Notes about DYN ship display
This page gives background information about the DYN ship display.
Overview of DYN
DYN enables you to calculate trip times for a particular distance travelled by choosing accelerations, and burn and coast times.
The first column contains the form. Enter and adjust your values here.
The second column displays the calculated values for your trip (total travel time, fraction of fuel used, etc.).
The third column has plots of velocity, distance, and delta-v as a function of time.
The first column contains the form. Enter and adjust your values here.
The second column displays the calculated values for your trip (total travel time, fraction of fuel used, etc.).
The third column has plots of velocity, distance, and delta-v as a function of time.
How to use the DYN travel time calculator
- Adjust the 'Ship deltaV(total)/ g day' field if necessary (its default is an estimate for the Rocinante).
- Enter the distance into 'Distance / au'
- Set the acceleration burn in ‘Accel / g’
- Set the deceleration burn in 'Decel / g'
This will evaluate the trip assuming there is no coast between the burns.
If you adjust 'T(accel) / day' to change the acceleration burn time and then click the 'T(accell)' button DYN will calculate an appropriate coast time for the distance to be travelled.
If you adjust 'T(coast) / day' to change the coast time and then click the 'T(coast)' button DYN will calculate appropriate acceleration and deceleration times for the distance to be travelled.
Warnings
Cells with white text and a red background are warnings.
‘REDUCE T(ACCEL)' warning - This occurs when the acceleration and its duration are too large and you would overshoot the destination. Reduce ‘Accel’ and/or ‘Taccel’ until the warning goes.
'Fuel fraction warning' - This indicates that the deltaV of the journey is greater than the total delta-v of the ship (i.e. the ship does not have enough propellant for the journey).
Increase 'T(coast)' or reduce 'T(accel)'.
Increase 'T(coast)' or reduce 'T(accel)'.
Delta-v value
Somewhere in The Expanse TV series (I forget which episode) Alex Kamal says that the Rocinante can run for 0.3 g for about 3 weeks. This gives a deltaV value of about 6 g days.
toughsf.blogspot.com has analysed the Roci and suggests 4000 km/s, which equates to 4.7 g day.
I used the latter value as a default. You will need to estimate delta-v values for other ships.
See https://en.wikipedia.org/wiki/Delta-v for more information about delta-v.
toughsf.blogspot.com has analysed the Roci and suggests 4000 km/s, which equates to 4.7 g day.
I used the latter value as a default. You will need to estimate delta-v values for other ships.
See https://en.wikipedia.org/wiki/Delta-v for more information about delta-v.
Comparison of travel times with Green Ronin's Expanse RPG
Earth to Jupiter
Consider a trip from Earth to Jupiter (4.2 au in the rules).
The rules give travel times of:
DYN gives the following, using the parameters for the Rocinante, using 4.2 au as the distance:
The numbers are in good agreement.
Using DYN we can see how long it would take using these acceleration and deceleration values, but putting in a coast so that we arrive with empty tanks (i.e. 100% propellant used).
Putting in the higher burns doesn't save you much time because most of the journey has to be coasting to conserve propellant.
The rules give travel times of:
- 257 hours at 0.3 g
- 141 hours at 1 g
- 53 hours at 7 g
- 41 hours at 12 g
DYN gives the following, using the parameters for the Rocinante, using 4.2 au as the distance:
- 257 hours at 0.3 g (68% propellant)
- 141 hours at 1 g (124% propellant!)
- 53 hours at 7 g (330% propellant!)
- 41 hours at 12 g (432% propellant!)
The numbers are in good agreement.
Using DYN we can see how long it would take using these acceleration and deceleration values, but putting in a coast so that we arrive with empty tanks (i.e. 100% propellant used).
- (257 hours at 0.3 g, using 68% propellant)
- 144 hours at 1 g, with a coast of 31 hours
- 95 hours at 7 g, with a coast of 79 hours
- 92 hours at 12 g, with a coast of 83 hours
Putting in the higher burns doesn't save you much time because most of the journey has to be coasting to conserve propellant.
Earth to Pluto
Consider now a trip from Earth to Pluto (38.53 au in the rules).
The rules give travel times of:
DYN gives (for 38.53 au):
If we do the numbers for 100% propellant use we get:
The rules give travel times of:
- 32.4 days at 0.3 g
- 17.8 days at 1 g
- 6.7 days at 7 g
- 5.1 days at 12 g
DYN gives (for 38.53 au):
- 32.4 days at 0.3 g (207% propellant)
- 17.7 days at 1 g (377% propellant)
- 6.7 days at 7 g (999% propellant)
- 5.1 days at 12 g (1308% propellant).
If we do the numbers for 100% propellant use we get:
- 41 days at 0.3 g, with a coast of 26 days
- 36 days at 1 g, with a coast of 31 days
- 34 days at 7 g, with a coast of 33 days
- 34 days at 12 g, with a coast of 32 days
