The Criticality of Thrust Measurement Testing in Aerospace

Interface is a force measurement solutions provider for many of the largest and most innovative aerospace and space systems organizations. Our measurement devices are utilized to test various aircraft and space vehicle components, including thrust testing for jet engines, gas turbines, and propulsion systems.

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Thrust measurement is critical when designing and developing aircraft and spacecraft. These critical measurements are used to build and test rocket engines for launch vehicles and missiles. Thrust testing is also vital for maintenance and quality inspections of vehicle engines and systems. By measuring thrust over time, engineers can identify any potential engine problems and take corrective action before they cause a failure.

Thrust measurements ensure rocket and airplane engines produce enough thrust to safely launch, fly, and land. If an engine is not producing enough thrust, it could lead to a catastrophic failure. Trust testing also helps to improve fuel efficiency and reduce emissions.

The force emitted by a thrust engine dictates the size and speed at which a payload can be lifted off the ground. Enormous amounts of thrust are needed to get a spaceship out of the Earth’s atmosphere or propel a jet engine to move faster than the speed of sound. Thrust must be measured precisely because applying too much thrust to an aircraft may damage it, or using too much thrust at a rocket’s liftoff can use too much fuel.

Interface force sensors provide extremely accurate data to assess the amount of force, helping engineers tune thrusters to provide the right amount of force for the size and speed needed to launch or lift their vehicle.

Thrust is measured by placing the thrust engine on a test stand. Then, as the rocket engine burns fuel and creates thrust, the force of the thrust creates compression force on the load cell sensor. As this happens, a mechanical signal is converted to a digital signal, and this data is sent back to the engineer through a data acquisition device, who can then assess, monitor, and record that data.

A load cell used to measure thrust force must be rated for extreme heat. A typical load cell could not provide an accurate measurement when placed in a temperature environment that the sensor’s materials could not handle. Interface offers a wide range of load cells rated explicitly for this type of testing.

To give you a sense of the power and environment of thrust testing, you can see the thrust test of a jet engine in action posted on the U.S. Defense News YouTube channel.

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Jet Engine Thrust Testing Application

A customer wanted to conduct a static jet engine thrust test that could accurately determine the engine’s thrust, burn time, chamber pressure, and other parameters, providing invaluable data to propellant chemists and engineers. They needed a high-accuracy load cell with excellent repeatability to withstand thrust forces in very harsh environments. From ignition to burn-out, Interface’s 1000 High Capacity Fatigue-Rated LowProfile™ Load Cell was ideally suited based on their performance for this application.

The load cell reacted to the thrust forces produced by the jet engine, and the signals were collected and recorded to create a “thrust curve” of the engine. The performance of an Interface LowProfile™ Load Cell allowed engineers to be confident in the data acquired from the static testing. Additionally, the repeatability of the load cell resulted in reduced time between tests, making static jet engine thrust testing more efficient. The 9330 battery-powered high-speed data logging indicator captured the data for analysis.

Thrust measurement ensures safety, reliability, and performance.

Interface is a long-time provider of  Aerospace and Defense Industry Solutions.  Here is another video to watch to learn more about Interface’s role in aerospace innovation.

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Go here to learn more about thrust and other force test examples in the aerospace industry.

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After analyzing the above mentioned test campaigns with the AST thrust balance a few issues appeared, and this triggered the development of the DEPB (DLR Electric Propulsion Thrust Balance) thrust balance. The ideas behind development of this new system can be listed as:

• to overcome the drawbacks of the AST device: its large size and heavy weight inhibits testing and upgrading the balance in one of DLR’s smaller vacuum chambers

• source code not available to DLR: the code cannot be modified or upgraded by DLR

• thermal drift: the observed drift must be lower for long term measurements

• issues with regulation loop: sporadic oscillations appear and the feedback loop optimization is tricky

The new system should fulfil the following specifications:

• device should fit in a small facility with 1 m diameter

• maximum load capacity: 50 kg

• maximum thrust: 500 mN

• full control over calibration procedures

• software code written inhouse by DLR

Thermal drift may be caused by a differential expansion of the bearings and crossmembers of the pendulum design (see Fig. 2(f). Employing design (e) of Fig. 2, merging crossmembers and bearings into one piece, and using a low expansion material (quartz) for these was the adopted option. The thermal expansion coefficient of quartz of 5.1⋅10−7 K−1 up to 100°C. This is more than one order of magnitude lower compared to aluminum which is extensively used in the AST balance. The assembly sketch of the main components of the DEPB device can be seen in Fig. 6. In the figure caption the components are names. The thruster plate is held by the quartz rods—four oriented vertically and four crossed for inhibiting sidewise movements. Technical details about the special features can be found in [22]. Figure 7 shows the open DEPB balance with its quartz rods as flex bearings, and an additionally mounted eddy current brake for oscillation reduction.

