Ideal T/W ratios for airless body launch (KSP .90)

[GoSlash27]

[TABLE]

[TR]

[TD][/TD]

[TD] Gilly[/TD]

[TD][/TD]

[TD] Tylo[/TD]

[TD=width: 86][/TD]

[/TR]

[TR]

[TD][/TD]

[TD] Mass[/TD]

[TD] Cost[/TD]

[TD] Mass[/TD]

[TD] Cost[/TD]

[/TR]

[TR]

[TD]LV-1[/TD]

[TD]2.5[/TD]

[TD]1.2[/TD]

[TD]1.8[/TD]

[TD]1.0[/TD]

[/TR]

[TR]

[TD]LV-1R[/TD]

[TD]2.5[/TD]

[TD]1.1[/TD]

[TD]1.8[/TD]

[TD]1.0[/TD]

[/TR]

[TR]

[TD]24-77[/TD]

[TD]2.9[/TD]

[TD]1.4[/TD]

[TD]2.1[/TD]

[TD]1.2[/TD]

[/TR]

[TR]

[TD]48-7S[/TD]

[TD]3.0[/TD]

[TD]1.7[/TD]

[TD]2.3[/TD]

[TD]1.4[/TD]

[/TR]

[TR]

[TD]LV-909[/TD]

[TD]2.1[/TD]

[TD]1.5[/TD]

[TD]1.6[/TD]

[TD]1.3[/TD]

[/TR]

[TR]

[TD]Mk-55[/TD]

[TD]2.3[/TD]

[TD]1.8[/TD]

[TD]1.8[/TD]

[TD]1.5[/TD]

[/TR]

[TR]

[TD]RAPIER[/TD]

[TD]2.4[/TD]

[TD]1.4[/TD]

[TD]1.8[/TD]

[TD]1.2[/TD]

[/TR]

[TR]

[TD]LV-T30[/TD]

[TD]2.5[/TD]

[TD]2.1[/TD]

[TD]1.9[/TD]

[TD]1.7[/TD]

[/TR]

[TR]

[TD]LV-T45[/TD]

[TD]2.3[/TD]

[TD]2.0[/TD]

[TD]1.8[/TD]

[TD]1.6[/TD]

[/TR]

[TR]

[TD]Aerospike[/TD]

[TD]2.2[/TD]

[TD]1.4[/TD]

[TD]1.7[/TD]

[TD]1.2[/TD]

[/TR]

[TR]

[TD]Poodle[/TD]

[TD]2.2[/TD]

[TD]1.8[/TD]

[TD]1.7[/TD]

[TD]1.4[/TD]

[/TR]

[TR]

[TD]LV-N[/TD]

[TD]1.4[/TD]

[TD]1.1[/TD]

[TD]1.1[/TD]

[TD]1.0[/TD]

[/TR]

[TR]

[TD]Skipper[/TD]

[TD]2.7[/TD]

[TD]2.0[/TD]

[TD]2.0[/TD]

[TD]1.7[/TD]

[/TR]

[TR]

[TD]Mainsail[/TD]

[TD]2.8[/TD]

[TD]2.1[/TD]

[TD]2.1[/TD]

[TD]1.8[/TD]

[/TR]

[TR]

[TD]KR2L[/TD]

[TD]3.2[/TD]

[TD]1.7[/TD]

[TD]2.4[/TD]

[TD]1.5[/TD]

[/TR]

[TR]

[TD]KS25x4[/TD]

[TD]3.1[/TD]

[TD]1.7[/TD]

[TD]2.3[/TD]

[TD]1.4[/TD]

[/TR]

[TR]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[TR]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[TR]

[TD]0-10[/TD]

[TD] inf[/TD]

[TD]1.3[/TD]

[TD] inf[/TD]

[TD]1.0[/TD]

[/TR]

[TR]

[TD]RV-105[/TD]

[TD] inf[/TD]

[TD]1.5[/TD]

[TD] inf[/TD]

[TD] 1.0

[/TD]

[/TR]

[TR]

[TD]Linear[/TD]

[TD] inf[/TD]

[TD] 1.3[/TD]

[TD] inf[/TD]

[TD] 1.0

[/TD]

[/TR]

[TR]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[TR]

[TD]PB-Ion[/TD]

[TD]1.1[/TD]

[TD]1.1[/TD]

[TD]1.0[/TD]

[TD]1.0[/TD]

[/TR]

[/TABLE]

- - - Updated - - -

All,

This table shows the ideal t/w ratios for launching from airless bodies using each of the engines available. For obvious reasons, I didn't bother listing the solid boosters.

Thanks to arkie 87's outstanding work on modeling DV losses on airless body launches, I was able to model each engine launching a suitably sized rocket and determine these crossover points.

T/W for ideal vehicle mass:

This is the point at which adding or removing engines to alter the t/w will result in an increase in overall launch vehicle mass.

T/W for ideal vehicle cost:

This is the point at which adding or removing engines to alter the t/w will result in an increase in total launch vehicle cost.

These numbers should not be considered design targets in and of themselves, but rather show where the crossover point exists. You would not want to add mass or limit your throttle just to meet these ratios.

A couple notes...

*All modeled launchers assume the most mass- efficient tanks for their type and those tanks are assumed to be infinitesimally scalable. This is not true in real life, so these results will tend to the high side for small scale launchers.

*The PB-Ion does not take into consideration the mass and cost penalties of additional solar panels and batteries required to support additional thrusters.

Best,

-Slashy

Edited December 30, 2014 by GoSlash27

[Red Iron Crown]

How can a 1.0 ratio be ideal? Doesn't it just burn fuel until TWR climbs?

[GoSlash27]

How can a 1.0 ratio be ideal? Doesn't it just burn fuel until TWR climbs?

That it does. But in some cases the cost and/or mass of the wasted fuel is less than the cost and/or mass of the additional engines. This is especially true for heavy, expensive, and highly- efficient engines.

Best,

-Slashy

[Red Iron Crown]

Interesting. Do you plan to expand the chart for other bodies?

[GoSlash27]

Interesting. Do you plan to expand the chart for other bodies?

I was at first, but decided against it. These 2 moons represent the absolute max and min for these figures. All other bodies will have ideal t/w between these 2. And of course I can't model drag.

Best,

-Slashy

[kerbiloid]

The less is T/W the greater is acceleration duration, the greater is the energy loss caused by the gravitation.

[GoSlash27]

The less is T/W the greater is acceleration duration, the greater is the energy loss caused by the gravitation.

This is also true. That energy loss must be paid for with additional fuel and tankage. This chart shows the point at which the penalty for low t/w in fuel and tankage offsets the penalty for high t/w in engine mass.

Best,

-Slashy

[smartech]

Minmus is interesting because you can use the Greater flats as long runways to accelerate to orbital velocity, and you dont really need TWR > 1.