Grav pipes, moss, and exponentially-increasing alien waves

Question

I am trying to remember a scifi book (or maybe novella?), about which I remember a rather diverse set of things (so it's entirely possible I'm inadvertently melding more than one story here, or have gotten thing(s) incorrect):

1. Main propulsion was 'grav pipes'. FTL jump drive, and I think for normal propulsion too? With increasing failure probability if you jumped too many times in quick succession.
2. At one point someone was (illegally) using said grav pipes to... clean dust off of moss so it wouldn't die? With appropriately-'why-would-you-do-that' reactions from other people for using a starship drive for gardening in the middle of a war.
3. We were fighting (in space) a seemingly-endless foe, who reacted to everything by sending exponentially-increasing waves. (I want to say 6-fold, with every ship carrying 6 of the next size down?)
4. Said foe was actually essentially bluffing. Yes, they had the ships, but they were fighting a two-sided war and couldn't spare the ships from the other front.
5. One weapon used was just essentially a spinning chunk of chain as a kinetic weapon.
6. I want to say that spaceship combat was essentially carrier-based?
7. At one point a chase ends in a pileup (multiple ships jumping the same FTL jump in sequence; the first ship couldn't get out of the way in time, which results in the second ship hitting the debris from the first ship, which results in more chaos, etc.)
8. At one point a main character after experiencing a gravpipe failure during a fight ends up writing out calculations on the window to see how long they had to live.
9. I want to say that the other front was essentially a static wall? This seems implausible however.
10. I want to say the conclusion was essentially 'take out enough of the other front that said foe actually talked to us'?

I would have read it somewhere in the range of five-to-twenty years ago or so; I suspect it might be substantially older than that.

Answer 1

It has been a while since I read the book, and I cannot find a complete online version, but I think this is Fire on the Border by Kevin O'Donnell, a novel from 1990.

The ships indeed employ FTL propulsion called "gravpipes". From a short sample provided at Baen, the gravtube operation is described as:

Mikhailaivitch checked the gravpipe lights. All green. Good-oh. Not that he could do anything if one went red besides scream for his chief engineer. The physics of even a simple gravity generator baffled Mikhailaivitch; those of the pipe made his head hurt.

Catman had fifty pipes. When he deployed them, each telescoped out of its housing into an iridescent tube a meter wide and a kilometer long. The hardware rode at the far end of the tube, in an aluminum sphere about the size of a basketball. According to the techies, that hardware took every gravity wave reaching the ship from every gram of mass in the universe, and bent it so it seemed to originate right at the end of the tube. Once the coherent gravity beam thus generated had accelerated the ship to threshold speed—eighty thousand kilometers or eighty kilokay per second—the pipe could create a finely calibrated, short-lived singularity through which the Catman could travel instantaneously to any spot in the galaxy.

One form of weapon is a shrapnel torpedo consisting of a spinning chunk of chain, "a chain link spiderweb 100 meters in diameter". I can also confirm that combat was essentially carrier-based, with carrier ships such as the Morocco and the Kathmandu.