"Puff" MonoPropellant Fuel Engine: producing no thrust on Eve

[FalloutBe]

Hello,

I've created a landing vehicle which has no electric wheels, but monopropellant thrusters.
I tested this on Kerbin and they worked fine. However, when activating them on Eve they produce smoke and sound but they give 0kN of thrust?!

- The thrust limiter is not on (at 100%)

- Fuel requirements are met and the engines do drain fuel.

- Status of the engines is Nominal.

How come? What's wrong?

[Snark]

Welcome to the forums! 

19 minutes ago, FalloutBe said:

How come? What's wrong?

It's because you're on Eve.  Specifically:  the Puff gets no thrust at high pressures such as are found there.  Details below:

Engines generally get less thrust when operating in atmosphere than in a vacuum (their Isp goes down).  The higher the ambient pressure, the greater the Isp loss.  Just how much it goes down depends on the engine-- they each have a different curve.

This makes a big difference on Eve, which has a very high surface pressure (5 times Kerbin sea level pressure).

If you want to see what the engine does, crack open its .cfg file and take a look at the "atmosphere curve".  Here is the config for the Puff engine:

http://wiki.kerbalspaceprogram.com/wiki/Parts/Engine/OMSEngine/omsEngine.cfg

This is the relevant part of the config:

atmosphereCurve
{
	key = 0 250
	key = 1 120
	key = 4 0.001
}

How to read this:  "At 0 atmospheres (i.e. vacuum), the Isp is 250.  At 1 atmosphere, the Isp is 120.  At 4 atmospheres, the Isp is 0.001."

Moral of the story:  you really don't want to try to use a Puff anywhere that has higher-than-Kerbin atmospheric pressure. 

 

[FalloutBe]

24 minutes ago, Snark said:

At 4 atmospheres, the Isp is 0.001.

 

Ooh! Well now it makes sense!

Tomorrow I'll try again with another engine, or perhaps with electric wheels, but they don't fold up :/ so the lander will get bigger than the big heatshield.

Thanks a lot for your fast reply!