Assembly sketch of the main components of the DEPB device. The eddy current brake not shown here. Parts: 1 thruster platform; 2 housing top plate; 3 upper moving platform; 4 housing frame; 5 flexible bearing rod assembly; 6 weigh cell; 7 weigh cell supporting frame; 8 calibration stage assembly; 9 Force transmission piece with screw; 10 base plate; 11 contact piece; 12 connection stud to thruster platform

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Open DEPB thrust stand. One can see some of the quartz rods holding the top thrust platform. Here, the eddy current brake is mounted: copper plate between two magnets

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Weigh cell sensor

The DEPB thrust stand uses a commercial weigh cell (load cell) from Sartorius®, a worldwide known company in the weighing, filtering and pharmaceutical market. The sensor has the reference WZA224, a full range of 220 g and a resolution of 0.1 mg. Its weight is about 2 kg, and it comes with an external control unit and an optional display unit. The communication with a computer is done via a RS232 interface. For simplicity this unit is short-named WZA in the following text. Its measurement principle is based on an actively compensated mechanism with an optical position sensor. The whole unit is well enclosed for different commercial applications. With this sensor we can rely on an often-used product with the profound background knowledge of a well-known company. The drawback is that this WZA unit is neither qualified for vacuum nor for a measurement of horizontal forces. Therefore, extensive qualification tests had to be performed before being able to use the WZA for our purpose. The first question was if the sensor would be able to function in vacuum, e.g. can the internal components withstand vacuum, and is this also valid for the spirit level mounted on the outside. The second question was if the WZA can operate in a position rotated by 90° degrees and measure a horizontal force. This is very different compared to a vertical weighing task, like in the arrangement it was designed for.

After completion of our qualification tests for the weigh cell these questions could be answered positively, although the force range in the vertical arrangement is largely reduced compared to the original orientation. This may be attributed to the internal complex mechanical design, a Sartorius® intellectual property design. We have a maximum measurable force of about 0.8N instead of 2N. But in conclusion, the WZA sensor can be used in the envisaged thrust balance setup.

DEPB calibration

The WZA cell in its tilted position in the DEPB device cannot directly be used as a calibrated force sensor, but only as a sensor giving a force-dependent signal. As the sensor is used in a tilted orientation its internal force calibration is not valid anymore. Furthermore, the sensor is mounted in a system of mechanical components that introduce additional back-pushing forces. This makes a calibration of the new system mandatory. For this task we have different options. The wire/weight/pulley method was discarded here due to high complexity and several issues, such as a very delicate mechanical construction, the slipping of the wire on the bearing and the introduced inaccuracy. A much simpler system was developed and tested separately and, after confirmation of its viability, implemented in DEPB. Figure 8 shows the principle. A spring on a linear motorized stage can produce a horizontal force on measurement device. Figure 9 sketches its implementation in the thrust balance. The horizontal spring force is directly applied to the thrust platform and acts like a thruster. The thrust platform propagates the force onto the WZA sensor, and the latter gives an arbitrary signal. A comparison of the WZA signal with the known force of the spring stage leads to the calibration factor.

Principle of the DEPB calibration. A calibration spring on its motorized motion stage produces a force F depending on the stage travel x, after contacting

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Calibration setup for the DEPB balance design. A motorized stage pushes the calibration spring against the thrust platform. The WZA sensor is show in its arrangement for horizontal force measurement

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Spring stage for calibration

In Fig. 8 a basic coil spring was shown, but such a design may fulfil Hooke’s spring law but could bend to one side if compressed. In our application the goal is not to tune the spring for getting Hooke’s law but to have a reproducible constant calibration behavior.

Several spring layouts were tested, including the standard cylindrical coil. The final shape is not a cylindrical wound wire but a straight spring steel wire with a 90° bent at the contact point. It turned out that this shape makes lateral bends much less likely. Figure 10 shows the bent spring wire on the motorized stage. The contact point to the thruster platform is the bent end on the left (see figure), which has a smooth rounded tip that does not punch into the contact surface.

The spring element of the DEPB balance is a single bent wire. The contact point is the bent end on the left, which has a smooth rounded tip

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The actuator for the spring stage is a Newport® motorized stage, and its specifications are listed in Table 3.

