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Old 03-05-2023, 06:55 PM   #11
Anthony
 
Join Date: Feb 2005
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Default Re: Reactionless drive ships orbital flight in 3rd ed physics?

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Originally Posted by David Johnston2 View Post
Don't think so. Lifting bodies can fly into space provided only that they have enough reactionless thrust to exceed the planetary gravity.
That's what non-lifting bodies can do. A lifting body just needs enough thrust to exceed the drag required to keep from falling, plus the ability to survive extended periods of hypersonic flight.
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Old 03-05-2023, 07:14 PM   #12
Fred Brackin
 
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Default Re: Reactionless drive ships orbital flight in 3rd ed physics?

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That's what non-lifting bodies can do. A lifting body just needs enough thrust to exceed the drag required to keep from falling, plus the ability to survive extended periods of hypersonic flight.
That's while they have enough air to generate lift (or drag). You'll hit practical vacuum eventually and if you're not at a viable orbital speed you'll fall again. Possibly to "skip" off the upper atmosphere and you might have to do this multiple times.

Only the unavailability of sufficiently powerful reactionless drives would make this a truly desirable option.
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Old 03-05-2023, 07:51 PM   #13
Anthony
 
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Default Re: Reactionless drive ships orbital flight in 3rd ed physics?

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Originally Posted by Fred Brackin View Post
That's while they have enough air to generate lift (or drag). You'll hit practical vacuum eventually and if you're not at a viable orbital speed you'll fall again.
Lift (and drag) scale with velocity^2 * atmospheric density, so you just stay at whatever altitude (and atmospheric density) required for your lifting capabilities. This will allow flying all the way up to orbital velocity, though you'll have a plasma sheath at a certain point, which will cause issues unless you have TL^ hull materials and sensors to go with your TL^ drive.
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Old 03-06-2023, 04:51 PM   #14
johndallman
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Default Re: Reactionless drive ships orbital flight in 3rd ed physics?

The Kármán line is the altitude at which going fast enough to support yourself by aerodynamic lift means you're at orbital velocity anyway.

Of course, that means there's enough drag that you'll slow down pretty fast if you stop thrusting. Flying into orbit like that isn't actually practical, just theoretically describable.
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Old 03-07-2023, 09:10 AM   #15
Ulzgoroth
 
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Default Re: Reactionless drive ships orbital flight in 3rd ed physics?

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Originally Posted by johndallman View Post
The Kármán line is the altitude at which going fast enough to support yourself by aerodynamic lift means you're at orbital velocity anyway.

Of course, that means there's enough drag that you'll slow down pretty fast if you stop thrusting. Flying into orbit like that isn't actually practical, just theoretically describable.
The Kármán line was calculated based on the aerospace capabilities of the 1950s, and presumably would be somewhat different if the same calculation was performed today...

Also, why would you stop thrusting? If you reach the Kármán limit flight profile, it doesn't mean you've finished orbital injection, it means that there's no longer much question of whether you can (assuming you're not going to run out of delta-V).
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Old 03-07-2023, 09:26 AM   #16
Fred Brackin
 
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Default Re: Reactionless drive ships orbital flight in 3rd ed physics?

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Originally Posted by Ulzgoroth View Post
The Kármán line was calculated based on the aerospace capabilities of the 1950s, and presumably would be somewhat different if the same calculation was performed today...

.
Yes, but the 1956 Karman line was calculated based on the capabilities of the Bell X-2. That had a thrust-to weight ratio of 3 to 5 fully loaded but better than 1 to 1 just before the fuel ran out.

We can do a little better than that today but not with a payload-carrying craft. The numbers are already far higher than the theoretical winged orbiters from Space 1e.
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Old 03-07-2023, 10:14 AM   #17
Anthony
 
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Default Re: Reactionless drive ships orbital flight in 3rd ed physics?

There's a reason this type of discussion is only used for reactionless drive; even if possible in theory, it's a pretty dumb idea with real world rockets.
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Old 03-07-2023, 10:56 AM   #18
Ulzgoroth
 
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Default Re: Reactionless drive ships orbital flight in 3rd ed physics?

The appeal, of course, would be to be able to use something much more efficient than normal rockets.

But most concept deep-space rockets likely wouldn't work in atmosphere anyway, or have enough thrust to sustain flight.
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Originally Posted by Fred Brackin View Post
Yes, but the 1956 Karman line was calculated based on the capabilities of the Bell X-2. That had a thrust-to weight ratio of 3 to 5 fully loaded but better than 1 to 1 just before the fuel ran out.

We can do a little better than that today but not with a payload-carrying craft. The numbers are already far higher than the theoretical winged orbiters from Space 1e.
I'm not sure thrust to weight is actually relevant to the calculation at all, which (going from the wiki here) was about lift vs. heating.
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Old 03-07-2023, 11:03 AM   #19
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Default Re: Reactionless drive ships orbital flight in 3rd ed physics?

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Originally Posted by Ulzgoroth View Post
I'm not sure thrust to weight is actually relevant to the calculation at all, which (going from the wiki here) was about lift vs. heating.
You need enough thrust to exceed your drag, or you slow down (thus failing to accomplish your purpose), and drag is equal to the weight of the vehicle divided by its lift to drag ratio.
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Old 03-07-2023, 11:31 AM   #20
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Default Re: Reactionless drive ships orbital flight in 3rd ed physics?

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You need enough thrust to exceed your drag, or you slow down (thus failing to accomplish your purpose), and drag is equal to the weight of the vehicle divided by its lift to drag ratio.
Yes, but I don't believe the Kármán line calculation cared about that. It was assessing how fast you could survive going and how fast you had to go to not drop out of the sky, not whether you could actually go that fast.
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