Gravity assist in Interstellar

[pavelcherepan]

I was re-watching Interstellar last week (my wife hasn't seen it before) and had a question about yet another physics inaccuracy in the movie... or so it seems to me.

 

In the end in order for Dr. Amelia Brand to get to the last planet in the system they haven't visited yet her and Coop fly Endurance and perform a powered gravity assist using central black hole for that purpose.

 

First issue is that you can't use central body of the system containing pretty much all the mass as a gravity assist, because in that stellar system FoR is would have zero momentum and hence you can't get any out of it. Although they did briefly mention a neutron star in the same system, so it's possible that the barycentre of the system is somewhere outside of black hole. But let's forget this for a moment.

 

The other issue is the way gravity assist itself was done. They fired engines of the spaceship and landing crafts at the closest approach to BH and then as the fuel ran out detached landing crafts... to get more momentum. This doesn't make sense to me. In the movie it looked like crafts were simply detached, not fired away and in that case why would Endurance get extra velocity from it?

 

[latex](m_e+m_c)*v=m_ev+m_cv[/latex]

 

where e is fore Endurance, and c is for landing crafts. I can't see why they had to drop Matthew MacConaughey into black hole apart from the fact that without it the plot would fail even to a greater extent than it had as it is.

Edited July 9, 2015 by pavelcherepan

[Janus]

It sounds like they wee taking advantage of the Oberth effect and not the not a gravitational slingshot effect. The Oberth effect relies on the fact that you can get a greater final Delta v by firing your engines when at the periapis of an orbit than you can at other points. It's an exchange of GPE for KE. When you fire your engines. I didn't see the movie, so I'm not sure of what exact sequence they used. But if the landing craft ran out of fuel before the main engines, then it did make sense to release them as they did so. So if they were still under some thrust when the craft were let go, there was an advantage to be gained.

[pavelcherepan]

Thanks! I didn't think of Oberth effect. Although I think in the movie when landing crafts got separated main engines were off already. I might be wrong though.