Usage Example

Imagine we build a light Mun lander, having a payload of 1320 kg. That is a Mk1 Command Pod, four LT-05 Landing Struts, a Parachute, a Heat Shield, a Stack Decoupler and Solar Panels. We want to have two stages: the upper one flying from Low Kerbin Orbit to Mun, landing there, and then flying back to Kerbin; and the lower one launching the lander stage from Kerbin Space Center to Low Kerbin Orbit.

After having determined the payload of the stage, we need to figure out Delta-v requirements, acceleration requirements and air pressure at the different flight phases.

In this case air pressure is easy: As the Mun does not have any atmosphere and the stage starts its way already being in orbit, it is clear that the lander will be designed to fly through vacuum only.

Needed Delta-v can be easily read at Delta-v maps or calculated by calculation tools found in the internet (see links section later in this document). We find out, that we need 1170 m/s from Low Kerbin Orbit to Low Mun Orbit, then 580 m/s for landing at Mun, 580 m/s for starting at Mun and later 310 m/s for returning to Kerbin. Additionally, in this example we want to have 700 m/s Delta-v as a reserve.

Now let’s think about acceleration. As we land and start on Mun, we indeed have constraints regarding minimum acceleration, because we need to counteract Mun’s gravity. In this example, we want to have at least 2g = 3.3 m/s² acceleration when starting to land at Mun (i.e. when having reached Low Mun Orbit), and 3g = 5.0 m/s² to launch at Mun, g being Mun’s surface gravity, which is about 1.65 m/s² as can be found out in the in-game knowledge base.

Do we have any preferences? Yes we do. We’re building a lander utilizing LT-05 Micro Landing Struts, which are quite bad, so it would be nice to prefer engines which have a short length. Thus, we add --length flag to kspalculator invocation. Additionally, our Payload has radial size small, so it would be cool if the propulsion system also had this radius. We add -R small. Note that adding preferences does not prevent the listing of solutions which do not meet these preferences, i.e. adding preferences always leads to more output.

Doing so, we get:

$ kspalculator 1320 -R small --length 1170 580:3.3 580:5.0 310 700
48-7S Spark
    Total Mass: 6145 kg (including payload and full tanks)
    Cost: 1670
    Liquid fuel: 840 units (4725 kg full tank mass)
    Requires: PropulsionSystems
    Radial size: Tiny
    Gimbal: 3.0 °
    Engine is short enough to be used with LT-05 Micro Landing Struts
    Performance:
    [...]

LV-909 Terrier
    Total Mass: 6320 kg (including payload and full tanks)
    Cost: 1190
    Liquid fuel: 800 units (4500 kg full tank mass)
    Requires: AdvancedRocketry
    Radial size: Small
    Gimbal: 4.0 °
    Engine is short enough to be used with LT-05 Micro Landing Struts
    Performance:
      1:  1170 m/s @ vacuum     9.49 m/s² - 13.42 m/s²    6.3 t -   4.5 t
      2:   580 m/s @ vacuum    13.42 m/s² - 15.92 m/s²    4.5 t -   3.8 t
      3:   580 m/s @ vacuum    15.92 m/s² - 18.90 m/s²    3.8 t -   3.2 t
      4:   310 m/s @ vacuum    18.90 m/s² - 20.72 m/s²    3.2 t -   2.9 t
      5:   700 m/s @ vacuum    20.72 m/s² - 25.48 m/s²    2.9 t -   2.4 t
      6:    51 m/s @ vacuum    25.48 m/s² - 25.86 m/s²    2.4 t -   2.3 t

[...]

LV-T30 Reliant
    Total Mass: 11008 kg (including payload and full tanks)
    Cost: 2825
    Liquid fuel: 1500 units (8438 kg full tank mass)
    Requires: GeneralRocketry
    Radial size: Small
    Engine generates electricity
    Engine is short enough to be used with LT-2 Landing Struts
    Performance:
    [...]

[...]

(Output was shortened)

Of the suggested designs, all are the best by some criteria. The first one, using Spark engine, is the one having the lowest total mass, but in this example we do not want to use it, for example because we did not research “Propulsion Systems” yet. We choose the Terrier design as we think it serves best our needs. Note that the tool also suggests the Reliant because of lower technology requirements, as well as some other nice designs which we skipped in this document to save space.

Now build the stage adding the 800 Unit Fuel Tank and the Terrier engine under your payload. Then add a stack decoupler (which weights 50 kg) as we’re building the launcher stage.

The payload for the launcher stage is 6370 kg (i.e. the lander stage plus 50 kg stack decoupler). Safe Delta-v and acceleration requirements for a launch to Low Kerbin Orbit have been found out to be 905 m/s with 13 m/s² at 1 ATM and then 3650 m/s with 13 m/s² at 0.18 ATM.

We want to use solid fuel boosters for the launch, so we add --boosters. Additionally, we prefer engines with thrust vectoring as it may be helpful to counteract turbulences during launch, so we add --gimbal. Small is still our preferred radial size. Now we determine best launcher designs:

$ kspalculator 6370 --boosters --gimbal -R small 905:13:1 3650:13:0.18
RE-I5 Skipper
    Total Mass: 89320 kg (including payload and full tanks)
    Cost: 18258
    Liquid fuel: 5600 units (31500 kg full tank mass)
    Requires: HeavyRocketry
    Radial size: Large
    Gimbal: 2.0 °
    Engine generates electricity
    Radially attached 2 * S1 Kickback SFB
    SFBs mounted on TT-70 Radial Decoupler, Advanced Nose Cone, 2 * EAS-4 Strut Connector each
    Performance:
     *1:   905 m/s @ 1.00 atm  13.30 m/s² - 21.35 m/s²   89.3 t -  55.6 t
     *2:   213 m/s @ 0.18 atm  23.59 m/s² - 26.08 m/s²   55.6 t -  50.3 t
      3:  3437 m/s @ 0.18 atm  15.55 m/s² - 47.68 m/s²   40.9 t -  13.3 t
      4:   107 m/s @ 0.18 atm  47.68 m/s² - 49.37 m/s²   13.3 t -  12.9 t

4 * Mk-55 Thud, radially mounted
    Total Mass: 108520 kg (including payload and full tanks)
    Cost: 19467
    Liquid fuel: 4600 units (25875 kg full tank mass)
    Requires: HeavyRocketry
    Radial size: Small
    Gimbal: 8.0 °
    Engine is short enough to be used with LT-05 Micro Landing Struts
    Radially attached 3 * S1 Kickback SFB
    SFBs mounted on TT-70 Radial Decoupler, Advanced Nose Cone, 2 * EAS-4 Strut Connector each
    You might limit SFB thrust to 79.5 %
    Performance:
     *1:   905 m/s @ 1.00 atm  16.42 m/s² - 26.35 m/s²  108.5 t -  67.6 t
     *2:   637 m/s @ 0.18 atm  29.12 m/s² - 39.36 m/s²   67.6 t -  50.0 t
      3:  3013 m/s @ 0.18 atm  13.15 m/s² - 36.68 m/s²   35.8 t -  12.9 t
      4:     2 m/s @ 0.18 atm  36.68 m/s² - 36.71 m/s²   12.9 t -  12.8 t

[...]

(Output was shortened)

The asterisks in the performance tables indicate that the phase of flight is done by solid fuel boosters. The SFB thrust limit suggestion is the minimum thrust required to fulfill your acceleration constraints.

Now build one of the launchers being suggested by kspalculator and we’re ready to do a giant leap for kerbinkind.