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 Post subject: Getting into orbit
PostPosted: Mon Aug 15, 2016 8:39 am 
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2300 AD (MgT) says it takes a rocket 2 hours to get to low orbit. Doesn't the space shuttle get there in six minutes (or used to)?

So where's the 2 hours coming from? Are they assuming some sort of docking maneuver that requires a lap around the planet? A craft that took off like a plane would probably take longer (those SABRE engines are pretty neat), but I'm guessing still not two hours.


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 Post subject: Re: Getting into orbit
PostPosted: Mon Aug 15, 2016 12:57 pm 
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Second edition 2300AD has a table (p.89 of the Adventurer's Guide) that breaks down travel time to and from orbit. It says that a rocket plane requires 1 hour in travel time ("actual flight time and maneuvering to position in orbit") and 1 hour for "cleanup" ("postflight checks and equipment shutdown"). An hour is also the smallest unit used on the table, so there is presumably some rounding involved.


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 Post subject: Re: Getting into orbit
PostPosted: Tue Aug 16, 2016 2:10 pm 
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thrash wrote:
Second edition 2300AD has a table (p.89 of the Adventurer's Guide) that breaks down travel time to and from orbit. It says that a rocket plane requires 1 hour in travel time ("actual flight time and maneuvering to position in orbit") and 1 hour for "cleanup" ("postflight checks and equipment shutdown"). An hour is also the smallest unit used on the table, so there is presumably some rounding involved.


I can accept a rocket plane taking a while. But an hour? Maybe not that long.

Though I'm an advocate of house rules, so make it your own way. :)

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 Post subject: Re: Getting into orbit
PostPosted: Tue Aug 16, 2016 5:54 pm 
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Don't forget it is more than just getting above the atmosphere. It is getting into a stable orbit at the angle that you want. That can take time. The Space Shuttle used to take a long time to dock with the ISS.

So, you really need 2 situations:

1. Time to get out of the atmosphere and engage the StutterWarp.
2. Time to get into orbit.

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 Post subject: Re: Getting into orbit
PostPosted: Fri Aug 19, 2016 3:09 pm 
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Agreed on all points so far. Except as far as I know once you've done your burn to get orbital velocity, you don't need constant thrust to get the exact orbit you want. I might be wrong (it happens often) but pretty sure getting your orbit right is more along the lines of little corrections and not using up a literal ton of fuel.

There is some assumption in the MgT 2300 that a ship would need an hour's worth of fuel to reach orbit. I mean the listed fuel efficiency is bonkers, so maybe that's their way of handwaving things and making the high-guard rocket rules work for 2300? I dunno


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 Post subject: Re: Getting into orbit
PostPosted: Fri Aug 19, 2016 9:15 pm 
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Matt Wilson wrote:
Agreed on all points so far. Except as far as I know once you've done your burn to get orbital velocity, you don't need constant thrust to get the exact orbit you want.


That's right. The typical thing is to do a burn to get into an orbit tangential with the one you want to be in, then coast for a while, then do another burn at the point/time of tangency. Though things can be a little more complicated if you need to change inclination.

That can very easily require 44 minutes of coasting from perigee to apogee of an orbit to gain altitude, or even more, if you don't have delta-vee to spare, and are doing Hohmann transfer orbits. But only a handful of minutes of total burn time.

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 Post subject: Re: Getting into orbit
PostPosted: Fri Aug 19, 2016 10:29 pm 
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The space shuttle was a marginal rocket-plane, which got itself into orbit in the extravagant manner of a rocket. A true spaceplane would presumably use its "plane" features (wings and a ramjet) to get to an altitude of 30 kilometres or so. Then, with a rocket burn or perhap a high-hypersonic scramjet it would put itself into an orbit with perigee at its current altitude and apogee outside the effective atmosphere, at altitude at least 160 km. Then it would coast to apogee, which would take 44-45 minutes, and perform a rocket burn to circularise its orbit.

That's a lot slower than a rocket's minutes-to-orbit trajectory, but requires much lower thrust-to-mass ratio (smaller, lighter, cheaper motors), uses aerodynamic lift to overcome the gravity drag (less delta-vee, dramatically less fuel), and uses air as oxidiser and reaction mass in the ramjet or scramjet (higher average effective specific impulse, dramatical saving on fuel and mass).

Spaceplanes are supposed to have low operating costs compared with rockets, not high performance.

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