Travel times surface to orbit?

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demonsbane
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Travel times surface to orbit?

Postby demonsbane » Wed May 16, 2018 11:24 pm

A number of quick questions.
1. How long should taking off to orbit take? (say world size 6 with atmosphere)
2. Also the reverse journey?
3. How long does it take to prep for launch?
NOLATrav
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Re: Travel times surface to orbit?

Postby NOLATrav » Thu May 17, 2018 1:57 am

Back in the CT days it was said an air/raft could reach orbit in 1 hr per UWP Size digit - so 6 hrs for a Size 6 world.

The Travel Times table on pg 153 of MgT 2e lists a distance of 10,000 km to orbit, taking 2,000 seconds (appx 34 minutes) at 1G Thrust. One assumes the return journey to be about the same. That same table gives times for various Thrust ratings.

I believe that readying a ship for departure takes 1space combat turn (6 mins) if everyone is onboard and at their stations. Can’t quote the exact reference however. You might rule that it’s only 1D minutes if the power plant and M Drive are already idling, or perhaps 1D minutes per assumed step (firing power plant, firing maneuver drive, charting course and gaining clearance from the port) so 4D minutes plus travel time?...
MonkeyX
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Re: Travel times surface to orbit?

Postby MonkeyX » Thu May 17, 2018 6:55 am

Also don’t forget for a safe jump the ship needs to reach a 100 diameters. That’s about a day or so for mid sized planets at thrust 1.
demonsbane
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Re: Travel times surface to orbit?

Postby demonsbane » Thu May 17, 2018 4:26 pm

Thank you for the answers. So are we taking about linear reduction of time, if the journey was at 2g? I just need to know what to tell the players when they start screaming for help from the ship they just sent back into orbit for safekeeping :cry:
AnotherDilbert
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Re: Travel times surface to orbit?

Postby AnotherDilbert » Thu May 17, 2018 8:10 pm

MT Imperial Encyclopedia had a table:
Image
NOLATrav
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Re: Travel times surface to orbit?

Postby NOLATrav » Fri May 18, 2018 5:47 am

The table above gives similar times as the table in the newest Mongoose rules so there you go. 23 minutes at 2G for a Size 6 world, one way. 46 minutes round trip.

Feel free to add time due to ion storms in the magnetosphere, heavy satellite traffic, star port authorization delays, ionization storms in the stratosphere, heavy commercial air traffic, etc.

You can give them a number that’s realistic but you can also decide when the help actually arrives. Could be sooner than expected, probably will be too late...
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Re: Travel times surface to orbit?

Postby steve98052 » Fri May 18, 2018 4:19 pm

I would say that the time to orbit is at least partially dependent on a maximum atmospheric speed. It's best to limit speed so that the hull doesn't get too hot; even if it can handle the heat, many cycles of hot and cold lead to fatigue that's an issue for a vehicle meant to fly as long as a B-52.

Even if we assume that the hull and even external features are sufficiently heat-resistant that they don't have much of a speed limit, there's the matter of atmospheric ionization that blinds most external sensors, including radio. If your spacecraft is only capable of entering atmosphere at close to orbital velocity, like the Space Shuttle or Apollo capsules, that radio blackout is a necessary difficulty. But a spacecraft with maneuver drives doesn't need to do that, and probably doesn't want to.

In the lower atmosphere, sonic booms are very annoying to people on the ground, and that argues for a speed limit around 85% of the local speed of sound.

Yet another consideration is whether the spacecraft has enough maneuver to directly levitate out of a planet's gravity (maneuver drive G rating higher than the planet's gravity) or requires some help from aerodynamic lift.
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Re: Travel times surface to orbit?

Postby Rikki Tikki Traveller » Tue May 22, 2018 1:41 pm

steve98052 wrote:
Fri May 18, 2018 4:19 pm
I would say that the time to orbit is at least partially dependent on a maximum atmospheric speed. It's best to limit speed so that the hull doesn't get too hot; even if it can handle the heat, many cycles of hot and cold lead to fatigue that's an issue for a vehicle meant to fly as long as a B-52.