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The spring stage must be calibrated as a system in order to have a function giving force versus stage travel F(x). This calibration is performed in an arrangement shown in Fig. 11. The motorized stage with the spring is mounted above a force measurement device. For convenience we use a second Sartorius® weigh cell, but this time in its nominal orientation, and this results in a valid calibration of measured forces. The motion stage moves the spring vertically downwards touching the WZA and producing a force. This is schematically shown in the inset graph on the right.

Setup for calibration the spring. A WZA load cell is used in its ordinary position (horizontal) and the motion stage pushed the spring against it. For simplicity sake we still show a wound wire spring. After getting contact the force increases with stage travel (see right inset graph)

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Figure 12 plots the recorded force data (crosses) versus the stage travel for an exemplary run. The travel range starts at 0 mm and goes up to 9.5 mm, which gives 500 mN of force. This is the maximum force that we need, according to the initially set specifications. The spring may work a little further, but the reversible behavior deteriorates beyond about 10 mm.

Spring force versus motion stage travel data. Shown are data (crosses), a polynomial fit of order 5 through the data points, and this fit shifted to left passing the origin in its quasi-linear part (solid line)

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Trying to fit the data shows that a linear approach can well be used from about 3 to 6 mm travel. Then we observe a reproducible kink with an increase in slope. This behavior is not what is expected by Hooke’s law, but here we may not expect an ideal behavior as mentioned above.

After this kink, a second more linear slope appears in the travel range 6 to about 9.5 mm. The kink may come from the geometry of the spring element in conjunction with its mounting. It may have been possible to connect two linear fits, but we would have to deal with the not well-defined position of the kink. Therefore, a polynomial fit was tried as a better and more soft approach. For an acceptable result we have to choose a fifth order polynomial of form:

$$\begin{aligned} F_{\mathrm{spring}} ( x ) = C_{s5} \boldsymbol{\cdot } x^{5} + C_{s4} \boldsymbol{\cdot } x^{4} + C_{s3} \boldsymbol{\cdot } x^{3} + C_{s2} \boldsymbol{\cdot } x^{2} + C_{s1} \boldsymbol{\cdot }x+ C_{s0}\ [N]. \end{aligned}$$

(1)

The need for 5th order comes from aiming at a correlation coefficient below 0.9999 for the fit to data. Data and fit (dotted line across the data) are shown in Fig. 12. The next step is to shift and normalize the fit so that \(\tilde{F}_{\mathrm{spring}} ( x=0 ) =0\), which results in finding the zero points. Polynomials might have more than one zero point, but we need the zero point closest to the data left side (the quasi-linear part). The zero point then identifies the point of contact of the spring to the WZA. With that we have the new function:

$$\begin{aligned} \tilde{F}_{\mathrm{spring}} ( x ) ={}& C_{s5} \boldsymbol{\cdot }( x+ x_{0} )^{5} + C_{s4} \boldsymbol{\cdot }( x+ x_{0} )^{4} + C_{s3} \boldsymbol{\cdot } ( x+ x_{0} )^{3} + C_{s2} \boldsymbol{\cdot }( x+ x_{0} )^{2} \\ &{}+ C_{s1} \boldsymbol{\cdot } ( x+ x_{0} ) + C_{s0}. \end{aligned}$$

(2)

For the data set shown in Fig. 12 the numerical values for Eq. (2) are:

$$\begin{aligned} &C_{s5} =-0.00004316\ \biggl[ \frac{N}{mm^{5}} \biggr], \\ &C_{s4} =+ 0.00127109\ \biggl[ \frac{N}{mm^{4}} \biggr], \\ &C_{s3} = - 0.01401273\ \biggl[ \frac{N}{mm^{3}} \biggr], \\ &C_{s2} =+ 0.07285354\ \biggl[ \frac{N}{mm^{2}} \biggr], \\ &C_{s1} =+ 0.11305771\ \biggl[ \frac{N}{mm} \biggr], \\ &C_{s0} =0\ [N]. \end{aligned}$$

The fit has been forced to pass zero, and this is shown by elimination of \(C_{s0} =0\). This constraint would not have been necessary but allowing a constant value only marginally improves the accuracy.

With this fit we are able to calculate the offset of x needed for Eq. (2):

$$\begin{aligned} x_{0} = 2.51378\ [ \mathrm{mm} ] . \end{aligned}$$

(3)

Pasting the above coefficients into Eq. (2) gives the spring force for x-values starting at zero, i.e. the point of contact. This is necessary because the motion stage travel up to a contact point is different in the spring calibration setup compared to its mounting position in the thrust balance.

