I'm missing something (Timing SRB burn out with Booster burn out)

[JoeSchmuckatelli]

I've been playing around with trying to set up a pair of the large SRBs to have them flame out - and then almost immediately after, have the Mammoth stage flame out - so that I can drop the SRBs then drop the booster and stage the sustainer engine.

Problem is - I'm getting the timing wrong.  Mammoth is flaming out before the SRBs.

Here's what I'm seeing: just a Mammoth, S3 14400 and S3 3600 (Large diameter cylinder tanks, the big and small one) show up as having 4524 m/s of DV.  The S2 33 Clydesdale shows up as having 4437 m/s DV.

Based on my rudimentary knowledge of maths, one of those numbers is smaller than the other... and so I'd think that the Clydesdales would burn out before the Mammoth.  Apparently, that's not how it works.

 

Anyone care to break this down Barney style for me?

[The Aziz]

ISP, thrust... There's more to engine burn time than just delta v, obviously? 

To clear this up a riddle:

I have two vehicles in space with the same amount of dv. One has Mammoth onboard with methalox tanks, the other has Nerv with its hydrogen.

Which one is going to run out faster and why?

[JoeSchmuckatelli]

44 minutes ago, The Aziz said:

I have two vehicles in space with the same amount of dv. One has Mammoth onboard with methalox tanks, the other has Nerv with its hydrogen.

Which one is going to run out faster and why?

Getting that part through my Neanderthal skull is the hard part for me.

So how do I figure out fuel consumption rates?  Other than just slapping on bigger tanks.  Is there a formula?  Or is it listed and I'm just not seeing it?

[determinationmaster]

KSP 1 had a burn time indicator in the VAB staging UI. Apparently, this isn't a thing anymore in KSP 2

[Streetwind]

1 hour ago, JoeSchmuckatelli said:

Is there a formula?

The relationship you're looking for is thrust = Isp * g0 * m, which you can rearrange into m = thrust / (Isp * g0).

You can insert sea level thrust and Isp here, or vacuum thrust and Isp; the result will be the same. Just don't mix sea level one with vacuum other. That'll screw it up.

The m is the mass flow rate, and should really be an m with a dot above it, but I can't type that with my keyboard, so kindly imagine it that way and call it m(dot) in your mind.

Once you have calculated your mass flow rate, you can look up the mass of fuel the engine has to work with, and divide it by the flow rate in order to get the burn time in seconds.

[The Aziz]

2 hours ago, JoeSchmuckatelli said:

So how do I figure out fuel consumption rates?

If you hit shift while mouseing over an engine in VAB part list you can see max fuel consumption. Unfortunately right now there isn't a stock way to tell how it changes when you play with throttle. (but I bet there's a mod for that already)

Edited March 21, 2023 by The Aziz

[JoeSchmuckatelli]

1 hour ago, Streetwind said:

The relationship you're looking for is thrust = Isp * g0 * m, which you can rearrange into m = thrust / (Isp * g0).

You can insert sea level thrust and Isp here, or vacuum thrust and Isp; the result will be the same. Just don't mix sea level one with vacuum other. That'll screw it up.

The m is the mass flow rate, and should really be an m with a dot above it, but I can't type that with my keyboard, so kindly imagine it that way and call it m(dot) in your mind.

Once you have calculated your mass flow rate, you can look up the mass of fuel the engine has to work with, and divide it by the flow rate in order to get the burn time in seconds.

Thanks!  I'm guessing g0 is the gravity of the object (Kerbin)? 

1 hour ago, The Aziz said:

max fuel consumption

I'll check it out when I get on again! 

[Streetwind]

g0 is called "standard gravity", and is a constant which is defined as ~9.80665 m/s².

[FleshJeb]

What Street said. ISP is kind of BS number. What we're really interested in is the "effective exhaust velocity", which is Ve = g0 * ISP. We just defined it as ISP = Ve / g0 because it's a useful shortcut for other calculations on Earth. These are not typically used for gameplay.

From wikipedia:

"Specific impulse, measured in seconds, effectively means how many seconds this propellant, when paired with this engine, can accelerate its own initial mass at 1 g. The longer it can accelerate its own mass, the more delta-V it delivers to the whole system."