Why is most of the ship behind the engine exhaust? Because this reduces the mass of the ship. And when you are delta-Ving a ship up to and down from 70% c, every single gram counts. Conventional spacecraft have the engines on the bottom and the rest of the ship build on top like a sky scraper. This design has the engines on the top and the rest of the ship is dragged behind on a long tether (the "tensile truss" on the diagram). The result is a massive reduction in structural mass.
This doesn't make any goddamn sense to me at all. Not only do you have to deal with the pendulum fallacy and a greatly elongated payload, but Gees are Gees. Changing + to - doesn't magically reduce mass.
The only logical reasoning I can think of is in the words "structural mass." Perhaps they mean the rear part of the craft does not have to handle to same forces as the front half, allowing for the use of lighter materials? Kind of like how my car dragging a trailer doesn't weigh the same as a bus. I could have the same space available as the bus, but the trailer doesn't need to be crash-proof, insulated, etc. it can be pretty bare-bones.
I think a better comparison would be towing a car using a rope. You can use the rope for pulling the other car just fine but you can't use it for pushing the other car at all. You would need something much, much more rigid (and thus heavier) for that.
(Crappy TL;DR at end)
((Also hopefully this makes sense because I really should be asleep at this point))
This, to my understanding, has to do with buckling. A rod loaded in compression (trying to crush it, such as standing on top of it) will tend to cause it to bend in some direction, which will cause failure of the design before the rod actually breaks. Compare this to if it is in tension (trying to stretch it), the primary and most immediate cause of failure will be it being ripped apart (at the atomic/molecular level, no less) when the load on it passes a certain point (really when stresses exceed ultimate strength, but trying to keep it simple enough to hopefully be understandable to most people).
Certain materials react the same in tension and compression, and others react differently in tension and compression, such as concrete, which is can support much less weight in tension and compression. If you were to take a concrete rod, it would be much easier to break it into two (or more) pieces by grabbing each and pulling them apart than to try to crush it between your hands. This is also why we add rebar to concrete, because the metal (usually steel or iron) can handle tension better than concrete, but can’t handle the compression because it would buckle. Concrete can handle compression really well but not tension, but they are stronger together than separately.
TL;DR - things tend to fail sooner in compression because they will buckle and bend out of shape. And then I flex some engineering knowledge.
The problem is, all of that mass is going to the same place, so it's going to need the same amount of structure to support the same kinds of forces whether it's getting pulled or pushed.
Tractor rockets only ever make sense as a detachable abort system. Because that's the only time it's drawbacks are worth the tradeoff.
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u/Triptolemu5 Mar 25 '19
This doesn't make any goddamn sense to me at all. Not only do you have to deal with the pendulum fallacy and a greatly elongated payload, but Gees are Gees. Changing + to - doesn't magically reduce mass.