Need some formulas

[Moonk]

Would be appreciated if u share your knowledge.

How can i translate TWR to acceleration?

How can i calculate aerobrake periapsis altitude? Can u give a couple of examples?

How can i calculate ship drag coefficient?

Thank you

Edited August 27, 2015 by Moonk

[mhoram]

If you mean acceleration by thrust, then the formula is simply

Acceleration = Thrust / Mass

where g is the local gravity at the location of the ship.

g can be calculated as

g = PlanetsGravityParameter / distance^2 between ship and Planets Center of Mass

Since TWR = Thrust / (Mass * g) you get the acceleration as

Acceleration = TWR_at_the_location_of_the_ship * g = TWR_at_the_location_of_the_ship * PlanetsGravityParameter / distance^2 = TWR_at_the_location_of_the_ship * G * PlanetsMass / distance^2

where G = 6.674*10^-11 N(m^2/kg^2)

You should be aware of the following additional things:

Thrust is not the only force that is applied to the ship. There are also Drag and Gravity.

TWR can mean different things:

- TWR at the sealevel of Kerbin

- TWR at the location of the ship

- TWR at the sealevel of the body, the ship is currently orbiting

Before KSP V1.0 aerobraking altitude was easy and there were tools like http://alterbaron.github.io/ksp_aerocalc/ or http://forum.kerbalspaceprogram.com/threads/30433

With the new aerodynamic system the periapsis altitude for aerobraking is not easily calculated, because it depends on multiple factors, like angle of attack, form of the ship or lift by wings: so it is mostly a combination of guessing and experience.

Now there is this addon: http://forum.kerbalspaceprogram.com/threads/104694

It helps eyeballing the periapsis but can not be exact because of the stated reasons.

As an example Returning from Minmus with a Lander or capsule I usually go for a periapsis altitude of around 32-38km. If the ship is not as robust, 45-60km is a safer option with multiple passes.

For Drag, there is some information in this thread: http://forum.kerbalspaceprogram.com/threads/119108-Overhauls-for-1-0

Edited August 26, 2015 by mhoram

[OhioBob]

How can i calculate aerobrake periapsis altitude? Can u give a couple of examples?

There's no easy solution to that problem. I've estimated it be using a computer simulation. Unfortunately the technique is way too involved to try to explain here in the forum.

How can i calculate ship drag coefficient?

1) Download and install the AeroGUI mod.

2) Perform a test flight with the AeroGUI display open on your screen

3) During the flight, periodically note the dynamic pressure, drag force, and Mach number.

4) For each Mach number, compute the drag coefficient, C_d = F_d * 1000 / (q * A), where C_d is drag coefficient, F_d is drag force (kN), q is dynamic pressure (Pa), and A is the area of the vehicle normal to the flow (m²)

Note that drag coefficient is a function of Mach number, so that's why you have to perform multiple measurements over a wide range of Mach numbers. You need to generate enough data to plot a graph of drag coefficient versus Mach number.

Edited August 26, 2015 by OhioBob

[Snark]

As previous posters have said, TWR is simple. Generally what you care about is acceleration in m/s2, so just divide your mass in tons by the kilonewtons of thrust of your engine(s).

There's no simple formula for aerobraking altitude. On the other hand, since atmosphere fade-out is exponential with height, this means that the actual characteristics of your ship (such as how it's shaped, or its mass) don't make a huge difference to aerobraking altitude. What it boils down to is that each body that has an atmosphere has an approximate "good height to aerobrake", and if you just learn that (or get it off a wiki), you're pretty much good to go. For example, on Kerbin, 35 km is good for just about anything-- if you just use that as your baseline (and maybe raise it slightly for things that are very "draggy", such as ships equipped with lots of airbrakes), you don't need to do any calculations.

As for drag coefficient: That's also complicated (read: impossible for humans) to calculate... I'm curious, though, what's the reason you want to know? Suppose you could calculate it, what would you use it for?

[Moonk]

Thank you all for answers

As for drag coefficient: That's also complicated (read: impossible for humans) to calculate... I'm curious, though, what's the reason you want to know? Suppose you could calculate it, what would you use it for?

I am trying to launch 4 kerbin vessel from Eva surface. The one ship that can do it looks ugly and his construction design is very dump. The main problem launching from Eva is drag. I cant find way to reduce drag from 2,5m fuel tanks. They produce enourmus drag no metter where i put them.

