Structure
The main functionality of the structure is to take on the system
loads from lift-off through flight. Within this particular design,
the structure has to fulfill three secondary functions:The structure
is an assembly of lightweight plastic parts and stabilizing Aluminum
and Stainless Steel poles. A thin-wall Aluminum tube represents
the housing, radiation protection, system ground, and antenna counter
pole. The structure skeleton is shown in Figure 2 as 3D model.
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Electrical Power System (EPS)
A battery pack I (9V Alkaline battery) is physically connected
to the circuit even before the removal of a remove-before-flight
pin (RBFP). The circuit layout though makes it impossible that any
devices are powered on before the second battery pack would be connected
at the RBFP removal. Furthermore the battery pack I is isolated
from the transmitter. This means, double safety preventing the transmitter
to be activated before removal of the RBFP.
After removal of the RBFP, the only way to turn the payload on
is to accelerate it by more then 10g, or to expose it to a strong
magnetic field. Any acceleration or electromagnetic field exposure
events that occur before RBFP removal are regardless for the time
after RBFP removal. Disconnecting battery pack II (as is the case
as long as RBFP is inserted) equals a system reset.Sensors
The following sensors were selected and integrated into the Sensor/C&DH
board:
- Accelerometer, Analog Devices, ADXL190
- Rate Gyro, Analog Devices, ADXRS300
- Temperature, integrated in ADXRS300
- Pressure, Motorola, MXP4115A
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Command & Data Handling (C&DH)
For command and data handling tasks we choose the so-called MIM
module. This is a complete telemetry TNC transmitter in less than
one cubic Inch. It has a serial data port, 5 analog and 8 digital
telemetry inputs. It sends audio AX.25 tones to the transmitter.
Figure 15 – MIM ModuleThe on-air packet telemetry format is
as follows:
T#sss,111,222,333,444,555,xxxxxxxx
Where “sss” is the serial number followed by five 3 digit
analog values (From left to right: A0 to A4) and eight binary values.The
A/D channels are connected as follows:
A0 Pressure
A1 Temperature
A2 Spinning Rate
A3 Acceleration
A4 N/C
COMM
Antenna
For this type of rocket the options for an appropriate antenna
layout are very limited. The rocket’s nosecone and fin assembly
are made of metal. Thus the antenna design has to be such, that
the transmitted RF is not being deflected by either of them. A body-mounted
antenna at the outside of the rocket is not possible, as no parts
may protrude from the rocket.
These constraints eventually lead to the approach of using the
nosecone as antenna. As counter pole for the antenna we decided
to use the Aluminum housing of the payload itself. To investigate
the feasibility of this approach, we conducted a gain pattern simulation.
The result was very good. As the analysis shows, this setup leads
to best results at a housing length of 19”. The DART nosecone
measures 13.38” in length. The optimum frequency is located
within the VHF range – we will be using 144.39MHz.
Transmitter
As the link budget indicated, 1Watt transmission power would be
more than sufficient utilizing the preceding current design. As
one of the smallest complete handheld radios on the marked, we choose
the Yaesu VX-2 for our design. The transmission power is about 2W
at 5.6V.
Ground Station
The Ground Station is mobile and easily to set up. As illustrated
in Figure 18 in consists of a Kenwood HT handheld radio that is
connected to a 144MHz Yagi antenna. The data is being received at
a 144.39MHz VHF frequency. Via the HT serial connector, the radio
will be directly connected to a laptop, running MS HyperTerminal.
The radio settings are: TNC packet mode, 144.39MHz. For successful
communication with HyperTerminal, the laptop’s COM port has
to be set to 9600baud. The Ground Station team should consist of
at least 2 people, and be located about 5 miles to the North or
to the South of the launch site. The Ground team shall be in radio
contact with the launch team, so as to being kept up-to-date about
the launch proceedings.
After launch, the acceleration switch will be triggered and the
payload will start transmitting. One person will hand point the
Yagi antenna towards the rocket. If the rocket is out of view, the
antenna shall be pointed to the point of best reception. The second
person shall watch and control the recording of data in Hyper Terminal.
The data will be recorded in packages of the form:
T#sss,111,222,333,444,555,xxxxxxxx
The data rate is one per second, limited by the performance of
the MIM module.
Stanford
University Metro-Rocket System Payload