If the pressure is 0.5 atmospheres, what is the Isp of an LV-N Atomic Rocket Engine?

[Gus]

Is it the average of the atmospheric and vacuum Isps? ie. (220 + 800)/2 = 510?

Thanks

P.S. Is this averaging of Isps mentioned in the Wiki? I can't find it.

Answered. Thanks.

Edited July 11, 2013 by Gus

[stupid_chris]

I'm not 100% sure of the equation, but I think it's around that. I know the ISP of the LV-N by altitude is on most of the Wiki's planet page for those who have an atmosphere.

[austin4050]

The ISP of an atmosphere 0.5 would be 50%, just turn the decimal into a percent.

[Kosmo-not]

It scales linearly with pressure. So, at 0.5 atm, it would be 510s.

If pressure is greater than 1 atm, it will remain at 220s.

[Gus]

Excellent reply Kosmo. "scales linearly with pressure" is a perfect description, esp. with your 1 atm. proviso.

Thanks.

Edited July 11, 2013 by Gus

[Gus]

The ISP of an atmosphere 0.5 would be 50%, just turn the decimal into a percent.

This doesn't take into account the "base" Isp value of 220. At 1% atm., you imply the Isp would be 8.8. Which it ain't.

[Gus]

I'm not 100% sure of the equation, but I think it's around that. I know the ISP of the LV-N by altitude is on most of the Wiki's planet page for those who have an atmosphere.

Ahhh... down there in the footnotes. Excellent. So the LV-N is still incredibly efficient even on the surface of Duna. Good to know.

[Gus]

My question has been "answered" but I don't know how to indicate this on my post.

[capi3101]

My question has been "answered" but I don't know how to indicate this on my post.

Edit your original post.

So...would the Isp of an LV-N still be 220 on the surface of Eve? You know, where the atmospheric pressure is 5 atmospheres?

[Gus]

[arq]

So...would the Isp of an LV-N still be 220 on the surface of Eve? You know, where the atmospheric pressure is 5 atmospheres?

Yes, ISP never goes below the atmo rating. On Eve, it will have 220 until it thins below 1 (kerban) atmo.

[Gus]

I just calculated that the LV-N starts to give you the best Isp of any stock rocket (390) at ~0.7 atm.. Which is apparently at the shockingly low height of 1,800 meters above Kerbin. However the LV-N's low thrust to weight ratio is it's downfall.

Edited July 11, 2013 by Gus

[capi3101]

Yeah I saw that. Doesn't seem right...I mean, you've got this big swing in the Isp from 1 to 0 atmospheres, it should work the same from 5 to 1.

Maybe that's just the way's it implemented in the game? I dunno. I suppose there could be a concept there that I'm misunderstanding.

[Gus]

Yeah I saw that. Doesn't seem right...I mean, you've got this big swing in the Isp from 1 to 0 atmospheres, it should work the same from 5 to 1.

Maybe that's just the way's it implemented in the game? I dunno. I suppose there could be a concept there that I'm misunderstanding.

Yeah, it looks like they went for a simple model of two baselines (at 0 and 1, like you say) and a "ramp" between them.

[numerobis]

The Isp ramp isn't really physically correct: if your rocket bell is wrong, you get worse Isp, sure, but then it melts.

The underlying implementation is not clear on what kind of curve is actually in use. For rocket engines, you have two values, and it's just linear between them. For jets you have three values, and it's some kind of curve that goes slightly higher than the highest value.

[Buzzed Aldrin]

ISP for a given pressure is given as:

(1-Pressure) / (1/ISPdif) + ISPsl

where ISPdif is the difference between the given values for the motor's sea level and vacuum ISP, and ISPsl is sea level ISP.

For the LV-N - ISPdif=800-220=580 and ISPsl=220

So:

(1-.5)/((1/580)+220= 510seconds

Edited July 12, 2013 by Buzzed Aldrin