- - - Updated - - -

too bad this desighn dont work becouse drag produced by lower parts. It looks fun anyway. I use pancake stages. Was hoping that long ship produce less drag. I failed )

But now i find that only 1st part after conus is protected from drag. And this part could be a big one.

Edited August 26, 2015 by Moonk

[Snark]

The cone should help with the central stack. What's up with all those radial air intakes off to the side? I don't see any jet engines anywhere to use them. Radially attached parts are gonna add drag. Also, those landing gear sticking out on the bottom are going to be draggy, too... what are they for?

[Moonk]

The cone should help with the central stack. What's up with all those radial air intakes off to the side? I don't see any jet engines anywhere to use them. Radially attached parts are gonna add drag. Also, those landing gear sticking out on the bottom are going to be draggy, too... what are they for?

Cone help with 1st part. not central, only the 1st part after cone is partialy protected from drag. Air intakes just to protect engines from drag, looks like they are little cheat currently becouse they less draggy then cones, and there are Aerospike undernith of them. Landing gear decouples when engines launch.

Any way i just did 4 kerbin launch to orbit with very simple 140t vessel.

[Snark]

Also, just a sanity check-- are you being careful about your ascent velocity profile?

Post-1.0, aerodynamic drag spikes up dramatically once you pass Mach 1, which is around 300 m/s at Kerbin sea level. If you simply max out your throttle and punch a hole in the sky, then your drag will go way up. On the other hand, if you're careful to limit the throttle so that you wait until you're above 10 km or thereabouts to go supersonic, then drag is much less of an issue.

Case in point: I've never found drag to be much of an issue, and didn't have to play games with air intakes on non-jet-powered craft or anything like that. I just try to design the rockets to be "reasonably" aerodynamic (i.e. don't try to hyper-optimize, simply avoid doing something stupid like having a big flat front end), stick to an ascent profile that avoids Mach effects until I'm above 10 km, and it works out fine.

Occasionally I'll want to launch something that's just really ugly and can't be made aerodynamic, (generally, either something big like a space station core, or else a large rover), and I know the aero is horrible... but even in those cases, it's reasonably manageable. I just make a point to keep the speed down a little longer and don't start the gravity turn until I'm up close to 20km and drag really doesn't matter.

[Moonk]

Also, just a sanity check-- are you being careful about your ascent velocity profile?

Post-1.0, aerodynamic drag spikes up dramatically once you pass Mach 1, which is around 300 m/s at Kerbin sea level. If you simply max out your throttle and punch a hole in the sky, then your drag will go way up. On the other hand, if you're careful to limit the throttle so that you wait until you're above 10 km or thereabouts to go supersonic, then drag is much less of an issue.

Case in point: I've never found drag to be much of an issue, and didn't have to play games with air intakes on non-jet-powered craft or anything like that. I just try to design the rockets to be "reasonably" aerodynamic (i.e. don't try to hyper-optimize, simply avoid doing something stupid like having a big flat front end), stick to an ascent profile that avoids Mach effects until I'm above 10 km, and it works out fine.

Occasionally I'll want to launch something that's just really ugly and can't be made aerodynamic, (generally, either something big like a space station core, or else a large rover), and I know the aero is horrible... but even in those cases, it's reasonably manageable. I just make a point to keep the speed down a little longer and don't start the gravity turn until I'm up close to 20km and drag really doesn't matter.

Yes, it works fine in Kerbin atmosphere

Edited August 26, 2015 by Moonk

[Red Iron Crown]

too bad this desighn dont work becouse drag produced by lower parts. It looks fun anyway. I use pancake stages. Was hoping that long ship produce less drag. I failed )

http://joxi.ru/GrqLbJRhvQ9YAz.jpg

But now i find that only 1st part after conus is protected from drag. And this part could be a big one.

Nice twisted candle, haven't seen anyone try that in the new aero.

[Tank Buddy]

A more direct answer to your question: if your local TWR is 1 and you are flying parallel to gravity, your acceleration will be 0. You can multiply the value of TWR-1 by local gravity fo find actuall acceleration. For example, 1.5 TWR @ Kerbin, 1.0 is counteracting 10m/s^2 (approximately) of gravity and .5 is free to accelerate your rocket. .5 * 10m/s^2 = 5m/s^2 or .5g. You can calculate acceleration when not parallel to gravity by using trig, but you don't really need that if your launch TWR is sufficient. When perpendicular to gravity (in orbit) multiply your TWR by the local gravity. You also could use Newton's 2nd law: F=ma => a=F/m (thrust/mass).