What is the average orbital speed of an unladen Scarab SRV?
Let's find out... (or scroll to the end to get to the practical answer)
I was recently introduced to the technique of SRV "flyving," where you use tilt-boosting, well placed bounces on the ground and plenty of SRV repair synthesis materials to gain far more speed than the SRV can reach driving on the ground. I even managed to drive my SRV into orbit around Enceladus, the results of which you can see in the linked video. I am definitely not the first to do this, there are other videos and a useful infographic around showing how to do the flyving.
Now, using wikipedia and math (or, a little bit quicker, Wolfram Alpha), we can figure out that Enceladus' orbital velocity at the surface is only 169 m/s. And sure enough, if I speed up my ship to 169 m/s above Enceladus, I don't fall down. Strangely enough, with the SRV I needed to get up to around 1100 m/s to keep a stable orbit. This is due to a downforce the SRV applies on low-g worlds to maintain a comfortable driving experience.
Well, orbit achieved despite that downforce, so mission accomplished. I didn't want to stop there though, I wanted to be able to predict for any world I visit how fast I would need to go to maintain orbit in the SRV.
SRV Assisted Gravity
The Scarab page on the Elite Dangerous wiki explains that this downforce results in an artificial, or assisted, gravity which is the average of 1 g and the planet's natural gravity. Before I found that article I also learned of this through a number of drop test experiments with the SRV from a high altitude on top of another CMDR's ship, which CMDR @Aeolessa was kind enough to help me with. (Thanks!)
So here's how to get the assisted gravity experienced by an SRV on a low-g world:
a = (g0 + g) / 2
Measuring gravity
Elite doesn't give us very accurate numbers for gravity on low-g worlds. If we want to get a decently accurate orbital velocity, we need to measure gravity more accurately. We can do this with a drop test in any ship:
g = 2*d / t^2
Orbital Velocity
To get the orbital velocity we need to know the acceleration due to gravity and the radius, then we can use the formula of circular acceleration:
a = v^2 / r
We can substitute for a and g0 here and add in a 1000 multiplier to make it ready to use for SRVs with a radius in kilometers:
v = sqrt( 1000 * r * ( 9.81 + g ) / 2 )
Putting it all together
So when you visit a planet and decide you might want to orbit it in your SRV, here's the practical version of how to get the orbital velocity.
1. Drop from a standstill from 900 meters to 400 meters (a 500 meter drop) and measure the time:
g = 2*500 / t^2
Or: If you have an accurate enough number in Gees (such as from a journal entry), convert the G to acceleration: g = 9.81 * G
2. Calculate the orbital velocity:
v = sqrt( 1000 * r * ( 9.81 + g ) / 2 )
For Enceladus, my drop test of 500 meters gave me a time of 96.9 seconds. Enceladus has a radius of 252.1 km.
Another takeaway from this when looking for a planet is that as long as the gravity is low, the biggest factor in the speed needed to reach orbit is the radius, not the planet's natural gravity. A very small, 200 km metal rich world with 0.08 g needs 1029 m/s to orbit where a larger 400 km ice world with 0.01 g needs 1408 m/s. Go for a small radius first and foremost for an easier time getting to orbit!
If you've gotten this far, thanks for reading! I hope you found this post interesting and/or useful and if you're inspired to try orbiting a planet yourself, flyve safe! o7
Let's find out... (or scroll to the end to get to the practical answer)
I was recently introduced to the technique of SRV "flyving," where you use tilt-boosting, well placed bounces on the ground and plenty of SRV repair synthesis materials to gain far more speed than the SRV can reach driving on the ground. I even managed to drive my SRV into orbit around Enceladus, the results of which you can see in the linked video. I am definitely not the first to do this, there are other videos and a useful infographic around showing how to do the flyving.
Now, using wikipedia and math (or, a little bit quicker, Wolfram Alpha), we can figure out that Enceladus' orbital velocity at the surface is only 169 m/s. And sure enough, if I speed up my ship to 169 m/s above Enceladus, I don't fall down. Strangely enough, with the SRV I needed to get up to around 1100 m/s to keep a stable orbit. This is due to a downforce the SRV applies on low-g worlds to maintain a comfortable driving experience.
Well, orbit achieved despite that downforce, so mission accomplished. I didn't want to stop there though, I wanted to be able to predict for any world I visit how fast I would need to go to maintain orbit in the SRV.
SRV Assisted Gravity
The Scarab page on the Elite Dangerous wiki explains that this downforce results in an artificial, or assisted, gravity which is the average of 1 g and the planet's natural gravity. Before I found that article I also learned of this through a number of drop test experiments with the SRV from a high altitude on top of another CMDR's ship, which CMDR @Aeolessa was kind enough to help me with. (Thanks!)
So here's how to get the assisted gravity experienced by an SRV on a low-g world:
a = (g0 + g) / 2
- a = assisted gravity
- g0 = standard gravity (1 g)
- g = planet's natural gravity
Measuring gravity
Elite doesn't give us very accurate numbers for gravity on low-g worlds. If we want to get a decently accurate orbital velocity, we need to measure gravity more accurately. We can do this with a drop test in any ship:
- Hover ship above the surface at 900 meters
- Set Flight Assist off and start a timer
- Stop the timer when you reach 400 meters. Then turn flight assist on so you don't crash into the surface.
g = 2*d / t^2
- g = acceleration due to gravity (m/s^2, natural gravity)
- d = displacement (m, 500 if you do the drop test above)
- t = time (s)
Orbital Velocity
To get the orbital velocity we need to know the acceleration due to gravity and the radius, then we can use the formula of circular acceleration:
a = v^2 / r
- a = acceleration (m/s^2, the assisted acceleration experienced by the SRV)
- v = orbital velocity (m/s, what we would like to find out)
- r = radius (m)
We can substitute for a and g0 here and add in a 1000 multiplier to make it ready to use for SRVs with a radius in kilometers:
v = sqrt( 1000 * r * ( 9.81 + g ) / 2 )
- v = orbital velocity (m/s)
- r = radius (km)
- g = acceleration due to natural gravity (m/s^2, this is the number we measure in a drop test)
Putting it all together
So when you visit a planet and decide you might want to orbit it in your SRV, here's the practical version of how to get the orbital velocity.
1. Drop from a standstill from 900 meters to 400 meters (a 500 meter drop) and measure the time:
g = 2*500 / t^2
- g = acceleration due to gravity (m/s^2, natural gravity)
- t = time (s)
Or: If you have an accurate enough number in Gees (such as from a journal entry), convert the G to acceleration: g = 9.81 * G
2. Calculate the orbital velocity:
v = sqrt( 1000 * r * ( 9.81 + g ) / 2 )
- v = orbital velocity (m/s)
- r = radius (km)
- g = acceleration due to gravity (m/s^2, natural gravity)
For Enceladus, my drop test of 500 meters gave me a time of 96.9 seconds. Enceladus has a radius of 252.1 km.
- g = 2 * 500 / 96.9^2 = 0.107 m/s^2
- v = sqrt( 1000 * 252.1 * ( 9.81 + 0.107 ) / 2 ) = 1118 m/s^2
Another takeaway from this when looking for a planet is that as long as the gravity is low, the biggest factor in the speed needed to reach orbit is the radius, not the planet's natural gravity. A very small, 200 km metal rich world with 0.08 g needs 1029 m/s to orbit where a larger 400 km ice world with 0.01 g needs 1408 m/s. Go for a small radius first and foremost for an easier time getting to orbit!
If you've gotten this far, thanks for reading! I hope you found this post interesting and/or useful and if you're inspired to try orbiting a planet yourself, flyve safe! o7
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