Starship Acceleration in The Mote In God's Eye

There are several instances in pages 40-50 of The Mote in God’s Eye where the people talk about constantly accelerating to their destination. I can barely continue for want of more information. These are starships. In space. Why would they need to constantly accelerate? And if they wanted to make the most speed and yet still come back to a standstill at the other end (wtithout smearing the humans inside), they’d need to accelerate about halfway there and decelerate the other half (assuming they are in space the whole time). That’s my understanding of space physics, anyway.

What is it that I’m missing about physics or this universe in particular that forces these starships to accelerate the whole way? It’s really bugging me and taking me out of the story every time someone says something like “But you don’t understand, we’ll be accelerating at high gees all the way to intercept.” and “Warships are designed for long periods of high gravity [due to their acceleration]” Every time, my mind starts yelling, “There’s no friction to speak of in space! Why are you speeding up the whole way there and how are you going to stop without smearing everybody?!”

Unless they are matching the destination velocity, they are accelerating all the way, relative to the ship. But you turn the ship around half way and decelerate relative to the destination so the vector is always the same relative to the ship.

(Unless you’re in The Forever War, where the crew is totally encased in liquid, so the acceleration vector can be applied in any direction. (By the description in the book, it was NOT a pleasant way to travel).)

As for accelerating the whole way, if you have the fuel, you can get to where you are going much quicker. You can do Apollo method, with massive acceleration at the beginning and coast. Or instead accelerate half way, turn around, and decelerate the rest of the way to the moon, having your relative velocity reach zero at touchdown. But your fuel tank would be huge. If you could do it at one-gee, it would probably take much less that the three days it took Apollo. If you could sustain 9 gees all the way, you could probably make it in several hours.

You’re right about space physics, but one additional factor is:

Accelerating as much as is survivable until halfway to wherever you’re going and then decelerating as much as survivable the rest of the way is expensive in fuel, and risks damaging the crew, but it’s by far the fastest way to get from one place to another, which is often a high priority for warships.

The thing is, they specifically say, “we’ll be accelerating at high gees all the way to intercept.” Which means they won’t be spending hardly any time decelerating as far as I can tell. And I would imagine that would smear everybody inside since they’d be going a ridiculously fast speed from constantly accelerating the whole way there and then doing a hard stop at the end.

They do mention that it burns massive amounts of fuel. They also mention they need to get places very fast. So accelerating at maximum human survivable Gs for half the trip makes sense to me, and those conditions match. But there’s no indication that they’re decelerating at maximum human survivable Gs the other half of the way. The only way they’d survive a quick deceleration right there at destination is if they’re dealing with some sort of slowdown or friction that the constant acceleration is overcoming (like in cars down on earth) so that their speed isn’t crazy high.

I haven’t read the books, but I think I understand the second quote, anyway.

As you said, for the first half of the trip the ship is accelerating at, let’s say, 1G. At the halfway point they (presumably) flip the ship 180˚ and start decelerating at 1G. When they arrive at their destination, they’ll be motionless.

But to the passengers, and to the structural members of the ship, the two halves of the trip are identical. The passengers feel Earth-normal gravity throughout. The ship is under exactly the same strain throughout.

ETA: Wow! Double-ninjaed.

Do they define what they mean by “intercept?” Is it necessarily referring to when they meet their target?

I just checked out the scene in question - what’s happening is that the human ship is trying to match speeds with an alien ship that is going very fast away from them - so forward acceleration all the way makes sense, because they don’t have to stop at the end of the trip - what they need is to be going at the same (very fast) speed as the alien ship.

I thought they had FTL in that novel. It’s been years since I read it.

Deceleration is acceleration; it’s just acceleration in the opposite direction. I would grant the benefit of the doubt and assume that’s what was meant.
ETA: Ah, I see Andy L has the answer. Overall the point stands, though: deceleration literally is acceleration.

I know you didn’t see my reply before you posted, but just to clarify.

If the ship is accelerating throughout the first half and decelerating throughout the second half, there is no difference, other than direction, between the two halves. The forces acting on the ship and the people are the same. So it’s easier to say, “We’ll be accelerating the whole way,” rather than “We’ll be accelerating half the way and then we’ll be decelerating the rest of the way and the same forces will be acting on us.”

ETA: Thank you, Andy L. Makes sense.

Although the OP said it happens throughout the book. In which case, what VarlosZ and I said.

AH-HA. OK, I think I understand what these folks are saying now. Deceleration is just defined as acceleration going the opposite way of the current vector - it’s still acceleration, just opposite. So yes, they would be “accelerating” the whole way there, just half way there they’d be doing it in the opposite way they were before. I was conflating my earth-term movements with space movements and thinking “acceleration” always meant forward (and forward is a relative term, at that).

Gosh, it would’ve been simpler if he just defined it as acceleration and deceleration for my 21st century earth mind. They probably got out of using “deceleration” as a term as soon as they developed these space engines, in his universe, but I wouldn’t know that.

ETA: VarlosZ you replied before I could finish typing out why it finally clicked for me!

carnivourousplant, they did have FTL, but it was very restricted. If you wanted to go from Star A to Star B, there was a single point somewhere in Star A’s solar system that you had to go to, then turn on the Alderson Drive, and you would instantaneously appear at a corresponding point in Star B’s system. You absolutely could not use it to just putter along wherever you wanted. They even said that depending on the geometry of the surrounding systems, it could be easier to get from two points in a system by going to transfer point AB, transiting from Star A to Star B, move to transfer point BC, transit to Star C, move to point CA, then transit back to Star A, winding up at a point closer to where you want to go.

And a significant plot point was the fact that the only point in the alien’s home system where you could use an Alderson drive would deposit you inside the corona of a star.


There is one part where they’re going from an Alderson Point to a refueling base, and they did mention deceleration: “MacArthur decelerated at nearly three gravities directly into orbit around Brigit”

On the next page someone mentions, “But you don’t understand, we’ll be accelerating at high gees all the way to intercept.” (this time in reference to arriving at or near an alien craft). At the time they hadn’t even mentioned what they were doing exactly. Later they mention that they will be directly intercepting the alien craft, which means matching speeds.

Then when they talked about intercepting the alien craft they talked about intercepting at either 4 Gs or 2.5 Gs. (“Right into the sun at 4 gees!” and “I can pull alongside in one hundred and seventy hours without ever going over 2.5 gees” are the exact quotes of the two choices) which confused me more - the alien ship can only be traveling at a single acceleration, and he just gave two different accelerations, so only one of those intercept accelerations could be correct. I expected a “deceleration” to be mentioned again somewhere in there since they were matching trajectories and the 4G one was implied to be faster than the alien ship was going.

So I can see why I got confused - deceleration is a term they use sometimes, so I didn’t get why they were only using it in what I perceived as some instances and not others. I should just relax about all the technicalities, I guess.

One of my favourite books!

You probably got confused whenever Pournelle was explaining it, and got it whanever Niven was doing the explaining. :wink:

It’s true, I never had trouble with Niven’s explanations in any of his other books. Pournelle’s the mind behind this universe and its space drives, right? Darn that Pournelle! :stuck_out_tongue:

Except that Niven’s grasp of optics is crap.

In the books I’ve read, anyway, using “acceleration” as the catch-all term is pretty standard. It’s something I’ve run across a lot.