Difference between revisions of "Cheat sheet"
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Revision as of 22:32, 5 February 2015
Kerbal Space Program rocket scientist's cheat sheet: Delta-v maps, equations and more for your reference so you can get from here to there and back again.
Contents
Mathematics
Thrust-to-weight ratio (TWR)
- → See also: Thrust-to-weight ratio
This is Newton's Second Law. If the ratio is less than 1 the craft will not lift off the ground. Note that the local gravitational acceleration, which is usually the surface gravity of the body the rocket is starting from, is required.
- is the thrust of the engines
- the total mass of the craft
- the local gravitational acceleration (usually surface gravity)
Combined specific impulse (I_{sp})
- → See also: Specific impulse
If the I_{sp} is the same for all engines in a stage, then the I_{sp} is equal to a single engine. If the I_{sp} is different for engines in a single stage, then use the following equation:
Delta-v (Δv)
Basic calculation
- → See also: Tutorial:Advanced Rocket Design
Basic calculation of a rocket's Δv. Use the atmospheric and vacuum thrust values for atmospheric and vacuum Δv, respectively.
- is the velocity change possible in m/s
- is the starting mass in the same unit as
- is the end mass in the same unit as
- is the specific impulse of the engine in seconds
True Δv of a stage that crosses from atmosphere to vacuum
Body | Δv_{out} |
---|---|
Kerbin | 1000 m/s |
other bodies' data missing |
Calculation of a rocket stage's Δv, taking into account transitioning from atmosphere to vacuum. Δv_{out} is the amount of Δv required to leave a body's atmosphere, not reach orbit. This equation is useful to figure out the actual Δv of a stage that transitions from atmosphere to vacuum.
Maps
Various fan-made maps showing the Δv required to travel to a certain body.
Subway style Δv map:
Total Δv values
Δv change values
Δv with Phase Angles
Precise Total Δv values
Maximum Δv chart
- This chart is a quick guide to what engine to use for a single stage interplanetary ship. No matter how much fuel you add you will never reach these ΔV without staging to shed mass or using the slingshot maneuver.
ISP (s) Max Δv (m/s) Engines 290 6257 LV-1
LV-1R300 6473 24-77 340 7336 KR-1X2 350 7552 48-7S 360 7768 Mark-55
Mainsail
KS-25X4
370 7983 LV-T30
LV-T45
Skipper380 8199 KR-2L 390 8415 Poodle
LV-909
Aerospike800 17261 LV-N
Math examples
TWR
- Copy template:
- TWR = F / (m * g) > 1
I_{sp}
- When I_{sp} is the same for all engines in a stage, then the I_{sp} is equal to a single engine. So six 200 I_{sp} engines still yields only 200 I_{sp}.
- When I_{sp} is different for engines in a single stage, then use the following equation:
- Equation:
- Simplified:
- I_{sp} = ( F1 + F2 + ... ) / ( ( F1 / I_{sp}1 ) + ( F2 / I_{sp}2 ) + ... )
- Explained:
- I_{sp} = ( Force of thrust of 1st engine + Force of thrust of 2nd engine...and so on... ) / ( ( Force of thrust of 1st engine / I_{sp} of 1st engine ) + ( Force of thrust of 2nd engine / I_{sp} of 2nd engine ) + ...and so on... )
- Example:
- Two engines, one rated 200 newtons and 120 seconds I_{sp} ; another engine rated 50 newtons and 200 seconds I_{sp}.
- Isp = (200 newtons + 50 newtons) / ( ( 200 newtons / 120 ) + ( 50 newtons / 200 ) = 130.89 seconds I_{sp}
Δv
- For atmospheric Δv value, use atmospheric thrust values.
- For vacuum Δv value, use vacuum thrust values.
- Use this equation to figure out the Δv per stage:
- Equation:
- Simplified:
- Δv = ln ( Mstart / Mdry ) * I_{sp} * g
- Explained:
- Δv = ln ( starting mass / dry mass ) X Isp X 9.81
- Example:
- Single stage rocket that weighs 23 tons when full, 15 tons when fuel is emptied, and engine that outputs 120 seconds I_{sp}.
- Δv = ln ( 23 Tons / 15 Tons ) × 120 seconds I_{sp} × 9.81m/s² = Total Δv of 503.2 m/s
Maximum Δv
- Simplified version of the Δv calculation to find the maximum Δv a craft with the given ISP could hope to achieve. This is done by using a magic 0 mass engine and not having a payload.
- Equation:
- Simplified:
- Δv =21.576745349086 * I_{sp}
- Explained / Examples:
- This calculation only uses the mass of the fuel tanks and so the ln ( Mstart / Mdry ) part of the Δv equation has been replaced by a constant as Mstart / Mdry is always 9 (or worse with some fuel tanks) regardless of how many fuel tanks you use.
- The following example will use a single stage and fuel tanks in the T-100 to Jumbo 64 range with an engine that outputs 380 seconds I_{sp}.
- Δv = ln ( 18 Tons / 2 Tons ) × 380 seconds I_{sp} × 9.81m/s² = Maximum Δv of 8199.1632327878 m/s
- Δv = 2.1972245773 × 380 seconds I_{sp} × 9.82m/s² = Maximum Δv of 8199.1632327878 m/s (Replaced the log of mass with a constant as the ratio of total mass to dry mass is constant regardless of the number of tanks used as there is no other mass involved)
- Δv = 21.576745349086 × 380 seconds I_{sp} = Maximum Δv of 8199.1632327878 m/s (Reduced to its most simple form by combining all the constants)
True Δv
- How to calculate the Δv of a rocket stage that transitions from Kerbin atmosphere to vacuum.
- Assumption: It takes approximately 1000 m/s of Δv to escape Kerbin's atmosphere before vacuum Δv values take over for the stage powering the transition.
- Note: This equation is an guess, approximation, and is not 100% accurate. Per forum user stupid_chris who came up with the equation: "The results will vary a bit depending on your TWR and such, but it should usually be pretty darn accurate."
- Equation for Kerbin atmospheric escape:
- Simplified:
- True Δv = ( ( Δv atm - 1000 ) / Δv atm ) * Δv vac + 1000
- Explained:
- True Δv = ( ( Total Δv in atmosphere - 1000 m/s) / Total Δv in atmosphere ) X Total Δv in vacuum + 1000
- Example:
- Single stage with total atmospheric Δv of 5000 m/s, and rated 6000 Δv in vacuum.
- Transitional Δv = ( ( 5000 Δv atm - 1000 Δv required to escape Kerbin atmosphere ) / 5000 Δv atm ) X 6000 Δv vac + 1000 Δv required to escape Kerbin atmosphere = Total Δv of 5800 m/s