A Photon-driven Microthruster
through Interaction of Ultraviolet light-emitting Diodes and Solid Polymers
By Koki Inoue,1) Takashi Sakaguchi,1) Tomoyuki Ikeda,1) Hideyuki Horisawa,1) Shigeru Yamaguchi, 1)
Yoshinori Nakayama,2) and Ikkoh Funaki,3)2)

1)
Department of Aeronautics and Astronautics, The University of Tokai, Hiratsuka, Japan
Aerospace Engineering Department, The University of National Defense Academy, Yokosuka, Japan
3)
Institute of Space and Astronautical Science, JAXA, Sagamihara, Japan(Received June 21st, 2019)
The objective of this study is to develop a microthruster, utilizing a reaction force of the molecular flow generated
through dissociation of molecular bonds of polymer targets by irradiation of ultraviolet (uv) photons, namely photochemical
reactions. The uv-photons, carrying higher photon energies, can directly dissociate the molecular bonds having lower bonding
energies than the photon energies. In this study, the uv-photons from two types light emitting diodes were irradiated to twotypes of solid polymer targets, nylon and polyvinyl chloride (PVC), to investigate each mass consumption rates. Furthermore,thrusts caused through the photo-dissociations of each polymer targets were measured using a torsional pendulum thruststand. The results showed that PVC target was capable of generating higher thrusts in lower propellant mass consumption
rates than nylon target.
Key Words: Electric propulsion, Ultraviolet photon, Photochemical reactionplans of developers and programs indicate as many as 3000
microsatellites will require launches from 2016 through20222,3.
Under these activities, the strong demands have arisen for thedevelopment of micro-propulsion devices that enable microspacecraft to perform more advanced missions. These devices
will be required to deliver very low thrust values and low
impulse bits and may feature engine masses and sizes ordersof magnitude smaller than available with current thruster
hardware. The benefit of using electric propulsion (EP) for thereduction of spacecraft mass will likely be even moresignificant for mass limited micro-spacecraft missions4,5.
Various potential EP systems for micro-spacecraft
applications have been proposed and are under significant
development for primary and attitude control applications.
However, further mass reduction and miniaturization of the
propulsion components including new technologies arestrongly required. Although use of EP can be one of the strong
candidates for the micro-spacecraft, simple miniaturization of
conventional EP components is not as physically simple due
to high-voltage or high-current requirements, etc.
For these severe requirements of micro-spacecraft, we
propose a novel, compact, lightweight, and low-power (lowvoltage) propulsion system. This system employs aninteraction of ultraviolet light-emitting diodes (uv-LEDs) andpolymer targets.
Since the invention of the first LED in 1927, the first visible
LEDs in 1962, and the first blue LEDs in 1972, the significant
improvements in photon energies (shortening wavelengths)
and luminous efficiencies of LEDs have been made. Recently,
the first high-brightness blue LED was demonstrated by Dr.
Shuji Nakamura of Nichia Corporation in 1994. After that, uvLEDs employing semiconductor

Nomenclature
cdEFhlV𝜀𝜆: speed of light: separation of electrodes: Photon energy: force: Planck’s constant: length of a side of an electrode: voltage, V: dielectric constant: wavelength of light

1.

Introduction
Micro-spacecraft segments of the satellite4, which has a wet
mass less than 100 kg, have been considered in the aerospacecommunity since the early 1980s and have gained increased
attention ever since. One benefit is the substantial reduction in
the overall life-cycle cost by making satellites less costly toconstruct, due to fewer components and the potential for mass
production techniques. Furthermore, the significance in
reducing launch masses has attracted growing interests inregard to a decrease in mission cost and an increase in launch
rate1. In recent years, a number of private sectors planning useof outer space for their activities have focused on theseadvantages. The micro-spacecraft segments of the satellite
launch industry have been growing rapidly for this reason.
Development activity for a mass less than 50 kg range has
been significantly exceeding that in the 50 - 100 kg range. In
the smaller range alone, the launch rate rapidly rose up to 92
satellites in 2013. Annual number of launches has been
exceeding 100 from 2014, which included some of morepractical missions. Projections based on announced and future1structures using aluminum gallium nitride (AlGaN) or
Indium gallium nitride (InGaN) were demonstrated. As
development of uv-LEDs and its manufacturing technologies
have made a significant progress in recent years, furtherimprovement in the efficiency and further cost reduction canbe expected in near future6. In general, the uv-LEDs aresignificantly compact and lightweight than previous uv-lightsources such as mercury lamps and excimer lasers. In addition,
it is not usually necessary to drive them with high-voltage
power supplies and with time delays for warming up during
operation.

