Are burn out velocity and speed shown in navball at the end of burn the same?

[kerbo_noob]

I am trying to learn the basics of rocket science using a single stage rocket in KSP. Using the rocket equation I calculated the burnout velocity of my single stage rocket to be around 2327 m/s.

Vbo=Isp*9.8*ln(Total Mass/Dry Mass) - g*burn time

The rocket has a Mk1 Command Pod (mass=0.84), a Mk16 Parachute(mass=0.1) and an RT-10 solid Fuel Booster(mass=3.7475 and fuel mass=3.25, Isp=225).

If I launch the rocket vertically upwards the fuel burns for about 29s (theoretical value is 28.7) but the maximum speed shown on the navball at the end of the burn is around 720 m/s. I am not sure if I missed something while calculating the burnout velocity. Is the speed shown in the navball at the end of burn called burn out velocity? If so why such a huge difference between the speed shown in game and the theoretical value? Could someone please let me know if I missed something?

[Rakaydos]

Atmospheric drag, I think. And Gravity losses.

[Camacha]

Gravity losses are easily calculated - although I think those are already included in the above equation - and drag in the stock model is doable too. The stock KSP atmosphere is very thick and soupy, so some significant losses are to be expected.

Your best bet would be to launch from a place without atmosphere. That way you can test the influence of the atmosphere.

Edited March 3, 2014 by Camacha

[brooklyn666]

Just out of curiosity, why are you using that equation? It's really only helpful for launching rockets straight up, which is not really useful for doing anything.

[maccollo]

You have to subtract gravity, drag and steering losses from the final velocity. Drag only applies to atmospheres obviously, but gravity and steering losses are always present.

Gravity losses are easily calculated - although I think those are already included in the above equation.

Gravity loss is not included in the rocket equation. The gravity factor in the equation is just a conversion factor to avoid confusion where people use different unit of measurement for the exhaust velocity, at least that's what I heard.

[Camacha]

Gravity loss is not included in the rocket equation. The gravity factor in the equation is just a conversion factor to avoid confusion where people use different unit of measurement for the exhaust velocity, at least that's what I heard.

The g * burn time part that you subtract from the first part looks an awful lot like accounting for gravity. You get a certain acceleration and then subtract the gravity acceleration per second times the number of seconds you burn. Acceleration up minus acceleration down equals end result, right?

Edited March 3, 2014 by Camacha

[maccollo]

The g * burn time part that you subtract from the first part looks an awful lot like accounting for gravity. You get a certain acceleration and then subtract the gravity acceleration per second times the number of seconds you burn. Acceleration up minus acceleration down equals end result, right?

No, it's there to force a consistent solution for the deltaV in meters per second, whether you use feet per second, or meters per second for the exhaust velocity.

I think that part was added because of the Mars Climate Orbiter that failed because one control system used imperial units and another one used SI units.

*Edit

Ignore that, I have really ****ty reading comprehension today XD

You are right, that is pretty much gravity losses.

Edited March 3, 2014 by maccollo

[Camacha]

I did a little test for you without atmosphere. The numbers go by pretty quickly, but what I could see was about 2270 m/s at the end of the burn. Pretty close to what it should be. The atmosphere obviously has a big influence and that is what is messing up your numbers.

No, it's there to force a consistent solution for the deltaV in meters per second, whether you use feet per second, or meters per second for the exhaust velocity.

Did you notice that g seems to appear twice in that equation? I am quite certain that everything after the minus is accounting for gravity, as that is the deduction you would expect. My little experiment confirms it and it would of course be strange for the numbers to line up if gravity was not taken into account in the formula, but the test was done with gravity present.

Edited March 3, 2014 by Camacha

[maccollo]

I actually missed that, which is the reason for my somewhat confusing statement.

[DonLorenzo]

That way of accounting for gravity losses will be a good approximation for short, vertical burns but be aware that the actual acceleration due to gravity will change with altitude. Much more rapidly on Kerbin than on earth.

[kerbo_noob]

Thank you all for the replies!! My main aim was to understand the equation of burnout velocity by matching the theoretical result with what I observe in KSP (I know it is crazy ). Thanks for pointing out that the atmosphere is reason I got a different result in KSP. I can sleep peacefully now because I thought I messed up the calculations (or the equation) and it was bothering me all along.

I want to try the test without atmosphere. Is there a way to turn atmosphere off in KSP? Once again, thank you all for the replies!

[kerbo_noob]

Hi,

I just wanted to match the theoretical result from the equation with actual result in KSP . I just want to get my basics straight as I am a newbie. Once I get the basics, I will try to design crafts myself to orbit Kerbin and then on to Mun and beyond! I really love this game.

[K^2]

I don't know if you can turn off atmosphere, but you can turn off drag for individual parts of the ship. If you go and find the config file for the parts, it has drag coefficients for them. Set these to 0, and atmosphere won't affect your ship. Of course, this only makes sense for a small ship with small number of parts.

Also, be careful with parachutes. You should be able to set both deployed and stowed drag. If you set both to zero, parachutes won't help you land. If you keep deployed drag as it is, you should still be able to use parachutes without worrying about drag while testing things out.

[Camacha]

It is not crazy at all Matching up observations with theory is a great way to learn and understand what is going on.

I want to try the test without atmosphere. Is there a way to turn atmosphere off in KSP? Once again, thank you all for the replies!

You can of course try it on another planet without an atmosphere, but I generally use HyperEdit for these kinds of tests. Although I never utilize it in actual missions, it is my way of emulating space agency testing facilities that are not available in the game (virtual simulators, vacuum chambers, sand boxes et cetera).

Edited March 4, 2014 by Camacha