STATIC TEST PAD FOR ROCKET MOTOR

Abstract

To design and simulate a Rocket Motor Static Test Pad for testing and acquiring the required data for the performance analysis of the high powered rocket motors

Full text

Internship Report on
STATIC TEST PAD FOR
ROCKET MOTOR
With STAR – Space Technology and
Aeronautical Rocketry
Brahadeesh Suresh
(Designing Intern)
(Systems Engineer)
National Institute of Technology Tiruchirappalli (NITT)
(B.Tech in Mechanical Engineering)
1 January 2021 to 30 January 2021
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PROBLEM STATEMENT:
To design a Rocket Motor Static Test Pad for testing and acquiring
the required data for the performance analysis of the high powered
rocket motors
STATIC TEST PAD
A. What is a Static Test Pad?
The Static Test Pad is a test stand designed to conduct full-scale
static test fires as well as other large subsystem tests of a solid
rocket motor.It is often performed with computer modeling to
characterize their thrust curves and Isp in static testing conditions
for the propulsion of a rocket motor.Some standard Models of a
Static Test Pad are illustrated below:
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B. Basic working principle of a Static Test Pad:
The static test pads are commonly used to test rocket motors and
engines of various sizes or types works in a systematic manner. A
program runs both the motor ignition and data acquisition systems
to produce time-resolved thrust data. It is rooted, so as to curb any
dynamic forces or pseudo forces as gravity acting on the body.
Now, the sequence by which the static test pad is initiated , is given
by –
1. All the connections are made and verified.The load cell is
mounted onto the central support beam, and the motor onto the
mounts. The I shaped base is rested on the ground if flat or the
legs are attached otherwise.
2. Then, the load cell is connected to the avionics bay, arduino and
amplifier are connected along with the thermo coupler.
3. A command is given through Arduino to start the ignition by
wifi module that allows us to activate relay remotely. When the
motor starts, it exerts the thrust along the horizontal axis
parallel; to the base, with exhaust towards the side.
4. The motor is fixed, due to the motor mounts. Hence, it won’t
displace, translate or rotate, as the degree of freedoms are
locked.The ball transfers allow axial movement to slide onto the
load cell.
5. The load cell assembly is mounted to a fixed thrust
measurement test stand securing the motor.As the motor is
operating, thrust is measured with the load cell.
6. The rocket engine test stand mount prevents any side loading on
the cell, ensuring the load cell is only receiving axial loads.
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7. Due to this, it vibrates to and fro, like an oscillator. The
vibrations generate stress in the load cell, as it is directly
connected to the motor mount. This stress would be converted
to a digital signal by the HX711 Board, which would be
connected to the Arduino uno via PCB as it is a force
transducer, and stress is force per unit area.
8. Mass flow and Drag are captured with load cells.Thrust data can
be live streamed and logged for monitoring and later review and
comparison against simulated models with external storage.
9. Throughout the test, the data is continuously converted from
analog to digital information, and we determine the calibration
factor, to see if there is any error in any of the sensors.
10.The testing automatically ends when the motor runs out of
propellant. Hence, now our static testing of the rocket motor is
done using the static test pad.
C. Why to make a Static Test Pad?
The last decade has seen an almost exponential increase in the
number of model rocket launches by hobbyists and high powered
rockets for academic, outreach purposes as well as experimental
motor design.The emergence of new technologies like rapid
prototyping, including 3D printing, is changing the approach to
rocket motor design. This has demanded series of small-scale static
fire tests of soon to be manufactured designs to explore the
performance of rocket motors.A simple means of testing thrust is
through a thrust measuring system (TMS), that uses load cells to
detect the amount of thrust produced. The test pad tests the rocket
engines in different regimes of operation by enabling the variation
of parameters like:
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1. Tank pressure
2. mass flow rate
3. Drag aka kick
4. firing time
5. Real time Thrust
D. Architecture
DESIGN
•Timeline: Conceptual-Draft-Final
•Being the systems engineer I was the chief of liaison between
design and avionics apart from a designer.
