Third Time's the Charm

In a little more than three weeks, Silver Spur 3 (a.k.a. Super Spur) was conceptualized, designed, built, and flown by USC's Rocket Propulsion Laboratory. As is usually the case with RPL, those three weeks were incredibly hectic and tiring, but any member that helped build Silver Spur 3 would say that every second was worth it. Because of this latest project, we have learned an enormous amount about our capability to design high-powered, incredibly high-speed rockets, and this information will be vital to our future endeavors.

Mitch and Alec busy machining

Silver Spur 3 was designed as a follow-up to the all-carbon motor case rocket, Silver Spur 2. We had many design objectives, among them:

- Design and fly a working composite motor-case rocket
- Increase max velocity from Silver Spur 2
- Reach as high a max Q as reasonable with our 4-inch hardware
- Design and test different thermal protection systems for the vehicle, both internal and external
- Break our previous altitude record
- Design and fly a vehicle that can withstand enormous initial and burnout G loads

All of these primary design objectives were met with great success. Even though both previous Silver Spur designs had failed in one way or another, we still wanted to push the design envelope, especially with our composites technology.

For a detailed look at SS3, let's start with the aft end and move forward:

SS3 Design
The nozzle was a hybrid 6061 aluminum, phenolic, graphite, and G-10 assembly. This general design had been tested before in our Trunnion series static fires, so this was the first iteration to prove flight worthy.


The fins were a first for Rocket Lab, and we are happy to say that our design proved to be very successful. Because of time and size constraints, we opted out of using any type of core material for weight savings and went with an 81-ply construction for each fin, using donated prepreg material. In an attempt to reduce the chances of fin flutter, the fins ended up being proportionally a bit smaller than the fins on previous RPL vehicles. But the "first" for RPL was the incorporation of an ablative leading edge - we machined pre-cured strips of linen phenolic into a tapered interface that would allow us to lay up each ply around the phenolic, holding the leading edge into place. We had never previously tested any leading edge material with a flight vehicle before, and as the photos at the end will show, they ended up working very well. The prepreg carbon fins were still in great condition when the rocket was recovered.

The layup proved a bit tricky, especially because our film adhesive bonding the carbon to the phenolic was a bit messy to work with. But after cure, they ended up looking better than we all thought they would. After machining and a little clean up:

The fins before they were scorched from mach 3+ flight

The four fins were then attached to the minimum-diameter motor case by a five-layer tip-to-tip carbon layup that cured in the oven the morning we left. Let's just say we sure know how to plan out our timing.

Above the forward motor bulkhead, we have the recovery section, which consisted of just enough space to house a 48" drogue parachute that would give near 100 ft/s descent rate. We were afraid of the rocket drifting too far and were pretty confident that it would be able to withstand a tough landing, so we opted for this approach. Not to mention, with the body being ~65" long, there wasn't too much room left over for a much larger parachute.

Avionics was mounted in the nosecone, which consisted of an ARTS board that was attached to a threaded rod extending from the nose tip. A G-10 bulkhead provided protection to the ARTS board from the ignition of the black powder charges. The threaded rod was attached through a bonded-in mount that held the nose tip on to the 8-layer wet-layup carbon nosecone.


And for the nose tip, we machined a ~1lb titanium death spike (nicknamed the "death nub" after further design iterations shortened its length and added a 0.25" radius to the tip). This was another first for rocket lab, as titanium had never before been machined for use in any other vehicle.


And the motor was an all-carbon motor case design that Rocket Lab has developed over the past few years. The motor was an O5000, with 7 six-inch long Bates grains housed in phenolic casting tubes.


Trip to Balls 19

On Thursday night, the whole lab was busy packing up everything we needed, thought we needed, and didn't think we needed into about 8 vehicles while the tip-to-tip layup was being cured in our oven. Most of us spent the entire night (and several nights earlier that week) awake, busy trying to get everything in order and finish any odd jobs that required attention. Shortly after the rocket was pulled from the oven at about 3 a.m., we all left for Balls 19 up in Black Rock, NV. This was a notable achievement for RPL - for once, we left to Balls mostly on time! This means that the dry lake bed was still barely lit by the setting sun once we got there, which didn't happen last year when we arrived at about midnight. This made a less hair-raising experience as we all roamed the enormous dry lake bed trying to find the flight line.

