EVA Pack Isp

[wrcsubers]

Does anyone know if the EVA Pack assumes a constant Isp?  I've been looking at the manual for the NASA MMU (Manned Maneuvering Unit) and it assumes a constant Isp, wondering if this is the same case in KSP...

Thanks!

[Pecan]

No idea what the Isp is, but it provides 600m/s dV IIRC, so if you look-up the mass of Kerbal you should be able to work it out.  Pretty much bound to be constant though, since anything with enough atmosphere to affect it is going to have loads more gravity than EVA RCS can cope with.

[OhioBob]

The mass of a Kerbal on EVA is 94 kg and he/she has 5 units of EVA propellant.  However, a Kerbal's mass does not change as EVA propellant is consumed (it always remains 94 kg regardless of whether the EVA pack is full or empty).  This makes the ISP infinite (which makes the OP's question moot).  However, if we assume that his/her mass does decrease, then we can compute the ISP.  We don't know the density of "EVA propellant," but if we assume it is the same as monopropellant (4 kg/unit), then we have 20 kg of propellant.  This means that the ISP is,

ISP = 600 / (LN(94/74)*9.80665) = 256 s

 

Edited December 16, 2015 by OhioBob

[fourfa]

1 hour ago, Pecan said:

anything with enough atmosphere to affect it is going to have loads more gravity than EVA RCS can cope with.

Splitting hairs, but the jetpack can lift a Kerbal on Duna.  Though Duna's atmosphere is essentially just a hair away from vacuum in Kerbal terms

[Snark]

Isp isn't irrelevant, even if the propellant is massless. When an engine's Isp changes, so does its max thrust.

So a different way of phrasing the OP's question would be, Does atmospheric pressure affect the max thrust and the dV of an EVA pack?

I don't know the answer. My guess is that it doesn't change, mainly because it would have been more work to code it, and it's clear that realistic EVA thruster performance is not an area that Squad has been prioritizing.

 

 

Edited December 16, 2015 by Snark

[wrcsubers]

4 hours ago, OhioBob said:

The mass of a Kerbal on EVA is 94 kg and he/she has 5 units of EVA propellant.  However, a Kerbal's mass does not change as EVA propellant is consumed (it always remains 94 kg regardless of whether the EVA pack is full or empty).  This makes the ISP infinite (which makes the OP's question moot).  However, if we assume that his/her mass does decrease, then we can compute the ISP.  We don't know the density of "EVA propellant," but if we assume it is the same as monopropellant (4 kg/unit), then we have 20 kg of propellant.  This means that the ISP is,

ISP = 600 / (LN(94/74)*9.80665) = 256 s

 

I would say this is very close, especially if you imagine (like I had originally) that Squad used a similar model to the RCS block's I_SP in Vac of 240.

3 hours ago, Snark said:

Isp isn't irrelevant, even if the propellant is massless. When an engine's Isp changes, so does its max thrust.

So a different way of phrasing the OP's question would be, Does atmospheric pressure affect the max thrust and the dV of an EVA pack?

I don't know the answer. My guess is that it doesn't change, mainly because it would have been more work to code it, and it's clear that realistic EVA thruster performance is not an area that Squad has been prioritizing.

 

 

You're correct Snark, that IS what I was trying to ask =) I'm going to agree with you that it doesn't change, just from eyeballing it on different bodies, atmosphere or not, it seems like it had the same relative effectiveness when you take local gravity into account.

As for realistic EVA thruster performance, the original NASA MMU has an I_SP of 60 and uses N₂.  dV with 0lbs of Cargo is about 42m/s and dV with 500lbs of Cargo is about 23m/s. 

I'm thinking that a 'Realistic EVA Thruster Performance' Mod wouldn't be very much fun lol

Edited December 17, 2015 by wrcsubers

[Snark]

59 minutes ago, wrcsubers said:

I'm thinking that a 'Realistic EVA Thruster Performance' Mod wouldn't be very much fun lol

Yeah, but I've been seriously thinking about writing a mod to nerf EVA to something reasonable.  Not "realistic", I think it's fine to be more powerful than the real-life EVA suits.  But 600 m/s is just ludicrous.  They're supposed to be for maneuvering around an orbiting ship, not sending a kerbal all the way home to Kerbin from the surface of Minmus without a ship. Sheesh.

[wrcsubers]

3 hours ago, Snark said:

Yeah, but I've been seriously thinking about writing a mod to nerf EVA to something reasonable.  Not "realistic", I think it's fine to be more powerful than the real-life EVA suits.  But 600 m/s is just ludicrous.  They're supposed to be for maneuvering around an orbiting ship, not sending a kerbal all the way home to Kerbin from the surface of Minmus without a ship. Sheesh.

Agreed, something more realistic would certainly be popular with the 'Hardcore' type of players.  Maybe cut it down to 1/3 or 1/4 of the current dV? Reduce the power as well.  I don't think you can change I_SP directly (at least from looking at the code), but I'm pretty sure you'd be just fine limiting the power and then limiting the fuel.

KerbalEVA KEVA;
KEVA.linPower = *float*
KEVA.rotPower = *float*
KEVA.PropellantConsumption = *float*

There is a 'Fuel' & 'FuelCapacity' but both are read only.  So between tweaking those three values above, I'm sure you could find a happy balance.

Edited December 17, 2015 by wrcsubers

[Snark]

10 hours ago, wrcsubers said:

Agreed, something more realistic would certainly be popular with the 'Hardcore' type of players.  Maybe cut it down to 1/3 or 1/4 of the current dV? Reduce the power as well.  I don't think you can change I_SP directly (at least from looking at the code), but I'm pretty sure you'd be just fine limiting the power and then limiting the fuel.

KerbalEVA KEVA;
KEVA.linPower = *float*
KEVA.rotPower = *float*
KEVA.PropellantConsumption = *float*

There is a 'Fuel' & 'FuelCapacity' but both are read only.  So between tweaking those three values above, I'm sure you could find a happy balance.

Yeah, I was thinking something along the lines of tweaking linear power to about 25%, and leaving everything else alone.  (Leave rotation power alone, because reducing that would make maneuvering really annoying; and leave consumption alone, on the theory that leaving it constant while reducing linear power will have the effect of reducing total dV).  That would still leave a comfortable margin for orbital maneuvering (the EVA pack would still have several times the dV of real NASA suits), but it would no longer be useful for making major orbital changes (no more free rides home from Minmus), and it would be weak enough that you can't fly around on the Mun with an EVA suit.

I'd also make propellant not "free" anymore-- when going EVA, it would require some monopropellant, and convert at some reasonable rate.  No more free refills. 

Plus it would also need some peripheral changes to make the above work.  In particular, would need to move ladders a bit earlier in the tech tree. ("Well, sir, we have the technology to land a crew on the Mun, but we're having trouble coming up with something grabby so that we can get back into the ship."  Really?)  Also, would need to add a smidgeon of monopropellant to the shipwrecks in rescue contracts, otherwise the rescued kerbals would be helpless and it would be a major pain.

I wonder why they made Fuel and FuelCapacity read-only?  Seems like an odd choice.