In this study, firstly, mass consumption rates of polymer
targets caused through photo-dissociation of surfaces by uvphotons from the LEDs were measured to verify
photochemical reactions (photo-dissociation) occurring on
polymer surfaces. Secondly, measurement of thrusts by atorsion pendulum thrust stand capable for sub-micro-newton
thrust ranges was conducted.
2. Experimental Procedure
2.1. Mass consumption rate measurement
Firstly, mass consumption rates of polymer targets causedby uv-photons from LEDs were measured. In this study, a uvLED (Nichia Corporation, NC4U133B (T)) of peakwavelength of 365 nm was employed. Specifications of theLED are listed in Table 2. Considering the wavelength of 365
nm, or photon energy, and bonding energies of molecularbonds, nylon with C-N bonds and polyvinyl chloride (PVC)with C-Cl bonds were selected for target materials. As shownin Table 1, the uv-photons of wavelength of 365nm emitted
from the LED, carrying higher energy than bonding energiesof C-N and C-Cl bonds, are capable enough to dissociatethese molecular bonds. Constitutional formulas of nylon andPVC are shown in Fig. 1. A schematic of experimental setup
is shown in Fig. 2. As shown in this figure, the measurement
was conducted in a vacuum chamber kept at 10 -3 Pa and a
distance of uv-LED and target was 4 mm. The uv-LED wasfixed to an aluminum heat sink in order to prevent
degradation of its light emitting performance caused by heatgenerations during long-time drive of the LED in a vacuum
environment.
To measure the mass consumption rates of thetargets during uv-irradiation, uv-photons emitted from theLED were irradiated on the targets for 6 hours continuously.
Using an electronic balance of the minimum resolution of 0.1
mg, mass reductions of the targets before and after uvirradiation were measured to estimate mass consumption
rates.

Table 1. List of bonding energies and dissociation
wavelengths for typical bonds.
Molecularbond

Bonding energy[kJ/mol]

Dissociation
wavelength
[nm]

C-N
C-Cl
C-C
N-H
C-H
H-Cl

266.5
321.9
352.9
384.7
408.6
426.6

448.9
371.6
339.0
311.0
292.8
280.4

The mechanism of thrust generation of uv-photon
propulsion technique proposed in this paper differs fromthose of conventional light propulsion concepts for spacecraft,such as laser ablation propulsion (LAP) and photon rockets(or lightsails)7. In the LAP, an intense laser beam strikes acondensed-matter surface (solid or liquid) and produces a jetof high temperature vapor or plasma, and just as in a chemicalrocket, thrust is produced by the resulting reaction force on
the surface. The lightsail utilizes the momentum of photons
to produce a thrust directly through reflecting photon beamsby pellicle mirror. Unlike the previous light propulsion
mechanisms described above, our uv-photon propulsion
mainly employs photochemical reactions, or photodissociations, induced by a large energy of individual photons,
instead of thermal dissociations or radiation pressure. In thiscase, given higher energies from the uv-photons, theintermolecular bonds of the polymer target will directly be
disconnected and exhausted from the surface. At the sametime, the thrust is induced as a reaction force. These
photochemical phenomena induced by uv-rays have been
applied in various fields, such as surface modification andoptical cleaning in industry8.
Relations of typical molecular bonds and their bonding
energies including corresponding dissociation wavelengths ofphotons, λ, are listed in Table 1. In this table, photon energies,
E, are calculated from the following relation,

Figure 1. Constitutional formulas of polymer targets.

𝐸 = ℎ𝑐/𝜆 (1), where h is Planck’s constant, and c is the speed of light.
Depending on the wavelengths, or photon energies, of the uvphotons, some organic compounds, or molecular bonds, canbe directly dissociated.

Figure 2. Schematic of experimental setup for massconsumption rate measurement.