The final calibration function for the mentioned spring calibration data is:

$$\begin{aligned} F ( x ) ={}&{ -}0.00004316\boldsymbol{\cdot } ( x + x_{0} )^{5} + 0.00127109\boldsymbol{\cdot } ( x + x_{0} )^{4} - 0.01401273\boldsymbol{\cdot } ( x + x_{0} )^{3} \\ &{} + 0.07285354 \boldsymbol{\cdot } ( x + x_{0} )^{2} - 0.11305771 \boldsymbol{\cdot } ( x + x_{0} ). \end{aligned}$$

(4)

One comment must be made here. One may be tempted to use the fitted function for extrapolating to larger x values. Using Eq. (4) and extending the calibration to values outside the data pool (solid line in Fig. 12) clearly shows that the polynomial will produce wrong results. Such a risk would be lower if we would have used a linear fit in the first place. But in our case the travel range must be confined to 7 mm after point of contact for avoiding the issue of wrong extrapolation. Travel values below zero are not possible due to the travel limit of the motorized stage.

In summary, with the motion stage mounted in the DEPB we are able to apply calibrated forces from 0 up to 500 mN to the thruster platform.

Software

The DEPB user interface is designed around the commercial components like the weigh cell and the motorized motion stage. It allows to perform the above-mentioned calibration which generates a specific file with the actual calibration data. The WZA features a tare command which is needed for its commercial weight balance applications. This option is not used here but the offset is corrected in the DEPB controller software in conjunction with the calibration function.

Long term data recording can be done within a user-defined interval. A second option permits a recording with maximum data acquisition rate, which is around 20 Hz depending on filter settings and interface communication speed. In this mode the software only offers a limited maximum number of data for preventing communication data storage overflow. When using fast data acquisition, the WZA internal setting must be selected for a very low noise environment, which means using the smallest averaging window. The feature of high-speed data acquisition should only be used with care and during fast response tests.

Cooled enclosure

For keeping the DEPB balance at a constant temperature the balance frame was equipped with cooled sidewalls. This cooling system is different from the approach used for the AST balance, which is controlled heated to a temperature above ambient. The balance with its coolers is shown in Fig. 13 during a test in a cleanroom for evaluating the thermal stability. The idea was to run the system over a few days and observe the balance behavior responding to the daily room temperature variation. Table 4 shows the results of balance operation without sidewalls, with sidewalls but no cooling water flow, and finally with a cooling water flow of 24°C. The evaluation parameter is the standard deviation of the recorded thrust measurement (in mN). Fitting sidewalls improve the stability of the measurement over time by an order of magnitude. The cooling water flow adds a further improvement of about 20%, and this is valid for an environment without additional heat sources.

DEPB device in a cleanroom tent with closed sidewalls and watercooled plates attached to the sides

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First results with the DEPB system

At the moment, the DEPB device is in a development status and did not yet run in conjunction with a real EP thruster. A very critical point is the routing of cables and feed lines from the fixed chamber environment onto the thruster platform. A wrong placement or tensioning of these leads to bias or other disturbing effects. Therefore, a first qualification was be done without a thruster. An electromagnetic push mechanism for thruster simulation was set up. Figure 14 shows this mechanism, which is made of a relay coil with an attached pushrod. The coil can be actuated by a DC current, and the pushrod simulates a thrust force. By tuning the DC current a force close to full scale can be obtained, and a value of 0.87N was selected. With this simple device basic response tests are feasible.

Force actuator made of a relay with a pushrod

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In Fig. 15 we have the data record of a step function of 0.87N lasting for about 6 s, produced by the coil pushrod device. It is interesting to see the rise time of the system for an estimation of the measurable pulses. In Fig. 16 the rise time is determined by the time needed to step from 10% signal up to 90% signal. The result is a time of 0.24 s.

Recording of a load step generated by the force actuator

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Determination of the rise time of the balance. The 10% to 90% rise time of the balance is 0.24 s

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A sequence of fast pulses was recorded in the fast data acquisition mode, and this is shown in Fig. 17. The balance is able to measure pulses down to a repetition rate of 1 Hz. Such repetition rates may be useful for testing thrusters in a pulsed firing mode. Pulses occurring at a higher rate can still be resolved, but absolute value is not correct which is explained by the rise time of 0.24 s.

Recording in fast mode with maximum acquisition frequency of 20 Hz. The spikes correspond to load pulses generated by the force actuator

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