want to calculate the amount of fuel in retrorocket to counteract the afterimpulse after stage separation. So, if the afterimpulse is P kgf×s and the Isp of retrorocket is I seconds, then you need to load P/I kilograms of fuel. The same goes for P in lbf×s and fuel mass in lb.

Edited January 18, 2018 by Pand5461

[Human Person]

@steuben @Pand5461 still doesn‘tmake sense to me. The mass cancels out, so you can use whatever unit you want for that and your delta v result will simply have the same unit as the one you used for the exhaust velocity. It‘s really simple.

this bugs me out, I made a topic on its own for this.

Edited January 18, 2018 by Physics Student

[steuben]

Quote

The mass cancels out, so you can use what ever unit you want

That's the point... or at the least the dominating side effect of it's definition.

[Spricigo]

2 minutes ago, Physics Student said:

@steuben @Pand5461 still doesn‘tmake sense to me. The mass cancels out, so you can use what ever unit you want for that and your delta v result will simply have the same unit as the one you used for the exhaust velocity. It‘s really simple.

Is a mathematical solution for a communication issue. The problem is that often people assume a value is given in some unit without checking or simple give a number without a unit assuming the other part will know the unit.  The idea behing g_o.I_sp is to make less likely to people make that mistake since Isp will be the same regardless of which unit of length is used OTOH people still need to agree on a unit of time.

 

[Pand5461]

@Physics Student ah, unit conversion may not really be the cause.

It's more about how you measure it and what you need it for. Beside being a characteristics of Delta-V, it's also how much thrust the engine produces when supplied with a certain fuel flow. It's especially useful for throttleable engines such as jets.

If thrust T is measured in kgf and fuel flow f in kg/s, then Isp = T/f is in kgf / (kg/s) = kgf×s / kg = s (×g₀). The Peenemünde team probably used kgf instead of Newtons for force, so they set the tradition both American and Soviet engineers have been following since.

[GoSlash27]

1 hour ago, Physics Student said:

serioussly, what‘s the point of the specific impulse? It looks like such a nonsense unit to me, why not give us the exhaust velocity right away?

Physics Student, 

Using Isp makes it easier to calculate stuff like parallel stages and figuring burn times. Other than that... not much point to it. I don't know if it's easier to think of efficiency in terms of Isp or if I've just gotten accustomed to it.

Best,

-Slashy

[Corona688]

I use this delta-v calculator for simple one-stage calculations:

http://www.strout.net/info/science/delta-v/

It just needs to know the full mass, the empty mass, and the ISP of your engine.

[Marschig]

When calculating dV, KER assumes that all engines are turned on. This works in most cases, but sometimes, when a set of different engines is installed, dV significantly depends on the order of switching on the engines. For special cases, it is better to use an Excel table in which various engine switching options will be calculated.

Spoiler

For example (CPlane2 = 2 * CR-7 + LV-N):

 

[Aegolius13]

5 hours ago, Spricigo said:

The problem is that often people assume a value is given in some unit without checking or simple give a number without a unit assuming the other part will know the unit. 

cough*MarsClimateOrbiter*cough