Travel - DropShips and co.

Pages: 1
MikeXL
05/05/20 10:45 AM
46.5.17.65

Edit Reply Quote Quick Reply
As I could not really find any (exact) information about it, I hope you guys can help me out.
My question is more a general one, concerning the travel speed and time from and to a jump point within one system (to a destination in the system). Information about it can be found here: DropShip (can't post a link, search in the wiki).

About travel time it says: T = 2 * sqrt( 2 * (D/A))
An example would be the dropship Achilles and the distance of Sun to Earth (= 1 AU), which would be a common jump point (below star) for Terra.

D = 74798935km (half of 1AU)
A = 4g (safe thrust, g = 10m/s, so its simple ) = 4 * 10 * 12960 km/h² (so it's km/h² and not m/s²) = 518400 km/h²
T = 2*sqrt( 2* ( 74798935km / 518400 km/h² ) = ~34h = ~1,4 days

There is a lot of information about how planets are certain distance in time from a jump point, for example: Abiy_Adi , it says 9.12 days, that would mean the planet is like 21AU (flying with Achilles) to the jump point and in turn - as most jump points are above or below a star - that distance from the star.

I'm not sure if I missed something somewhere, can you guys help me out and fill the gaps please.

- What is the planet information (jump point distance) and why is it a fixed time?
- Does ships sheet thrust g actually mean gravitational acceleration ~9.8 m/s² ?
- Are my assumtions so far correct? Did I miss something?



Thanks in advance.
ghostrider
05/05/20 12:05 PM
66.74.60.165

Edit Reply Quote Quick Reply
I can only give your some answers to this, but not that technical.
The distance to the jump point is giving for a normal 1 g burn, and not always under thrust.
Dropships will produce thrust for so long, then coast. It will turn around to have it's bottom/backside towards the planet then start slowing down with another 1 g force. Done correctly, you end up at your destination.

Most civilian transports will not go faster then their 1 g burn and normal people are not used to it, and some may even get hurt. In an emergency, they will probably exceed it though.

Military can do a higher burn, but it does effect the body when doing so for a long period, such as a transit from jump point to a world.
Something to think about when using higher then 1 g burns. Troops are fatigued when they get back into the 1g/0g state like orbiting a world or even landing on one. It also burns more fuel, which for more then a few ships isn't something you tend to have a lot of. But the fuel issue is more economic, or if you are really pushing the normal game, something that could strand you on world.
Most civilized worlds have some fuel for sale, though prices vary, but since the Achilles can't land, you better hope they have refueling ships that can dock with it to refuel it.

The G-force is from the acceleration or decelaration of the ship. It is the same concept of being thrown into a wall if the ship thrusts to one side for some reason.

Hope that answers you questions. If anyone else has a correction or more information, I hope they add it here.
CrayModerator
05/06/20 09:30 PM
71.47.151.234

Edit Reply Quote Quick Reply
Quote:
As I could not really find any (exact) information about it, I hope you guys can help me out.
My question is more a general one, concerning the travel speed and time from and to a jump point within one system (to a destination in the system). Information about it can be found here: DropShip (can't post a link, search in the wiki).

About travel time it says: T = 2 * sqrt( 2 * (D/A))



Yep. That's a form of equation 3b in this link:
Engineering Toolbox Acceleration and Velocity Equations

It's applied to the special situation of a spaceship that accelerates for half its flight distance, flips, and brakes for the remainder.

Quote:

An example would be the dropship Achilles and the distance of Sun to Earth (= 1 AU), which would be a common jump point (below star) for Terra.

D = 74798935km (half of 1AU)
A = 4g (safe thrust, g = 10m/s, so its simple ) = 4 * 10 * 12960 km/h² (so it's km/h² and not m/s²) = 518400 km/h²
T = 2*sqrt( 2* ( 74798935km / 518400 km/h² ) = ~34h = ~1,4 days



Looks correct at first glance. I seem to recall 1G takes about 2.something days for 1AU, twice as long as 4Gs.

Quote:
There is a lot of information about how planets are certain distance in time from a jump point, for example: Abiy_Adi , it says 9.12 days, that would mean the planet is like 21AU (flying with Achilles)



If you're still at 4G, yes, that's correct. BattleTech's published transit times assume 1G, and you're given the above equation if you want to try a different acceleration level for a rushed military assault.

Quote:

- What is the planet information (jump point distance) and why is it a fixed time?




The standard jump points, as you noted, are above and below the poles of the stars, while the planets orbit the equator. For a fairly circular orbit, the planet's distance from a standard jump point isn't going to vary. For example, Earth is always 1AU from Sol; Sol's zenith and nadir jump points are always 10.2AU from the poles of the sun. The distance, ignoring the slight eccentricity of Earth's orbit, is constant.

So, for a 1G transit from the jump point to planet the travel time is a fixed 9.12 days.

In fact, because of that square root factor there's not a lot of variation with increasing orbital distance for the planets. Strategic Operations p. 133 looks at transit times for Terra's sister planets from the zenith/nadir points at 1G. (Page 132 has a map for the transits below, and p. 259 has a detailed set of examples of system transit calculations.)

Transit time to Mercury: 9.10 days
Transit time to Venus: 9.11 days
Transit time to Terra: 9.12 days
Transit time to Mars: 9.15 days
Transit time to Jupiter: 9.23 days
Transit time to Saturn: 10.66 days or 2.21 days from the nearest edge of the proximity limit
Transit time to Uranus: 13.29 days or 6.69 hours. Uranus is outside Sol's proximity limit and has its own, tiny proximity limit.
Transit time to Neptune: 16.06 days or 6.96 hours.

Quote:
- Does ships sheet thrust g actually mean gravitational acceleration ~9.8 m/s² ?



Yes. Standard transit times assume 1G at 9.8m/s/s. You can use the equation you cited to calculate alternate transit times, but above 2Gs isn't really bearable for days on end.
Mike Miller, Materials Engineer

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
MikeXL
05/13/20 09:15 PM
46.5.17.40

Edit Reply Quote Quick Reply
That's exactly the information I was looking for, thank you very much for your efforts.
MikeXL
05/14/20 06:09 PM
95.208.250.253

Edit Reply Quote Quick Reply
After playing around with the numbers i found an error and had to verify everything. The formula stated in the wiki seems to be wrong.

What it says in the wiki: T = 2 * sqrt( 2 * (D/A))
What it should say: T = 2 * sqrt(D/A)

The reason here is simple, we take only half of the way as accelerate after which we deaccelerate, so it is basically 1/2 * D resulting in removal of the 2. The first 2 is now our second part of the way (time to accelerate = time to deaccelerate).

I verified it with the numbers from Strategic Operations p. 133 as Cray mentioned above.
CrayModerator
05/14/20 09:03 PM
71.47.193.139

Edit Reply Quote Quick Reply
Quote:
What it should say: T = 2 * sqrt(D/A)

The reason here is simple, we take only half of the way as accelerate after which we deaccelerate, so it is basically 1/2 * D resulting in removal of the 2. The first 2 is now our second part of the way (time to accelerate = time to deaccelerate).

I verified it with the numbers from Strategic Operations p. 133 as Cray mentioned above.



Fixed, I think: https://www.sarna.net/wiki/DropShip#DropShip_Operations
Mike Miller, Materials Engineer

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
Pages: 1
Extra information
1 registered and 39 anonymous users are browsing this forum.

Moderator:  Nic Jansma, Cray, Frabby, BobTheZombie 

Print Topic

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is enabled
      UBBCode is enabled

Topic views: 2158


Contact Admins Sarna.net