Objective

The central objectives of the load cell apparatus are:

• quantify tensile and compressive forces acting on the rocket in flight, with an emphasis on the deployment of the recovery system, to realize the strain the system experiences in flight and to prepare for future missions accordingly
• to gather experimental data that augments theoretical insights to ensure survivability of Hermes II avionics, payload, structures, etc. in future flights under similar or more intense conditions

In order to fulfill these goals, a load cell will be placed at the interface between the Hermes II flight test rocket body and the recovery system, to measure tensile and compressive forces experienced throughout the test flight. An emphasis is placed on the deployment of the recovery system itself, when we expect to endure an exceptional amount of strain (~1,500 lbs maximum, see below) as measured by the load cell. An associated data logger, preferably by the same manufacturer, will be placed with the load cell to record information throughout the flight.

System Requirements

To ensure integrity of our data, the load cell and its associated data logger need to be able to withstand several conditions that limit pre-configured systems we can choose from: high re-entry temperatures, high accelerations, near or complete vacuum, and also the load on deployment of the recovery system itself. In addition, the system must have appropriate dimensions, solid construction, and a minimal weight - and last long enough to record data throughout the flight, in terms of both memory and battery life.

Load

On deployment of the recovery system, the Hermes II flight test is expected to endure a maximum of 1,500 lbs. of force. We set this as the upper bound of our prospective load cell's measurement range: maximizing measurement accuracy while staying within requirements.

Temperature Conditions

In the center of the nosecone, temperatures may reach 370K - or 100 degrees Celsius. While the load cell won't be placed here on the rocket, this number serves as a valuable example: our system should be able to withstand remarkable temperatures no matter where it is placed. In addition, the cell must be able to withstand brief exposure to fairly low temperatures in the upper atmosphere. For these reasons, we will narrow our range of options to only consider high-tolerance load cells with operating conditions between -40 and 90 degrees Celsius.

With regard to the data recorder, we assume that it can be insulated to where most adverse temperature impacts are mitigated. Regardless, we'd still like to search for options that can withstand temperatures between -30 and 80 degrees Celsius.

Acceleration

The maximum acceleration we expect the rocket to endure on ascent is roughly 20g, or ~700 ft/s^2 in imperial units. In addition, previous data taken from older experiments indicates we may experience spontaneous vibrations with up to 40g peaks. These numbers may change later in the design, but for now we seek devices conformant to this specification.

This specifically is fairly negligible to the load cell: for a maximum load of 1,500 lbs. the load cell would need to weigh 34.07kg for ascent acceleration to cause an overload. However, the accompanying data logger must have an adequate shock rating to handle these conditions as well.

Vacuum

At the top of its trajectory, the final Space Shot vehicle will be briefly exposed to near-vacuum conditions - and generally decreased atmospheric pressure throughout the flight. Accordingly, the load cell and data recorder must be able to withstand this.

The data logger is of little concern in this regard: temperature is a greater issue when choosing this purely non-mechanical system, and the device should be able to be insulated to partially mitigate any effects regardless. However, this is critical to the operation of the load cell: a failure to de-gas on ascent and acclimate to significant pressure changes was described as "fatal" by an Applications Engineer at OMEGA. This limits us to certain varieties of load cells - particularly non-hydraulic, purely strain-based types, e.g. S-beam cells - that are able to record accurate measurements in both vacuum and even increased-pressure environments.

Dimensions and Construction

Both devices must be small enough to fit comfortably within designated slots in the Hermes II flight test rocket. We're leaving this vaguely up to interpretation, as we can adapt as needed depending on the chosen components - but the system should be fairly compact and lightweight. In addition, both the data logger and load cell should be constructed of sturdy material, able to withstand violent motion and potential jostling.

Longevity

The minimum specification for general instrumentation is two hours of on-pad life. To permit for configuration, testing, and possible immediate data taking after the flight, we'd like to aim for a minimum of one day of battery life for the data logger.

In addition, on-board memory of the data logger is crucial to consider when assessing on-pad life. Accordingly, for a reading rate of 4 Hz (standard fare for high-accuracy load cells), the device must be able to continuously take data for up to 7,200 seconds - or for 28,800 readings.

Chosen Devices

The following devices were chosen as conformant to the above specifications:

Load Cell: LC101-1.5k

Price: $360.00 (available in 4 wks.)

Excitation: 10 Vdc, 15 Vdc max
Output: 3 mV/V ±0.0075 mV/V
Linearity: ±0.03% FSO (0.1% 40 K)
Hysteresis: ±0.02% FSO (0.1% 40 K)
Repeatability: ±0.01% FSO (0.05% 40 K)
Zero Balance: ±1% FSO
Operating Temp Range: -40 to 93°C (-40 to 200°F)
Compensated Temp Range: 17 to 71°C (60 to 160°F)
Thermal Effects:
   Zero: 0.002% FSO/°C
   Span: 0.002% FSO/°C
Safe Overload: 150% of capacity
Ultimate Overload: 300% of capacity
Input Resistance: 350 ±10 Ω
Output Resistance: 350 ±10 Ω
Full Scale Deflection: 0.25 to 0.51 mm (0.010 to 0.020")
Construction: 17-4 PH stainless steel

More information: https://www.omega.com/pressure/pdf/LC101.pdf

Data Logger: OM-CP-BRIDGE101A

Price: $530.00 (in stock!)

Reading Rate: 4 Hz to 1 every 24 hours
Memory: 1,000,000 readings; software configurable memory wrap 333,000 readings in multiple start/stop mode
Memory Wrap Around: Yes
Start Modes: Immediate start, delay start up to 24 months, multiple pushbutton start/stop
Stop Modes: Manual through software timed (specific date and time)
Multiple Start/Stop Mode: Start and stop the device multiple times without having to download data or communicate with a PC
Multiple Start/Stop Mode Activation:
  To Start the Device: Press and hold the pushbutton for five seconds,
  the green LED will flash during this time. The device has started logging
To Stop the Device:
  Press and hold the pushbutton for five seconds, the red LED will flash during this time.
  The device has stopped logging
Real Time Recording: The device may be used with PC to monitor and record data in real-time
LED Functionality:
  Green LED Blinks: 10 second rate to indicate logging; 15 second rate to indicate delay start mode
  Red LED Blinks: 10 second rate to indicate low battery and/or full memory; 1 second rate to indicate   an alarm condition
Password Protection: An optional password may be programmed into the device to restrict access to configuration options. Data may be read out without the password
Engineering Units: Native measurement units can be scaled to display measurement units of another type. This is useful when monitoring voltage outputs from different types of sensors such as strain gauges and load cells.
Battery Type: 3.6V lithium battery (included); user replaceable
Battery Life: 10 months typical, at a 1 minute rate with 350Ω load; 2 years typical, at a 1 minute rate with 1000Ω load
Time Accuracy: ±1 minute/month at 20ºC (68ºF), stand alone data logging
Computer Interface: USB (interface cable required); 115,200 baud
Software: XP SP3/Vista/7 and 8 (32 and 64-bit)
Operating Environment: -40 to 80ºC (-40 to 176ºF), 0 to 90% RH non-condensing
Dimensions: 36 H x 64 W x 16 mm D (1.4 x 2.5 x 0.6")
Weight: 24 g (0.8 oz)
Material: ABS plastic

More information: https://www.omega.com/pptst/OM-CP-BRIDGE101A.html