1. This model of the Static Test Pad, that we built, is in a
horizontal orientation. It has a symmetric shape with a butterfly
wing inspired base, and it is designed to withstand a thrust of
unto 500N. The process of designing this model was
progressive and experimental. We first made a conceptual
prototype design, and then made a draft design, and finally, the
final revision.
(Prototype) !
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2. After modeling this design, we had a rudimentary idea of how
we were going to build our model. To further improve the
model, we listed out the requirements.
3. Portable, flexible-use stand for statically testing rocket motors
and demonstrating elements of rocket propulsion. With its
simple design, the goals of modularity and mobility are
achieved, while procuring accurate performance data. The
robust design of this test stand allows it to be easily transported
and to accurately test different rocket sizes and types.
4. Kulvir submitted his dimensions for the components bay that he
drafted and I attached with a glass cover.
5. Following this, we tried to design load cell mounts for which
Heet came to the rescue.It was planned to be able to vary height
as per the motor with a bolt and sliding tracks.
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6. I came up with the idea for the motor mounts and made an
initial design inspired by lathe machine.
7. The motor mount assembly consists of two identical structures
with three ball-transfers on each. These ball transfers are
equally spaced 120 degrees from each other and will allow the
engine to only move in the axial direction against the load cell.
8. The plates are designed to be a single piece with shafts out of
the back of the ball transfers allowing normal movement to
adjust to motor. Originally, the body tube could be slid into the
series of mounts and tightened.
9. The ball transfers were chosen for their capacity as well as their
capability to work at any angle. They have four ball transfers to
support the weight of the motor and body tube.
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10.Then I was up with the challenge of adapting the base to be
used on uneven terrain.I drafted a leg design based on its use on
the dog robots of Boston dynamics but left it to be altered in a
more effective way by my mates.
11.These insect like feet though appealing had stability issues so
we agreed upon a more down to earth approach.My team mate
Heet came up with an idea taking my reference of a tripod stand
leg.
12.As soon as the design came together my team mate Ashreya
took upon using all bits and building it from scratch to get a
better understanding of materials to be used.
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13.I then came up with the name “metamorpha”, owing to its
similarity to a butterfly’s mutating , changing and restyling
principle in life.This made use of a futuristic concept of
modularity.
•Materials and Components :
We selected the most viable and easily obtainable components from
a plethora of options with consult from Ashreya,Heet and Manish
The basic components that we used for building the static test pad,
are:-
1. Stainless Steel(SS) Square Tube/Beam : The Beam is used as
a primary support structure. It is connected to the base and
reinforced by a 45 degree tube for support, and it provides
support to the mount that holds the load cell. We figured that a
square rod as the vertical rod would provide us better stability,
due to less smoothness on its surface.
2. SS Bottom Plate: This plate forms the majority of the base that
is weighted to provide a firm foundation.
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3. SS Mount frame and Omni Wheels:The frame connector
reduces weight, durable and makes it more compact.They are
created by large rollers, which deform it into the required shape,
shapes, angles, tubes, and so on. As steel softens at high
temperatures, which can cause structural collapse, frames
require have fire protection.Omni wheels rotate and give
flexibility of side movement as well. The rollers are aligned
perpendicular to the wheels axis.
4. Leg assembly: It consists of the Leg , Leg Base , Pin and
Rubber Pad for grip.
5. 3D Printed and Misc Parts; Avionics Bay (PLA), glass cover
and Load Cell Mount.
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•Pricing and Selection:
DESIGN – 19,250 INR / 264 USD
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AVIONICS
To design a Rocket Motor Static Test Pad for testing and acquiring
the required data for the performance analysis of the high powered
rocket motors the avionics is vital.
As I was given the role of systems engineer I assessed the members
who were part of the avionics team and handed over the tasks to
Jatin and Kulvir with routine checks on their progress.To my relief
they were phenomenal on their own with very little occasional
inputs from me.
•Components in the Avionics Bay
1. Arduino Uno R3:The Arduino Uno R3 is a microcontroller
board based on a removable, dual-inline-package (DIP)
ATmega328 AVR microcontroller
2. Amplifier:An amplifier, electronic amplifier or amp is an
electronic device that can increase the power of a signal. We
may need to amplify the digital signal of the data collected
through the load cell, and for that, we can use a AD595
Amplifier.
