So, the main question is what parachute(s) would folks recommend for main and drogue? The other question I have is in calculating descent weight, do I include the weight of the parachutes themselves? And then the third question, aside from weight and how much space it takes up, are there any drawbacks to using a slightly oversized parachute (one that I could potentially use for heavier rockets down the road)?
Congrats on the build completion and here's hoping for a successful flight for you. I'll be skipping around the order in which you asked the questions, because linear thought is not my forte.
You mentioned a payload, as well, at times. So, my recommendation is to size the main, in particular, for the descent weight. This will be the airframe and recovery components, burnt-out motor and casing, and payload. If it's at 8 now, without the chutes, motor, and payload, you're going to be looking at a recovery weight of somewhere around 12lb, if the figures you mentioned hold true.
As far as a particular recommendation: "Chute XXX from manufacturer YYY" that's a little harder for me to do for you. There's a good many styles out there to choose from. Topflight, Crossfire, and Rocketman have already been mentioned. Others one might consider are--in no particular order-- Spherachutes, Skyangle, and Fruity Chutes. They each have their own advantages and disadvantages, fans and detractors, and levels of cost and performance. But they all work.
Comparing them is not quite as straightforward as one might initially think. Not everyone uses the same reference measurements for their Cd calculations, and not everyone uses the same recommended landing speed. Some use 15-20. Some use 17-22. If you like the look of Chute A, with a recommended landing speed of 17-22, but you want to hit the ground at 15 --or let's say you have a delicate project, and want to land even slower... 12 or 13... it often is just guessing, based upon the published data.
That all said....
Try this:
Cd * REF * 0.5 * rho * Velocity^2 = Mass.
Where all values are in FEET and LB.
REF is reference area. For circles, r^2 *pi. But, being as chutes are typically sized on diameter (D), it may be best to think of it as (D/2)^2 *pi. If chute size is in INCHES (as most are in rocketry, with Rocketman being a noted exception), then you must divide this all by 144 (sq inches to sq ft.)
rho is 0.002377 slugs/ft (I know; it's an odd name for a measurement value, but it is, actually, a real thing it means "the mass of accelerated by 1 ft/sec when 1 pound-force is applied. In air, at sea level, this value is 0.002377)
Let's look at a 48" chute from Nooneinparticular. They say it's good for 13.2 lb at 22FPS, but do not publish their Cd. You want to know what this chute will recover at 13 FPS.
So:
Cd * REF * 0.5 * rho * Velocity^2 = Mass.
Cd * ((48/2)^2 * pi)/144 * 0.5 * 0.002377 * 22^2= 13.2
Cd * 1809.216/144 * 0.5 * 0.002377 * 484 = 13.2
Cd * 12.564 * 0.5 * 0.002377 *484 = 13.2
Cd * 7.23 = 13.2
13.2/7.23 =Cd
13.2/7.23 = 1.825
Cd = 1.825
Now that we know the Cd, we can use it to find the recoverable mass at any given speed. In this case, 13 FPS.
Cd * REF * 0.5 * rho * Velocity^2 = Mass.
1.825 * 12.564 * 0.5 * 0.002377 * 13^2 = Mass
1.825 * 12.564 * 0.5 * 0.002377 * 169= Mass
4.60 = mass.
So that chute will recover 4.6 lbs at 13fps. Seems rather low, given the relatively high Cd, but the recoverable mass is dependent upon the square of the velocity, remember.
I know--it's a lot of math, but maybe it will help you in your chute quest.
Later!
--Coop