Space Travel Physics

[Cinisajoy]

Not a book but a show.
Have any of you seen Farscape?
It seems to be all Sci-fi rolled into one show. We tend to play now where did they get that plotline.

[davidfor]

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.

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!

I can't remember anyone justifying this on fuel, but know that the long curve have been justified for other reasons. The two reasons I can think of are ship and engine design.

The engine design was a factor because of the difficulty of stopping and starting the engine. In that case, turning the ship to thrust in the other direction becomes an interesting manoeuvre. I know I've read a book that used this as an important point, but I can't think of what it was.

Something like the Bussard Ramjet used in Larry Niven's "Protector" has other problems. The ship was made of three separate sections connected by long cables. It's been a while since I read it, but my memory has that in normal operation, the only way to turn was using long sweeping turns. Some of this was the design of the ship, having to keep the cables taught. But, the ramjet design only works at reasonably high speeds, so other fuels or methods have to be used to get up to operating speeds.

[pdurrant]

Originally Posted by davidfor

I can't remember anyone justifying this on fuel, but know that the long curve have been justified for other reasons. The two reasons I can think of are ship and engine design.

If there are physical reasons for the manoeuvres, that's fine. It's when it's just because it hasn't been thought about that I have a problem.

[wodin]

Yep, I usually just chalk it up to literary license and move on to enjoy the story. Sir Terry Pratchett and Douglas Adams are extreme examples of this.

[crich70]

Not space travel but in Well's "The Invisible Man" the invisible man is totally transparent which would limit the story a bit since it is light impinging on the retina that allows us to see. So either the invisible man is blind or others would see a pair of eyes floating about with no body. Of course Well's had his 'cavorite' to screen a ship from gravity too. At least Verne tried to keep within known science in his stories. Still Mr. Well's books are fun to read even with the errors.

[Greg Anos]

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.

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!

I can think of two reasons.

1. Authorial laziness.

2. Deliberately modelling of an earlier form of transportation to better fit the average reader's (not you Paul) understanding. (They know how ship and airplanes work. . .)

[pdurrant]

Originally Posted by Ralph Sir Edward

I can think of two reasons.

1. Authorial laziness.

2. Deliberately modelling of an earlier form of transportation to better fit the average reader's (not you Paul) understanding. (They know how ship and airplanes work. . .)

From my reading of it, it's 1, sadly.

[ApK]

Originally Posted by Ralph Sir Edward

I can think of two reasons.

1. Authorial laziness.

2. Deliberately modelling of an earlier form of transportation to better fit the average reader's (not you Paul) understanding. (They know how ship and airplanes work. . .)

In Star Wars, it was 2. The starfighter battles were copied from movies of WW II aerial dogfights.

[geekmaster]

Originally Posted by pdurrant

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!

Unless that curved course dips down into a gravity well. That gravity slingshot is what got the Apollo 13 astronauts safely home, without burning a bunch of fuel to do so.

[geekmaster]

Originally Posted by crich70

Not space travel but in Well's "The Invisible Man" the invisible man is totally transparent which would limit the story a bit since it is light impinging on the retina that allows us to see. ...)

Except in the metaphorical sense (i.e. when viewing God): https://en.wikipedia.org/wiki/Transparent_eyeball

[crich70]

Originally Posted by geekmaster

Except in the metaphorical sense (i.e. when viewing God): https://en.wikipedia.org/wiki/Transparent_eyeball

Wrong forum for that discussion.

[pdurrant]

Originally Posted by geekmaster

Unless that curved course dips down into a gravity well. That gravity slingshot is what got the Apollo 13 astronauts safely home, without burning a bunch of fuel to do so.

I could argue that in some sense that was still a straight course.

But the story in question concerned manoeuvres far from any gravity wells.

[geekmaster]

Originally Posted by crich70

Wrong forum for that discussion.

The post I replied to was also "not space travel". I do not see many of the "social fences" that divide threads and forums, so I reply to existing posts. Geeks enjoy playing ball in any field.

[ApK]

Originally Posted by geekmaster

The post I replied to was also "not space travel". I do not see many of the "social fences" that divide threads and forums, so I reply to existing posts. Geeks enjoy playing ball in any field.

Space travel or not was not the issue.

[pdurrant]

Originally Posted by geekmaster

Geeks enjoy playing ball in any field.

But if the ball is politics or religion, you'll need to do it in the P&R forum, not the lounge.