Space Travel & Gravity Fields

[GeeKay]

I'm not sure if this is the correct forum to ask this question, given that it's more concerned with science fiction than science fact. Still, it does bring into play Newton's field equations. So here goes: assuming a spaceship or probe of some kind is travelling between (say) the Earth and the Moon and it happens to have a suitably advanced propulsion system that enables it to maintain a continual thrust throughout the voyage; this being so, would the craft make its mid-voyage 'turnover' at the halfway point between the Earth and Moon? Or would it be more advantageous to perform this manoeuvre at the gravitational 'null point' between these two bodies - that is to say, where their gravity fields cancel out? Or is the answer, as I suspect, rather more complicated than that?

 

 

[BearOfNH]

If your propulsion system is designed to keep a constant earth-like gravity it must accelerate at 1 g for half the voyage and decelerate at 1 g for half the voyage, i.e., turnover at the halfway point. Making a turnover at any other point would mean not having earth-like gravity for most, if not all, of the mission.

 

So one question that pops up is: how important is it to maintain an earth-like gravity?

 

And ... what do you mean by "more advantageous"? Faster trip time? Less fuel consumed? Optimum passenger comfort?

[GeeKay]

Yes, I should have said something about what I meant by 'advantageous'. My main concern is fuel consumption, followed by journey time. I'm less concerned about the rate of acceleration, other than it remaining constant.

 

Another qualification: I cited the Earth and the Moon to keep things as simple as possible. However, what I really had in mind was an interstellar voyage that has a spaceship venturing out from the Solar System at a constant acceleration of 0.33 m/s² to reach a nearby (and wholly fictitious) dwarf star, whose mass is half that of the Sun. If my calculations are to be trusted, and assuming the distance between the two stars is 2,120 AU (again this is the stuff of fiction), it appears that the gravitational null point lies about 1,400 AU outwards from the Sun - or roughly two-thirds of the distance to the destination star. (NB. this fraction appears to hold true, regardless of the distance between the two stars). Now, is it more fuel efficient for the spaceship to make its so-called 'mid-course' turnover here, or at the true (geometrical?) mid-point between the two stars: namely at 1,060 AU? Here, though, it's worth pointing out that the Sun's escape velocity would be approximately 1,300 m/s while that of the other star is a tad over 900 m/s - assuming my maths is correct, of course. I'm sorry to have laboured this to death, but one comes across 'mid-voyage turnovers' quite a lot in science-fiction novels and as such, I'm intrigued as to what this means exactly. Gravity fields are akin to gradients, after all, and as anyone who's ever ridden a bike knows, there's definitely a calorific difference is when it comes to cycling up a steep hill and freewheeling down the other side!