Even if we assume that the hull and even external features are sufficiently heat-resistant that they don't have much of a speed limit, there's the matter of atmospheric ionization that blinds most external sensors, including radio. If your spacecraft is only capable of entering atmosphere at close to orbital velocity, like the Space Shuttle or Apollo capsules, that radio blackout is a necessary difficulty. But a spacecraft with maneuver drives doesn't need to do that, and probably doesn't want to.

In the lower atmosphere, sonic booms are very annoying to people on the ground, and that argues for a speed limit around 85% of the local speed of sound.

Yet another consideration is whether the spacecraft has enough maneuver to directly levitate out of a planet's gravity (maneuver drive G rating higher than the planet's gravity) or requires some help from aerodynamic lift.
With gravitic drives, atmospheric speed is not really the issue, the ship simply goes straight up. A car, at 100kph, could read the orbit of the space station in about 2 hours. A streamlined ship should have no problem with normal acceleration rates. Should the ship need to dogfight or chase/be chased, then the maximum speed would become involved. Unfortunately older additions of Traveller tied maximum airspeed to acceleration and that is not the case. It should be tied to hull shape (Streamlining) and probably TL to account for advances in aerodynamics etc. Concepts like super-laminar airflow etc. are not even possible until TL8 (we are just getting into that engineering now...). Acceleration affects how fast you can get to your maximum speed, but not what that speed actually is.

Sorry for the Aerodynamics lecture, but this has always been a pet-peeve of mine!

Rather than define the time to orbit in minutes, I would define it in terms of COMBAT ROUNDS. Likely the only time you need to know to that level of detail is when a Star Destroyer is on your tail as you blast out of Mos Eisley spaceport headed for Alderan.
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steve98052
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Re: Travel times surface to orbit?

Postby steve98052 » Wed May 23, 2018 5:43 pm

Rikki Tikki Traveller wrote:
Tue May 22, 2018 1:41 pm
. . .
With gravitic drives, atmospheric speed is not really the issue, the ship simply goes straight up. A car, at 100kph, could read the orbit of the space station in about 2 hours. A streamlined ship should have no problem with normal acceleration rates. . . .
Yes, just levitating to space with a gravitic drive is a simple solution. Even an unstreamlined ship could travel through atmosphere on a slow gravitic elevator ride.

The catch is if a world's gravity is stronger than the ship's maneuver drive. A Maneuver-0 station-keeping drive couldn't lift off from (or land softly on) even a large asteroid. A fractional-G slow cargo shuttle could only use gravitic levitation through the atmosphere of small, low-gravity worlds. A Maneuver-1 ship couldn't use gravitic levitation on a world with higher than 1 G surface gravity. And it takes a fairly stout maneuver drive to do wilderness refueling with gravitic levitation; Jupiter's gravity at its cloud tops is about 2.4 G.

But with maneuver drive and aerodynamic lift, a ship can land on worlds with stronger gravity. In that case, streamlining becomes necessary, and the ship must fly fast enough to generate lift, as well as slow enough to avoid thermal damage to external features.

Gas giant wilderness refueling should probably have tighter rules than it does; larger ships should probably be built in the form of high-maneuver streamlined breakaway hull fuel shuttles, with the bulk of the ship waiting in orbit.
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Re: Travel times surface to orbit?

Postby Rikki Tikki Traveller » Wed May 23, 2018 6:44 pm

Even a ship with an M-Drive less than the gravity of the planet can still takeoff and land, as long as there is an atmosphere.

The Streamlined feature should include lifting surfaces that can generate lift that exceeds the thrust. A typical aircraft (streamlined with Aerofins) can generate 20:1 ratios, so a 1G trust would have no problem with any planet, or gas giant, that Players are likely to encounter.

Personally, I would use 5:1 for Streamlines and 2:1 for Partially-Streamlined, and 1:1 for Unstreamlined. This is in a Standard density Atmospeheric pressure. For Thin atmospheres, halve those values, (not less than 1:1) and for Dense Atmospheres, increase by 1.5. Very Thin would be 1/4 for example.

These are just quick estimates of course if you need something quick.

If you have actual Atmospheric densities and gaseous composition, there are formulas in Aerodymaic texts that can be used to determine Lift to Drag ratios.

