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.