ISP/Weight tradeoff on engines (graph)

[impyre]

The graph below shows the trade-off associated with ISP, weight, and dV. Since the weight is listed as the percentage of the vessel's mass composed of fuel, the slope along that axis may change dramatically for different weight profiles. For a very heavy vehicle, an extra ton's worth of engine won't change the fuel ratio much; however, for a very light vehicle the same tradeoff would decrease the percentage mass of fuel by much more. By itself, this graph just helps illustrate the relationships involved... to be of any real use you have to take your own situation into consideration.

EDIT: Here is another that I've attempted to animate to show the effect of overall vehicle weight on this tradeoff. It loops through each frame, and it starts with the highest vehicle/fuel weight. Each frame reduces the vehicle and fuel weight linearly, and fuel/vehicle mass is a constant proportion (more true for heavier vehicles). As you should be able to see, as the mass of the rest of the vehicle goes down, the effect of the engine's weight becomes much more important (as it becomes a bigger contributor to the total mass ratio). The effect of the engine's weight drags down dV quickly once total vehicle weight gets below 5-8 tons, (changing the fuel/ship mass ratio can alleviate or exacerbate this effect depending on the direction of the change). You'll also notice that the ISP/dV curve doesn't move as much when the engine weight is very low (the corner that doesn't move much). Basically, making the other parts of the craft (including fuel) heavier stretches this graph out along the engine-weight axis, which has the consequence of all but eliminating the penalty paid for engine weight. At higher vehicle/fuel weights, the dV loss between an engine of 500kg and 2000kg has become pretty negligible (the highest vessel/fuel mass is about 70t in this series of frames) while ISP gains remain relatively in-tact.

http://postimg.org/image/onpa7tzud/

Edited March 21, 2015 by impyre

[Rematog]

Graph does not go up to 800 for LV-N

[impyre]

Yep, but since the graph demonstrates the linear relationship that ISP has with dV, you can take a pretty good guess at what it would look like. At an ISP of 300, and a fuel mass percentage of about 70% of gross vehicle weight, you're looking at somewhere around 3600-3800 m/s dV. Since it's linear, just solve for slope. 3600/300 = 12, so an ISP of 800 would be around 12*800, or approx. 9600. Again, just an estimate. The actual calculation gives 9439.15 for the given ratio. Also, if you drop weight on the return trip this will change the ratio further. The fact that the nuclear engine's ISP is soo much higher than the others is what makes it's weight relatively inconsequential by comparison.

Also, I mistakenly posted this in the wrong forum. If someone takes a mind to move it, please do. Thanks, and sorry.

- - - Updated - - -

I'm actually putting together another graph that labels some points for common KSP engines for a few vehicle weights. So if you have a 20t vehicle, you can see how each engine compares in terms of how much mass it contributes to the vessel mass and how much difference its ISP makes. This should give a better feel for the tradeoff, and how it is affected by the ratio between the engine's mass and the mass of the rest of the vessel.

[NathanKell]

Very cool!

[impyre]

Thanks!

[GoSlash27]

Yep, but since the graph demonstrates the linear relationship that ISP has with dV, you can take a pretty good guess at what it would look like. At an ISP of 300, and a fuel mass percentage of about 70% of gross vehicle weight, you're looking at somewhere around 3600-3800 m/s dV. Since it's linear, just solve for slope. 3600/300 = 12, so an ISP of 800 would be around 12*800, or approx. 9600. Again, just an estimate. The actual calculation gives 9439.15 for the given ratio. Also, if you drop weight on the return trip this will change the ratio further. The fact that the nuclear engine's ISP is soo much higher than the others is what makes it's weight relatively inconsequential by comparison.

Also, I mistakenly posted this in the wrong forum. If someone takes a mind to move it, please do. Thanks, and sorry.

- - - Updated - - -

I'm actually putting together another graph that labels some points for common KSP engines for a few vehicle weights. So if you have a 20t vehicle, you can see how each engine compares in terms of how much mass it contributes to the vessel mass and how much difference its ISP makes. This should give a better feel for the tradeoff, and how it is affected by the ratio between the engine's mass and the mass of the rest of the vessel.

Good stuff.

I think people overestimate the efficiency of the nuke due to the Isp being so high, while forgetting that it weighs over 2 tonnes.

In anything under approx. 10 tonnes total mass (where you'll be making a hair over 1/2G acceleration), you're almost always better off using something else, like the 48-7S.

Thanks for doing this!

-Slashy

[impyre]

Added the new graph, decided to do something a bit more comprehensive rather than just doing a few points for each engine/vehicle weight combo.

[bakanando]

A label in the second graph indicating the wet mass/dry mass ratio for each frame would be nice, so it's easier to know which frame represents what amount.

Good work.