Liquid Bi-Prop Recovery System: VLAD (Very Low Altitude Demonstrator)

VLAD (Very Low Altitude Demonstrator)

The VLAD project

VLAD (Very Low Altitude Demonstrator) exists as a means to qualify the recovery & flight avionics system for the liquid bi-prop rocket 'Eureka-2' under real (albeit lower apogee) flight conditions. Rather than build a relatively analogous but smaller system, VLAD is literally the top section of E2 (same avbay, same parachute hardware, same separation mechanism), just with an added fin can and COTS solid motor mounting hardware. Now that it has been successfully launched and recovered, the fincan & solid motor mounting bulkheads can be removed, and it can be mounted atop Eureka-2.

Design

CAD views

Cutaway

Recovery

Tests of the recovery system at FAR

Recovery Subsection

The recovery subsection is fully removeable from the fiberglass airframe, mounted on rails as seen. This architecture made iteration and ground testing very easy, with all internal components readily accessible and plenty of places to mount or swap new components. The parachute subcompartment, which itself does not need to contain any pressure from the black powder ejection system, is a 0.8mm thick FDM printed shell & exists to organize and contain parachutes and lines.

Assembled recovery subsection- the printed elements spanning between rails are nonstructural, they merely contain the parachute and prevent snagging during integration. Aside from the obvious fixturing benefits for testing, building the system on rails like this greatly reduced the amount of holes needed to be drilled in the fiberglass (which is normally a time bottleneck), and furthermore made it possible to iterate on the system without altering the fiberglass.

Separation Mechanism

Rather than pressurizing the entire parachute bay, a common strategy, this design pressurizes a relatively small piston in front of the parachute bay. This keeps the parachute hardware relatively isolated from the violent pressure and temperature changes associated with detonating a black powder charge- this made doing some more delicate things possible, like running wires down our kevlar lines. This architecture proved to be capable of great consistency in testing, and ultimately flight.

Separation mechanism assemblies-
Left: top of the puck, female side of the piston

Middle: Top of the UBH (upper bulkhead), note the kevlar tie-down pins
Right: Bottom of the UBH, showing the male side of the piston

Separation sequence 1:

Black powder detonated inside chargewell, pressure generated

Separation sequence 2:

Shear-pins sheared, upper bulkhead + nosecone ejected, recoil force absorbed by puck backstop & fiberglass airframe tube

Separation sequence 3:

Tension in kevlar lines connecting upper bulkhead and 'puck' re-distributes momentum, pulls out puck, which in turn pulls out the chutes

Chargewell

Passthrough V1

The first method that was used for black powder ignition in the chargewell was a nichrome bridgewire. The wire passthrough was machined out of delrin, which provided insulation for the passthrough screws while being sufficient for holding in the pressure generated by the charge. In version 1 (on the left), the delrin was exposed to the blasting compartment. Later, a replaceable lasercut 1/16" wooden barrier was added to extend the life of the delrin passthrough (on the right).

Passthrough V2

The goal with the nichrome bridgewire was that it wouldn't need to be replaced after every firing, but this did not end up being the case- furthermore, there was a noticeable delay between the time the command was sent to blow the charge and the charge actually blowing, which was the time it took the nichrome to impart sufficient heat to the black powder grains. Given these disadvantages, the chargewell was reconfigured to simply accept an E-match in the place of the bridgewire. This was the final configuration used for qualification testing and ultimately flight.

Installed chargewell on the puck

Manufacturing

Fiberglass

Length Jig

A big part of making the system compatible with the E2 bi-prop structures was utilizing the same fiberglass tubes for the external airframe components. These fiberglass tubes are COTS, and come in rough increments, so it is necessary to cut them to length. I implemented a lasercut/ threaded rod jigging system for all the fiberglass operations. For length cutting, the procedure was to simply measure out the length desired between the wood surfaces visible in the image at the four points, adjust the position by moving the nuts up or down the threaded rod, and then using the resultant position as reference.

A similar jig was used for radial holes. Instead of being re-position able on the fiberglass tube, each side has a flange, and nuts on the threaded rod are used to apply a clamping force on the tube, sufficient to lock radial position.

A printed radial jig which holds bushings for repeated use spans between the threaded rods. The distance between jig and the flange can be measured, and adjusted with the nuts.

I performed a wet layup over a printed shell in order to fabricate the nosecone. Baked into the printed shell was a heat set insert flange, which eliminated the need to drill any radial holes into the fiberglass post-layup.

Waterjet

All of the planar components in VLAD, including the fins, were manufactured on the waterjet- this was a convenient way to avoid relying on the machineshop, which has historically been a bottleneck.

Testing

Separation mechanism testing setup

Main chute release charge testing

Fully integrated nose deployment test

October 7th Flight

Raw Data (csv)

VLAD annotated telemetry (altitude AGL in meters, derived from barometric pressure)