Scope

Overview

This document will analyze the flight of Hermes 1 at Friends of Amateur Rocketry in July of 2018. The document will outline the flight performance of the vehicle, calling specific attention to any deviances. These flight events will be discussed in roughly chronological order, by subsystem. Under-characterized or poorly understood behaviors will be noted for future efforts. A section at the end of this document will discuss lessons learned.

Methodologies

Times are indicated as T+ or T- from the first vertical motion on the rail (liftoff). Times are indexed off of the Telemetrum data.

Timeline

Flight Statistics

Simulation Accuracy

The last simulation before flight predicted an apogee of 31600', which proved to be very similar to the true performance of the rocket. At 2.5% error, it seems our RAS Aero model is accurate enough to predict future flights of comparable rockets. After the flight all of the data was plotted. The results are shown below. A discussion of the pre-flight vehicle descent velocity mis-estimation is in the recovery section. The University of Victoria payload altimeter cut out during vehicle descent. The cause of this failure is unknown.

Avionics

Payload

See the Hermes I Payload Analysis page

Propulsion

Thrust Curve

See the motor data page.

Long Startup Time

The motor took significantly longer to light than previous tests, including the second P motor test that used an identical igniter with significantly more propellant surface area. During static fires the igniter was retained by gravity until the motor had built up enough pressure to eject it. Video evidence shows that the igniter fell out of the motor within a second of firing.

Future efforts could faster-burning igniters (pyrogen or BKNO3), higher surface area propellants (pixie dust), or additional mechanical retention. While chuffing represents a minor issue on this flight, successful, rapid, and complete ignition is a necessary technical milestone for multistage flight. Care must still be taken to not over-pressurize the case or clog the nozzle with a large igniter.

Recovery

The Telemetrum was configured as the primary altimeter, and the Raven was configured to fire 2 seconds after the Telemetrum for drogue, and 500 ft lower for the main. This was seen reflected in the post flight data.

Parachute Drag Coefficients

The drag coefficients of the parachutes were calculated–drogue was calculated using data between 7k and 8k meters, while main was calculated at landing. These drag coefficients are effective for the flight, they also factor in the drag on the mission package as well as interactions with the fin can.

Drogue Effective C_D between 7k m and 8k m: 0.48

Main Effective C_D at landing: 1.33

The expected drogue Cd was .36. With a correction for the drag of the body, it was expected to be between .36 and .40. This error is probably due to a conservative body correction or a poorly characterized fabric porosity.

Knake gives hemesphericals a Cd of about 1. It seems that both the main and the drogue Cd are 33% high. It is possible that the glide ratio is .33.

E-match voltage anomaly

Post flight data review notes abnormal voltage readings on Telemetrum apogee channel. Despite commanding an apogee event, the Telemetrum continued reading 4.2V across the channel for the remainder of flight. The e-match should register a significant increase in resistance within 10 mS of exceeding it's no-fire current of 300 mA. This suggests that the initiator misfired.  A fishbone analysis was conducted to classify the nature of the issue. Evidence collection is ongoing, however it appear probable that the Telemetrum e-match is internally shorted, either autogenously or to the piston.

We checked continuity post-flight and neither e-match had continuity. This is not a conclusive test because the wires could have been jostled during landing or travel, however this suggests that the e-match was not internally shorted to the piston.

In the future, component level testing should include resistance checks of the e-matches before and after firing. A design revision of the firebolts should also include insulation as a design requirement.

Structures

Videography

Flight Cameras

The initial design for the Hermes avionics bay supported 4 cameras, 3 radial facing, and one downward facing. Due to integration time constraints, only two radial cameras were integrated. Due to a configuration error, the cameras took a picture when powered on. The cameras were Hawkeye Firefly Q6 cameras recording in 4K at 24 fps. It is recommended that successful video capture is considered part of a successful ground test. The cameras were also not configured with the correct time stamp. The value of the time stamp as an engineering reference should be weighed against it's 'tackiness'.

Ground Cameras

The ground cameras were laid out as shown.

The high rate of failure of ground cameras suggests that we should put additional effort into our ground camera array. Doug's Rail has provided invaluable footage in the past.

Integration

Integration began at roughly 1:30 pm PST and the rocket arrived on the pad at roughly 3:00 pm PST.

Lessons Learned

• Having a good checklist would have been beneficial, even though we likely wouldn't have been able to integrate the rocket in the amount of time that we were given before the end of the launch window if we had been following a strict checklist.
• Regular batteries are NOT enough to fire 2 e-matches in parallel. We ran into this problem during the ground test, as well as a previous static fire.