3. LM35 Temperature Sensor:Thermocouple is used to find the
flame temperature from which we can calculate the specific
impulse and the chamber pressure. We can use AD595
thermocouple amplifier to amplify the signals from the
thermocouple
4. Alternative Data Storage System (SD Card):We have to store
and process the data collected through Arduino. Also, we need
to have a backup, in case of data loss, and due to that, we need
SD Card and SD Card holder
5. WiFi Module:For the purpose of calibration and activating
relay wirelessly, we have used a wifi module. The Wifi Module
we can use is ESP8266 Wifi Module.
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6. Printed Circuit Board (PCB):This Board would be used to
integrate all the electrical subsystems like Arduino Nano,
Amplifier, and the HX711 Board together. Using this, we won’t
have to use connecting wires
7. Arduino Circuit Breaker:This is used to break the circuit if
increase of high voltage supply or burn of any IC or
malfunction of the sensors.
8. LED, Load Cell and Buzzer
9. Systems Check & Ignition Controls
•Working
1. When a motor is affixed to the load cell, and the igniter is
placed and wired, AND the momentary pushbutton is activated,
a 10-second warning time begins, with flashing of a bright RED
LED.
2. When the 10 seconds is up, the relay is closed allowing the full
12V from a gel-cell battery to be applied to the igniter.
Immediately afterwards, the Arduino logs the time, and begins
acquiring force data at 80 samples per second.
3. After 800 readings are acquired, the relay shuts down,
acquisition stops and the stop time is logged. The data acquired
by the Arduino is transmitted to the computer using wireless
modules.
4. We are also planning to attach a SD Card to the Arduino so that
the data can be stored for later purposes.
5. The data acquired from the Arduino is logged into the excel
sheet and the graph is plotted. We can calculate the specific
impulse , thrust and the graph of thrust v/s time.
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•Algorithm Flowchart:
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1.ACTIVATE
SAFETY &
2. 10 second
Warning
3.Ignition Relay
Closes
3.Measurement
Starts :
- 80 Reading Per
Second
4.Number Of
Measurements == 800
5.Relay
Closed
6.Data
Logged
END
- Led Flashes In
10 second
Countdown
- Data can be
stored in a SD
card for
Backup and
also transmitted
to a computer

•Circuit Diagram
•Data Collection System:
Its extremely reliable, safe, and easy to use data acquisition system,
tabulated output in a csv file can generate detailed graphs for the
test.
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•Pricing
AVIONICS – 2,260 INR / 31 USD
SUBTOTAL – 21,510 INR / 295 USD
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E. List of Software that we used to build our Static
Test Pad –
1. ArduinoIDE–This is the software with which we did all of our
programming with the electronics systems. It was the main
system through which we would command the static test pad.
2. Proteus Professional – Proteus was used in simulating the
working of the electronics part of our project. We built circuits
and connections, for relay through the wifi module, and for
calibration factor, and for storing data in SD Card.
3. Solidworks – Solidworks was used for designing the final
computer-aided designing (CAD) model of our project. All of
our components were built again on solidworks. Most of our
simulations were done on Solidworks, while few on Fusion 360.
We ran the linear static stress simulation on Solidworks.
4. Fusion 360 – Another CAD software that we used in our
designing is Fusion 360. I designed all of the initial concept
components and prototypes on Fusion 360, and hence, all the
designing from my side was done on Fusion 360. We ran the
static stress simulation for the concept in it but most advanced
tests couldn't be run on my MacBook.
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F. Assembly and Disassembly
•The whole assembly is assembled and disassembled using a few
M6 nuts and bolts. There are several parts that make up the
entire structure; bottom plate, tube frame, load cell mount, load
cell, motor cap, motor mounts and the avionics bay.
1. Firstly, the bottom plate attaches to the tube frame using 8 sets
of M6 nuts and bolts.
2. Next the motor mounts are fixed according to the required
motor dimensions.
3. The fixed end or the one closer to the load cell is fixed with 2
sets of M6 nuts and bolts.
