First Flights - MPR Scratch Build
Thursday, March 24th, was a near-perfect day for launching rockets. It could have been a little cooler—the Windfinder forecast called for a high of 79 degrees, but by 3 p.m., it was 101. But the wind was predicted to be 1 to 2 mph—and it was—and that was really all I cared about. That, and a clear, blue sky.
I was joined by my stepson Zac, who helped set everything up for the launches. He also shot the video from a smartphone. And I've got to say, he nailed the framing on the ascent every time, with the rocket never flying out the top of the video.
I had concerns about the first flight. First, would the parachute deploy fully—it's a big chute, with long shroud lines. And I made it that way so I could get soft landings, which was my second concern—would the fins be damaged hitting the hard surface of the lake bed. The answers were yes, the parachute deployed beautifully, and no, the fins weren't damaged, at least on the first two flights. They were absolutely perfect.
First flight, on a G74-9W:
View attachment MPR first flight 3-24-22 G74-9W.mov
I want to say first that it was pretty awesome to see this rocket fly. I've never mentioned it in the thread, but this is my first build since I was a 15-year-old kid with an Estes Saturn 1B on a cluster of four B8-4s. That was in 1966. And it was the same old feeling of holding your breath until the parachute popped, then just enjoying the hell out of it—but this time on a G motor. Seeing this rocket streaking like a shot into that dark blue sky and disappearing out of sight, was really freaking cool.
The rocket landed approximately 300 feet away with no damage to the fins. Not even a chip. I attribute that to the descent speed of 10 fps, and the coat of Pledge Floor Gloss I applied the day before. That stuff is pretty amazing.
Data from FlightSketch Mini:
Apogee: 1559 feet
Max Speed 303 mph
Time to Burnout 1.1 secs
Time to Apogee: 10.2 secs
Time to Eject: 9.9 secs
Apogee to Eject: -0.3 secs
Total Flight Time: 165.4 secs
Average Descent: 10 fps
Compare that to the OpenRocket simulation I ran the night before with 2 mph of wind:
Apogee: 1636 feet
Max Speed 296 mph
Time to Apogee: 9.45 secs
Total Flight Time: 166 secs
Average Descent: 9.5 fps
That tells me that when you weigh every component that goes into the build, OpenRocket is pretty darn close to the actual flight. The only variable really is drag, and in the end that boils down to finish—regular, smooth or polished. And those can be pretty subjective.
Even though mine was coated with Pledge, and pretty close to having a polished finish, I simmed it as smooth because of the vinyl lettering. A lot of edges there. Sealed, but edges nonetheless. And looking at the OR-to-altimeter comparison, I'd say it's somewhere between smooth and regular.
Second flight, also on a G74-9W:
View attachment MPR second flight 3-24-22 G74-9W.mov
The second flight was a carbon copy of the first, except it landed half the distance away from us—about 125 feet. Again, no damage whatsoever from landing on the hard lake bed.
I think the thing that struck me the most in both these flights was how straight the rocket hung beneath the parachute on descent. No circling at all. I think the spill hole is the key there.
The third flight, on a G80-10T, was a different story. At least from deployment, forward. The flight up was even more spectacular than the previous launches, of course, because of the bigger motor. But the parachute fouled, and the rocket came in hard, fins down. I can't be certain, but I believe it was the way I packed the lower shock cord cord in, S-folded next to the Nomex blanket bundle. Same way as I'd done it earlier, but also a little hastier, not being as meticulous. It was 3 p.m. and the temperature was 101, and Zac and I were frying. I just got in a hurry.
My rushing through the preflight prep and checklist for that flight also resulted in my failure to arm the altimeter for launch. So I have no data from that flight. But if the FlightSketch comparison to OR sims with the G74 held true with the G80, the apogee would have been around 2400 feet, with max velocity about 400 mph. Total flight time around four minutes.
Third flight, G80-10T:
View attachment MPR third flight 3-24-22 G80-10T.mov
Incredibly, two hairline cracks along the fin fillets were the only damage.
Thankfully, this rocket will fly again. Lessons learned: Don't rush the process. Ever. Don't skip the pre-launch checklist just because you've done it once before. And finally, just because you have two perfect flights in a row, it doesn't mean you're going to have three. Prep the rocket for each flight as if it's the rocket's first time up.
In the end, there was nothing wrong with the rocket. It was built strong, and flew beautifully. It crashed because of my failures. On the brighter side, it's got its first dings, so I can quit worrying about marring the finish.
All in all, it was a great day. Zac and I had a blast launching this rocket together. He shot some excellent video. And we both got sunburned. We can't wait for the next time.