Good point. I guess my decision to cluster engines was to achieve enough thrust to attain a minimum altitude of 1000 ft. The sims I've run in OpenRocket show an estimated altitude of 950' using E9-8's. It's not a big deal if I don't cluster. I'll just use what I can legally use and accept the altitude it attains. Then, when I'm ready to do a qual flight, I can use an engine of proper impulse to qualify.
I haven't started construction yet, precisely because I have a lot of things to learn. I've already bookmarked a discussion thread on CG/CP, and am looking at several build threads. Once I'm set on the final design, maybe I'll post a build thread of my own.
The E-9 is just about the porest choice you could make for a 4 motor cluster on a rocket that might weight more than 1.5 kg. The E9 has a peak thrust of on 20 Newtons. Even if all 4 light, the peak thrust is only 8o Newtons. Your rocket probably weights 15 Newtons, so if all motors light the peak thrust to weight ratio is only 5.3:1, and the average lift-off thrust to weight ratio is half that or a mere 2.7:1. The peak acceleration is only 4.3 g, and if there is a breath of wind, the rocket will weathercock severely, and will not reach the expected apogee.
Here's the sim results using 4 E11-4s in a 3" minimum diameter rocket.
Total launch mass = 1500.0 Diameter = 75.0 millimeters, Initial Cd = 0.750
Stage 1 has 4 engines of type Estes E9-4 for an impulse of 111.47 Newton-Seconds
Total Impulse = 111.47 Newton-seconds Ejection 8 seconds after launch. Parachute diameter = 1000 millimeters.
Flight duration 35.7 seconds.
Max altitude = 165.99 meters (544.6 ft) at 7.3 seconds.
Peak speed = 45.28 meters per second (101.3 MPH).
Maximum acceleration = 4.32 g.
Turn over at 7.3 seconds
Chute deployment at 7.06 m/S, shock = 1.00 g.
Impact at 5.9 m/s.
With one motor not igniting, the altitude is halved, and the ejection is 2 seconds after apogee, however if the rocket weathercocks, it will crash before ejection. If 2 or more motors fail to ignite, the rocket crashes.
This rocket needs motors with a minimum average thrust rating of 30 Newtons to have a safe flight if one engine doesn't light.
Bob