• 0

# How does thrust change as a function of pressure?

## Question

As the title says: we have listed values for vacuum and ASL thrust, but how is the Isp/thrust interpolated from there (or extrapolated for higher atmospheric pressures)?

## Recommended Posts

• 1

There's a thrust curve for each engine, defined in its config file as a set of key-value pairs. The thrust for a particular pressure is interpolated along a curve defined by those points.  It's only surfaced in the UI by giving you a sample of two points on that curve (for vacuum and ASL), though you can observe it in flight for other pressures by right-clicking on the engine.

##### Share on other sites

• 1

That is definitely not true. Linearity is not at all guaranteed (infact, given how float curves normally look, I'd be rather surprised if any of them were given they aren't using tangents).

fourfa: KER calculates exactly what the engine would give you under those cirumstances. It would actually be significantly more complex to try and do it linearly because the samples are already packaged up into "floatcurves"

##### Share on other sites

• 0

Ah, and there it is. Thanks!

##### Share on other sites

• 0

Happen to know if the VAB/SPH readouts in KER follow those individual configs?  Or is it a dumb interpolation?

##### Share on other sites

• 0

For rocket engines the relationship is simple: {actual thrust} = {vaccum thrust} - {some engine-specific constant} * pressure, i.e. it's just linear, and ASL number is just plugging in the pressure at Kerbin sea level. That should also be how KER calculates thrust at different altitude on different planets.

Jet engines follow a more complicated curve, and often includes velocity as one of the parameter as well.

##### Share on other sites

• 0

For rocket engine I wasn't talking about curve in KSP but IRL (http://www.braeunig.us/space/propuls.htm, note A_e is fixed for a particular engine, so it should be linear to ambient pressure). I didn't look at the actual curve in KSP, but I would be surprised if it's not the case - I thought they've fixed that back in 1.0 when they don't change fuel mass flow for simulating atm ISP change.

##### Share on other sites

• 0

Ah, missed the bit where we weren't looking at KSP engines.

```atmosphereCurve
{
key = 0 300
key = 1 280
key = 9 0.001
}```

^^ example isp curve definition from the Twin Boar (key = pressure, ISP). That third point isn't linear off the first->second, and the tangents aren't being used so the first->second point will curve a little as well to keep the curve smooth. The change in 1.0 was to make fuel flow constant and thrust vary based on the fraction of vac_ISP the engine currently had

Since thrust is linear with ISP changes and ISP depends on pressure, a non-linear relationship between pressure and ISP makes a non-linear relationship between pressure and thrust (probably nozzle related losses caused b non-optimal pressures causing the non-linearity that equation doesn't account for)

Edited by Crzyrndm
##### Share on other sites

• 0
On 2/1/2016 at 8:10 PM, FancyMouse said:

For rocket engine I wasn't talking about curve in KSP but IRL (http://www.braeunig.us/space/propuls.htm, note A_e is fixed for a particular engine, so it should be linear to ambient pressure). I didn't look at the actual curve in KSP, but I would be surprised if it's not the case - I thought they've fixed that back in 1.0 when they don't change fuel mass flow for simulating atm ISP change.

I'm assuming you're referencing F = mdot*Ve + (Pe - Pa)*Ae

You do realize Ve is not constant either? It changes non linearly, see equation 1.22 in the link you posted.

Here is a more complete equation for thrust with various efficiency variables:

$F&space;=&space;\lambda&space;C_d&space;C_v&space;\Gamma&space;\left[\frac{2\gamma}{\gamma-1}\left(1-\left(\frac{P_e}{P_c}\right)^{\frac{\gamma-1}{\gamma}}\right)\right]^{1/2}&space;P_c&space;A_t&space;+&space;A_e(P_e&space;-&space;P_0)$

Sforza, P. M. Theory of Aerospace Propulsion. Waltham, MA: Butterworth-Heinemann, 2012. Print.

On 1/31/2016 at 7:28 AM, FancyMouse said:

For rocket engines the relationship is simple: {actual thrust} = {vaccum thrust} - {some engine-specific constant} * pressure, i.e. it's just linear, and ASL number is just plugging in the pressure at Kerbin sea level. That should also be how KER calculates thrust at different altitude on different planets.

Jet engines follow a more complicated curve, and often includes velocity as one of the parameter as well.

You're going to need a source for that equation.

##### Share on other sites

• 0

I stand corrected. My first reply is based on my second reply, and I didn't realize V_e is not constant. I didn't read too far below in that article, and that's solely my fault.

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.