In reply to Kate:
Don't wash your flysheet in a top-loading machine. During the spin cycle, the water pressure is likely to force the fabric through the door, leading to tearing.
I did this once many years ago, trying to do a simple rinse and spin.
Even with a front-loader, I'd recommend a short, slow spin.
Detergent won't damage the fabric, but it will reduce the effectiveness of any water repellent treatment, so should be avoided; use water only, or a soap-based cleaner.