Boron Lynx

Overview

Boron Lynx was originally conceived as a smaller variant of the Mojave Sphinx design, with a 3” OD X 24” long tank and an uncooled chamber producing approximately 120 lbf for 2.5 seconds.

Tank

Diameter : 3”

Wall: .065”

Length: 24”

Bolt pattern: 12X 1/4-20

Chamber

The Boron Lynx chamber is made from 2” OD X .12” wall 304 stainless steel tube, with an aluminum nozzle carrier and copper nozzle throat insert. The chamber is sealed to the nozzle with a graphite gasket, compressed by four threaded rods which clamp the nozzle carrier, chamber, and injector between two steel flange plates. 

The selection of stainless tube for the chamber rather than a thicker aluminum wall was driven primarily by the challenge of packaging the chamber within the rocket’s 3-inch diameter. A sufficiently thick aluminum chamber would have resulted in too small of an inside diameter to maintain a reasonable contraction ratio (~>4), given the nozzle throat diameter dictated by the target thrust level and chamber pressure. Stainless steel exhibits significantly better strength retention at elevated temperature than aluminum, allowing a thinner chamber wall for a given total heat input, with a similar mass due to steel’s higher density. It is important to note that the combustion gas temperature and heat flux must be relatively low for stainless to be a viable choice for an uncooked chamber, even for short burn durations. While stainless steel has a much higher melting point than aluminum, it’s thermal conductivity is an order of magnitude lower, limiting the rate at which heat is transferred from the inner surface of the chamber to the rest of the material comprising the wall. A nominal mixture ratio of 1.9 was therefore selected for Boron Lynx to ensure survival of the chamber. 

The Boron Lynx injector is a scrintle similar to that used on Mojave Sphinx, with the oxidizer orifices located above the screw head to radially deflect the nitrous, and the fuel orifices distributed in an outer ring, fed by a fuel manifold groove. The initial injector was an 8-pair scrintle, but was modified after hotfire testing to double the number of oxidizer orifices. A severe overestimate of oxidizer discharge coefficient combined with a slight underestimate of fuel Cd resulted in an extremely low O/F ratio (0.9-1 estimated) when Boron Lynx was first hotfired. The second set of 8 oxidizer orifices was added in a radial pattern clocked 22.5 degrees to the original orifice pairs; this may have resulted in less-than-ideal impingement between the deflected oxidizer fans and axial fuel streams, although it is inconclusive whether the lower efficiency of Boron Lynx compared to Mojave Sphinx is due to injector mixing effectiveness, or simply a result of the physically smaller chamber.

Valves

Boron Lynx uses the same 1/4” COTS ball valves as Mojave Sphinx, but with a more compact servo actuator assembly. Prior testing determined that the torque required to actuate the ball valves when pressurized with nitrous was approximately 3kg*cm, indicating that the 25kg*cm servos used in the standard HCR SABV are somewhat overkill (note that this isn’t a bad thing when it comes to reliability). The first iteration of the compact SABV designed for Boron Lynx used 12kg*cm servos with the same mounting interface as the 25kg version, but only about half the height thanks to the smaller motor and gearbox. Unfortunately, the generic 12kg servos could not be actuated when Y-harnessed together, due to some form of electrical interference causing them to move out of sync with one another. These were replaced by slightly more expensive 15kg*cm servos, which could be Y-harnessed, but were found to be susceptible to occasional uncommanded twitches of a few degrees in either direction. One such event was sufficient to move the fuel valve past its cracking position, resulting in fuel being inadvertently dumped during nitrous fill for what would have been the 4th Boron Lynx hotfire. While the cause of this issue is not well understood at the time of writing, it appears to be unique to the “short body” style servos in the 12-15kg range.

Airframe

Boron Lynx’s airframe consists of a single tube mated to the forward tank bulkhead retaining ring, with the separation point at the nose cone shoulder and the avionics housed within the nose cone itself. Both the drogue and main parachutes are packed in the single recovery compartment and ejected together at apogee, with cable cutters keeping the main bundled until reaching the desired deployment altitude during descent. This layout reduces overall length and improves airframe stiffness compared to a traditional dual-deployment configuration with separate drogue and main compartments on either side of an avionics bay/coupler.

Launch 1

On the first launch of Boron Lynx, the fuel valve did not fully open before the GSE servo cable disconnected. This was attributed to slight misalignment between the servo and valve causing high torque and slow actuation, along with insufficient slack length in the servo cable. The increased pressure drop across the partially-open valve resulted in a drastically higher mixture ratio than in the static test conducted earlier the same day, producing a brighter exhaust flame and increasing performance significantly. The increased combustion temperature at this mixture ratio partially melted the aluminum nozzle carrier, although the copper throat insert and stainless chamber survived intact. The launch reached an apogee of XXXX ft, compared to YYYY ft predicted for nominal performance.