[Main results]

This last post on the ZOE GPS proto board in a small 35 mm rocket application deals with the main results of launch trials of the MER-35-ZOE model rocket.

Two launches of this rocket were executed on 9 November 2020 at Laren (Gld.), The Netherlands.
Weather was excellent for launching.

For an overview and detailed figures see the attached presentation.

Given the used Klima C6-5 rocket motor with a specified peak thrust of 15 N and a total rocket mass of 130 + 21.5 ~ 152 g it is clear that the acceleration limit of 4G in the dynamic platform model of the ZOE-M8B will be exceeded at least for a short time interval during the boost phase.
It depends on the model estimates inside the ZOE if this will be detected resulting in missing position fixes or not.
 
Main results

Herewith some main values for launch #1 resp. launch #2, see also slide 9 of the attachment:

-   Maximum forward acceleration (corrected for gravitation): 10.2 G, 10.0 G
-   Average forward acceleration during boost phase: 4.4 G, 3.9 G
-   Maximum altitude (above ground level / barometric): 135 m, 134 m
-   Maximum altitude (above ground level / GNSS): 134 m, 134 m
-   Trajectory length (by double integration of forward acceleration): 146 m, 135 m
-   Trajectory length (GNSS, step by step): 146 m, 137 m

The maximum forward accelerations are somewhat higher than expected (~9 G).

Average forward acceleration at launch #1 was higher than that of #2 resulting in max vertical velocities of 54 m/s and 49 m/s respectively. However, max altitudes were nearly the same (135 and 134 m). Launch #2 was more vertically oriented leading to a much smaller difference between trajectory length and altitude as was also visible on site and on videos.

Model rocket trajectories

The different trajectories can be visualized by the following figure, in which GNSS longitude (x), latitude (y) and altitude (z) are depicted relative to launch point:


(Figure 1, see attachment, sheet 11)

Launch #2 is much more vertically oriented as can also be seen from rotated versions of this figure (see attachment). In the trajectory of launch #1 six position fixes are missing, presumably because of too high forward acceleration, "too much > 4G".

Altitude and other measurements

Altitudes (corrected for ground level at launch) measured by both barometers and GNSS are rather close to each other as can be seen in the next figure for launch #1. Correction for time alignment was not necessary.


(Figure 2, see attachment, sheet 15)

The barometric altitudes have a more noisy character. It would help a bit to take the average of the outputs of the two barometers or to use more averaging per barometer.

The GNSS altitude curve is rather smooth, but it is striking that the altitude after touchdown is more negative than the height of the launch platform. So the GNSS altitude also includes some random walk effect.

The two barometric outliers relate to measured pressure peaks (so altitude dips) during ejection of the nose cone section including parachute.

For figures showing forward acceleration and roll rotation please refer to the attachment.

Because of the canted fins which are designed for a roll rate of 2000 deg/s (~5.6 Hz) at a forward velocity of 40 m/s, it is expected to see the roll rate increase with increasing forward velocity. This is indeed the case.
Also note that the LSM9DS1 gyro output saturates just above 2000 deg/s conform specification.
This happens in fact at ~6.4 Hz.
Magnetic field attitude readings are used to fill in the saturated gyro part.

Conclusions

It has been shown that the ZOE GPS prototype board can be successfully applied in a small 35 mm diameter model rocket.

ZOE setup modification through UART only took some effort, but proved to be done in a reliable way.

The trials were successful with 5Hz GNSS and ~50 Hz sensor data.

The ZOE-M8B gave six missing position fix data during the boost phase of launch #1, the one with the higher forward acceleration.

As next step it is planned to implement "Enable position output for invalid fixes" using the UBX-CFG-NMEA command in the ZOE setup in order to avoid missing position fix data in follow-up trials.


Regards,

Willem