Integration/Flight Prep

We woke up at about 7 a.m. to begin prepping the rocket. We still had some things to do, namely, start sanding up the tip-to-tip layup, actually test-fit the grains into the case (and hope they fit properly), integrate the avionics and recovery system, attach the nose cone and shear pins, 5-minute epoxy the death "nub" onto the front of the nosecone, and assemble the launch rail.

Integrating the rocket out at Black Rock, NV

Well, at least this year the launch rail assembly was painless. Because we built the entire rocket so quickly, very little time was available before we got out to the desert to actually assemble the rocket and test the recovery system. This is one thing that separates us from the rest of the guys out at Balls every year. We are always frantically trying to assemble (and by assemble I mean build) the rocket that we don't know will even fit together properly out in the desert. But it always seems to work out well for us (minus last year - but the weather also played a large part in that one).

Rocket is fully integrated and being walked to the launch rail ~1/2 mile away from the flight line

Flight


David Reese was right - as long as it made it off the rail, it would fly.

And fly it did! There were so many things that we didn't know (or didn't think) would work, but we proved ourselves wrong after SS3 took to the skies. It came perfectly straight off the launch rail, and took off like a bat out of hell. The initial thrust of the rocket was enough to make a sizable crater in the ground and throw dirt clods 14 feet in the air.

The flight path was nearly perfect, minus a little bit of what looks like precession right near burnout.

But the one thing that didn't work as planned was recovery. The nosecone never came off the body, and thus the parachute did not deploy. As we were frantically panning the skies with the Rocket Hunter, a group of guys drove up to our group in their ATV saying that they saw a black rocket sticking about a foot out of the ground roughly a mile or so away. That would explain why we didn't get a signal from the avionics...

Amazingly, the fins survived intact. The best thing about them were the burn patterns and the completely scorched phenolic leading edges. They were charred completely black from mach 3+ flight. There was a little fraying from the tip-to-tip carbon, but they looked perfect besides that.

The rest of the rocket was a fun project to dig out of the ground. It took us a while (thanks for the shovels!) to get the everything out, but what we found was very interesting. The nozzle slid all the way up the body tube and stopped at the front bulkhead, making the hybrid nozzle look like a SolidWorks expanded assembly view. About 50% of the body tube was recovered, and the nosecone caved in on itself. But luckily, the carbon was just crushed enough to save most of the ARTS avionics board that we might be able to get some data off of it! And the best part was the titanium death spike. It survived flight at over mach 3 and ended up plowing into the ground over 6 feet at sonic speeds and never took a scratch. It will definitely be a permanent fixture in our RPL "museum" that we keep in lab.

- You can see the charred remains of the phenolic leading edges and the frayed carbon around them

Because we still need to wait on data, we can only approximate what the real flight path was. After analyzing video from the flight, the burn time was approximately 5 seconds, which was a tiny longer than we predicted. Backing out the acceleration from how many frames it took the rocket to clear the pad, acceleration was ~42 g's with a 2000lb initial thrust spike. The speed of the rocket, using these numbers, would come out to be roughly mach 3.3 at burnout, and max altitude around 50,000 feet. And the impact would occur 2 minutes later at 660 ft/s. But again, these numbers are speculative! Hopefully usable data will come out of the ARTS board in a few weeks, and the official stats will be released later.


All in all, this was the most remarkable project RPL has completed to date. The rocket was built at breakneck speed (as is usual) - but the best part is that it worked. All of our new implementations: our carbon case, our ablative LE's, our titanium nose spike, our 4-part nozzle - all worked! It's so exciting to have something like this come off as a success. And it will be vital to the continuation of our next high-altitude project, because we know we are capable of building a rocket that can survive incredible speeds and high altitudes, even if it lands a little faster than we expect. And we're working on that one, too.

Late-night digging party in the desert to get the rest of what was left out of the ground

-Flight On!

P.S. - expect more data/pictures soon. And an updated Projects page to the website.