FYI the Space Shuttle had an L/D ratio of 1.5 - barely Partially Streamlined - Astronauts called it a Flying Brick.
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Condottiere
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Re: Travel times surface to orbit?

Postby Condottiere » Fri May 25, 2018 1:26 am

https://www.youtube.com/watch?v=S3QfbtrfBjA

“Stratolaunch” is a 500,000-pound beast with twin fuselages and a wingspan of 385 feet- and it just made another huge step in the space industry!
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Re: Travel times surface to orbit?

Postby steve98052 » Sun May 27, 2018 4:29 pm

Rikki Tikki Traveller wrote:
Wed May 23, 2018 6:44 pm
Even a ship with an M-Drive less than the gravity of the planet can still takeoff and land, as long as there is an atmosphere. . . . [emphasis mine]
Good point about atmosphere. Airless worlds with more than 1 G surface gravity are probably rare, but don't try to land on one without enough maneuver drive!
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Re: Travel times surface to orbit?

Postby Rikki Tikki Traveller » Wed May 30, 2018 2:21 pm

steve98052 wrote:
Sun May 27, 2018 4:29 pm
Rikki Tikki Traveller wrote:
Wed May 23, 2018 6:44 pm
Even a ship with an M-Drive less than the gravity of the planet can still takeoff and land, as long as there is an atmosphere. . . . [emphasis mine]
Good point about atmosphere. Airless worlds with more than 1 G surface gravity are probably rare, but don't try to land on one without enough maneuver drive!
I would argue that ATM E is also problematic. Deep atmosphere in the trenches, but near-vacuum at the surface. These could be common on larger worlds - although I agree it should be pretty rare for large worlds (larger than Earth) to not have a decent atmosphere.
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steve98052
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Re: Travel times surface to orbit?

Postby steve98052 » Thu May 31, 2018 3:38 pm

Rikki Tikki Traveller wrote:
Wed May 30, 2018 2:21 pm
. . . I agree it should be pretty rare for large worlds (larger than Earth) to not have a decent atmosphere.
If I had time, I might look through the world generation formulas to see whether it's even possible to get an atmosphere too thin for aerodynamic lift on a large world.

If I really had a lot of time, and knew how to read the master list of canonical worlds, I might dump them into a spreadsheet to find whether any actually exist in canon.
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Re: Travel times surface to orbit?

Postby Tom Kalbfus » Fri Jun 01, 2018 6:32 am

demonsbane wrote:
Wed May 16, 2018 11:24 pm
A number of quick questions.
1. How long should taking off to orbit take? (say world size 6 with atmosphere)
2. Also the reverse journey?
3. How long does it take to prep for launch?
Depends on how quickly you are accelerating to reach orbital velocity.
Image
G = 6.674×10^−11 m3⋅kg−1⋅s−2
M⊕ = 5.976×10^24 kg.
R⊕ = 6,378,000 m
Velocity = 7907.8084 meters per second.
It takes 791 seconds rounded up to accelerate to that velocity at 1g or about 13 minutes.
steve98052
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Re: Travel times surface to orbit?

Postby steve98052 » Mon Jun 04, 2018 4:50 pm

Tom Kalbfus wrote:
Fri Jun 01, 2018 6:32 am
. . .
It takes 791 seconds rounded up to accelerate to that velocity at 1g or about 13 minutes.
Does that include overcoming the pull of gravity toward the planet?
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Re: Travel times surface to orbit?

Postby AnotherDilbert » Mon Jun 04, 2018 5:38 pm

steve98052 wrote:
Tom Kalbfus wrote: . . .
It takes 791 seconds rounded up to accelerate to that velocity at 1g or about 13 minutes.
Does that include overcoming the pull of gravity toward the planet?
No, nor atmospheric resistance.
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Re: Travel times surface to orbit?

Postby Tom Kalbfus » Tue Jun 05, 2018 4:14 am

AnotherDilbert wrote:
Mon Jun 04, 2018 5:38 pm
steve98052 wrote:
Tom Kalbfus wrote: . . .
It takes 791 seconds rounded up to accelerate to that velocity at 1g or about 13 minutes.
Does that include overcoming the pull of gravity toward the planet?
No, nor atmospheric resistance.
Assuming there is an atmosphere, not all planets have one.

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