Space Travel Physics

[pdurrant]

A thread to discuss what authors get wrong with space travel.

Prompted by the Jack Campbell "Lost Fleet" series.

[pdurrant]

Well, Campbell gets lots and lots wrong, but the main thing for me is that he uses limitations of sea-going vessels or of aircraft for his spaceships.

The main one is his implicit assumption (eventually stated explicitly) that space ships save fuel by changing course gradually, so that their 'speed' relative to whatever situation they're in remains high. The most wasteful course, according to Campbell, is to directly reverse course.

This is sort-of true for waterships and aircraft. But not spaceships. They don't have anything for their wings or hull to press against, and so for spacecraft the most efficient was to move back the way they came (outside gravity wells) is to just flip end for end and accelerate directly in the opposite direction.

A curved course for a spacecraft is wasteful way to reverse course, as you're accelerating perpendicular to the way you want to go, twice!

[wodin]

Originally Posted by pdurrant

... They don't have anything for their wings or hull to press against...

Of course they do, how do you think hyperdimentional improbability drives work, anyway? They push on the quintessence!

[pdurrant]

Originally Posted by wodin

Of course they do, how do you think hyperdimentional improbability drives work, anyway? They push on the quintessence!

And if an author wishes to have space ships that push on the quintessence, I'll happily accept it.

It's not getting the physics right when you're describing well-known physics that irks.

[dwig]

Originally Posted by pdurrant

Well, Campbell gets lots and lots wrong, but the main thing for me is that he uses limitations of sea-going vessels or of aircraft for his spaceships.
...

For me, this is a characteristic of "Space Opera", which tends to be more fantasy than fact concerning space travel.

One common "error" in such Space Opera that gives me a chuckle is how the authors have spacecraft "flying around" as if they were aircraft.

These don't disturb me when I know in advance that I'm dealing with Space Opera. They annoy me when the authors work in fantastic and false reasons for the behavior trying to make the story into "hard science".

[pdurrant]

Originally Posted by dwig

One common "error" in such Space Opera that gives me a chuckle is how the authors have spacecraft "flying around" as if they were aircraft.

I wouldn't class it as Space Opera, just as badly done science fiction.

[dwig]

Originally Posted by pdurrant

I wouldn't class it as Space Opera, just as badly done science fiction.

Yes, when the spacecraft fly like airplanes while in space appears in otherwise "realistic" SF it just signals "bad author, bad author, no donut!", and it annoys me. In true Space Opera I expect such things and they don't disturb.

One in the "Bad SF" errors that I found laughable, even as a kid during the first run, is the repeated crisis of the "decaying orbit" in Star Trek/TFG. Here's an otherwise sensible SF structure (space craft that are true spacecraft and not single surface-to-interstellar space vehicles like the steel behemoths often seen in 1950s pulp, ...) where the navigators can't place the craft in a high enough orbit so that the decay rate is measured in decades or centuries instead hours, which would mean they were actually skimming the very high atmosphere.

[wodin]

Originally Posted by dwig

... which would mean they were actually skimming the very high atmosphere.

Or maybe the quintessence.

[pdurrant]

Originally Posted by dwig

Yes, when the spacecraft fly like airplanes while in space appears in otherwise "realistic" SF it just signals "bad author, bad author, no donut!", and it annoys me. In true Space Opera I expect such things and they don't disturb.

One in the "Bad SF" errors that I found laughable, even as a kid during the first run, is the repeated crisis of the "decaying orbit" in Star Trek/TFG. Here's an otherwise sensible SF structure (space craft that are true spacecraft and not single surface-to-interstellar space vehicles like the steel behemoths often seen in 1950s pulp, ...) where the navigators can't place the craft in a high enough orbit so that the decay rate is measured in decades or centuries instead hours, which would mean they were actually skimming the very high atmosphere.

Yes, that was very weird. Unless, perhaps, they needed a very low orbit for transporters to work? (I know, I'm trying to justify a plot device with hand-waving talk about another plot device.)

[dwig]

Originally Posted by pdurrant

Yes, that was very weird. Unless, perhaps, they needed a very low orbit for transporters to work? (I know, I'm trying to justify a plot device with hand-waving talk about another plot device.)

To hand wave your hand waving...

... but the transports work at any time. This means they either work over a great enough range that the ship can be in orbit on the opposite side of the planet or that the ship is in Clark orbit (I think geostationary should only refer to Gaia/Earth). If the planet has adequate mass (a given since all planets they transport down to have near Earth-like gravity) such orbits are rather large and if the atmosphere extended out that far, in order to degrade an obit, the pressure at the planet's surface would be too extreme for the away team to survive.

[Ravensknight]

Originally Posted by pdurrant

Well, Campbell gets lots and lots wrong, but the main thing for me is that he uses limitations of sea-going vessels or of aircraft for his spaceships.

This is sort-of true for waterships and aircraft. But not spaceships. They don't have anything for their wings or hull to press against, and so for spacecraft the most efficient was to move back the way they came (outside gravity wells) is to just flip end for end and accelerate directly in the opposite direction.

A curved course for a spacecraft is wasteful way to reverse course, as you're accelerating perpendicular to the way you want to go, twice!

But doesn't their mass keep going on Course X? So even if they flip over, they have to overcome their inertial mass to reach zero movement on Course X, then accelerate on Course -X?

Excuse my ignorance here. I find this really interesting and thank you for starting this thread up.

[crich70]

One bit of physics that was shown wrong I think was in the one Star Trek movie where they show a bottle of champaign breaking against the hull of the ship. I mean the bottle was stoppered in a one atmosphere environment and in orbit there is no atmospheric pressure so why didn't it explode? Granted I might be off on that somehow, but it doesn't make sense to me. And even if the glass is able to take the pressure difference the champaign was liquid when the bottle broke and even orbital space is some -200 degrees. I also have to wonder about the engines of the ship in 2001. Why so large? Surely ion engines would be small and the ship would accelerate over time. Not sure if it was the script writers or the film makers who got those things wrong.

[BenG]

Isaac Asimov once wrote that he doesn't believe faster-than-light travel is possible, and he really respects those writers who limit their ships to sub-light speeds, but he uses FTL ships in his own fiction because it makes for a good story.

[pdurrant]

Originally Posted by Ravensknight

But doesn't their mass keep going on Course X? So even if they flip over, they have to overcome their inertial mass to reach zero movement on Course X, then accelerate on Course -X?

Excuse my ignorance here. I find this really interesting and thank you for starting this thread up.

Yes, they do need to overcome their momentum in their current direction and reverse it to go the other way. But the most economical way to do this is to directly reverse course, not to start accelerating sideways, keeping the same speed but changing your direction of travel until you've turned 180 degrees and started moving in the opposite direction, to do that requires the same acceleration in the direction you want to go *and* the sideways acceleration one way and back again.

[ApK]

Originally Posted by pdurrant

Yes, that was very weird. Unless, perhaps, they needed a very low orbit for transporters to work? (I know, I'm trying to justify a plot device with hand-waving talk about another plot device.)

That was always how I rationalized it to myself. They needed to be geostationary, but much lower than a geostationary orbit, so "standard orbit' was actually a very low, highly powered flight mode. It helped me get to sleep.

My personal peeve is space ships coming to a 'stop' when the engines quit.

To be fair, the realities of space flight and orbital mechanics can really make for a dead slow story. I mean, 'The Martian' did it, but that one transfer orbit took years...not every story can support that....

Maybe I'll do story that establishes:
"The ship activated its FTL drive and jumped into hyperfield underspace, where, by an amazing coincidence, ships handled almost exactly like airships in an atmosphere...."