4. The other mount can be fixed at one of the five positions
provided on the frame with 2 sets of M6 nuts and bolts
according to the dimensions of the motor.
5. The load cell mount is attached to the vertical tubes with 2 sets
of M6 nuts and bolts.
6. Before that, the load cell needs to be attached to the load cell
mount, that has countersunk holes to prevent interference with
the frame.
7. On the other side of the load cell a motor cap is attached to
concentrate the load on the specified area on the load cell.
8. The avionics bay attaches to the frame with 6 sets of M6 nuts
and bolts at the specified position.
9. Finally the motor can be placed in the motor mounts and
centered with respect to the load cell as required.
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G. Results of Linear Static test study:
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H. Observations and Conclusions:
•After carrying out some simulations, we had the data from which
we had to make our ratiocinations . We made few notable
observations. Ashreya did the linear static test study.
1. The more force we applied, the lesser safety factor it was
showing.This is pretty logical, because if we put too much
force, then the beam supports could break, and the motor would
fly off.
2. Linear Static Stress Study ->Max Stress 4.627e+10 N/m2, for a
thrust of 500N.
3. We can also observe that for linear static stress simulation ,the
displacement is much less. This may be due to the fact that
stiffness matrix is constant in the linear analysis.
4. Linear Static Stress Study -> Max Strain 9.369e-06
5. The model has much higher reaction force in the linear static
stress analysis due to the fact that in the case of linear analysis,
the relation between applied force and displacement is linear,
whereas in the case of non-linear analysis, the relation is non-
linear.
6. Linear Static Stress Study -> Max Displacement 1.464E-02mm
7. The yield strength of the model material was 50 times the stress
generated under 500N thrust.S theoretically the structure can
withstand a lot more.But for the problem statement 500N was
more than enough.,
8. It is designed to withstand the 500N thrust of our test rocket
which is also limited by the capacity of our load cell.This
should be sufficient to withstand the force of future high power
rockets designed. If a larger or smaller capacity is required, a
simple replacement of the load cell is all that is necessary
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•From the observations made after studying the simulations,
finally I was able to draw these conclusions about our static test
pad. These conclusions are –
1. Our Static Test Pad would be able to handle the thrust of
500N ,as the safety factor closes on the value of
40-50.Generally when we design any component we start from
a specified FOS, and then go ahead from there.But since we
went the other way, the design ended up being way to strong for
our requirements which is well adapted.
2. Our design is future proof and durable. The total estimated
weight for our design comes out to be around 13kg.
Corresponding this to the safety factor, we can safely say that
we have obtained a good strength/weight ratio.
3. We have used PLA for our avionics bay, due to which it is very
unlikely to get damaged when the motor is tested.
4. We have used nuts and bolts to assemble the parts of the design.
Hence, assembly and disassembly of the design would be very
easy also adding scope to alterations.
5. We have used a Wifi Module in the avionics bay with the
Arduino to remotely activate relay, hence reducing the risk of
being near to the test stand.
6. We have used linear static stress study, hence our simulations
are well with minimum error.
7. We have a SD Card for smooth storage of data. It also serves as
a great backup for the future.
8. In case of motor explosion, the avionics bay is protected by a
thick slab of glass, that would alleviate the damage.We have the
wifi module that helps us control the calibration and relay
wirelessly, while buzzer and led for emergencies and updates.
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I. Precautions:
1. Make sure that the static test pad is able to take wide range of
motor thrusts than the maximum thrust you intend to use.
Adequate Safety factors are really essential in such cases.
2. Double check on the placement of the static test stand you
erect by using a spirit level and clear the ground off any
combustible materials.
3. Place the Static test pad on a rooted object of greater mass
than the thrust of the rocket motor you are testing plus also
keep in mind the centre of mass must be closer to the ground.
4. Ensure to check all the electronic connections on the PCB,
and the nuts and bolts before initiating the command to fire
the ignition.
5. Keep an eye on the environment and local weather to avoid
any damages in case of any inhibitors.
6. In the unfortunate case of fumes, exhaust and sparks being
uncontrollable have a fire extinguisher at close proximity.
7. Always maintain a safe distance from the static test pad
during operation.
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