22.2. Thrust measurement
An experimental setup for thrust measurement is shownin Fig. 3. A thrust stand consists of a horizontal torsion
pendulum was installed on a vibration isolator with rubbercushions in a vacuum chamber. The vacuum chamber waskept at 10-3 Pa during thrust measurement. Displacement of
the pendulum arm was measured by a laser displacement
sensor (LDS) with the resolution of 0.1 µm. A propellant
target was fixed on an edge of the pendulum arm. Photons ofa uv-LED were irradiated to the target surface with aseparation of 4mm. In order to make fine adjustment of theneutral position of the arm, an end of the flex pivot of thependulum was connected to a rotary stage with an actuator.

Results of mass consumption rate measurement are listed
in Table 3. After irradiation of uv-photons for 6 hours invacuum, mass reductions of nylon target and PVC target were
9.8 mg and 3.5 mg, respectively. From these results, massconsumption rates for nylon and PVC targets were 0.45 μg/s
and 0.16 μg/s, respectively. On the other hand, from theresults of a preliminary experiment conducted to survey mass
reductions caused by the outgassing of each polymer targets,it was verified that each target mass decrements induced
simply by being exposed to a vacuum environment for 6hours were about 0.04% of their initial masses. Considering
this effect, mass reductions of nylon target and PVC target
induced by irradiation of uv-LED were estimated to be 9.3
mg and 2.9 mg, respectively.
According to the above results, it is confirmed that the mass
reduction largely caused by photochemical dissociations istaking place on the target surfaces by irradiation of uvphotons, then intermolecular bonds with the targets are
disconnected and the molecules are discharged. It is also
confirmed that the mass consumption rate of PVC target was
about one third of that of nylon target.
3.2. Results of thrust measuremont
Typical waveform of the thrust stand displacement isshown in Fig. 4. In this figure, it can be seen that a thrust force
was detected in a duration of irradiation of uv-photons on thetarget surface for 4 s.

Figure 3. Schematic of experimental setup for thrustmeasurement. Viewed from above.
A calibration of the thrust stand was conducted using anelectrostatic actuator consist of square copper plates. An
electrostatic force F was generated when a voltage V wasapplied to two parallel square plate electrodes. In this case,the electrostatic force considering fringing effect isdetermined by the following equation,(2)
13 2 𝑉2𝐹 = 𝜀0 (𝑙 + 𝑑) ( )28𝑑, where ε, dielectric constant, l, a length of a side of theelectrode, and d, a separation of the electrodes.
The thrust generated through an interaction of uv-photons
and target was calculated from the relation between thedisplacement and the electrostatic force estimated from thetheoretical equation (2).

Figure 4. Thrust waveform with nylon target. UV-LED
was irradiated to the target surface between 10 s and 14 s.

3. Experimental Results
3.1. Results of mass consumption measurement
Table 3. Results of mass consumption ratemeasurements.
Target material

Nylon

PVC

Initial mass [mg]1178.21376.5

Final mass [mg]1168.41373.09.8

3.5

0.450.16

Decrement [mg]
Estimated mass consumption rate[µg/s]

Figure 5. Calibration waveform with electrostaticactuator. Applied voltage = 30 V.
3

Examples of results of calibration by an electrostaticactuator are shown in Figs. 5 and 6. Results of calibrated
thrusts calculated from the calibration are listed in Table 4.
From this table, it was confirmed that the thrusts were0.103±0.037 µN and 0.414±0.046 µN, for nylon and PVC
targets, respectively. Considering resolution of the LDS,
variations of the electrode separation and applied voltages,
the accuracy of the thrust calibration was within ±5 nN.
From Table 4, it can be seen that generated thrusts tend to
become lower with the repetition of LED irradiation in bothcase. One of several causes for this phenomenon could be thatthe exhausted substances from targets adhered to the LED
surface and caused lowering of uv-light intensity. Anotherpossible cause is that adsorbed impurities on the target
surfaces were discharged from target materials, and increased
thrusts in the early stages of uv-irradiation. From these results,
cleaning of the target surface before irradiation of uv-LED
may be effective to obtain stable thrusts.
The specific impulses estimated from the massconsumption rates and the thrusts for nylon target were
estimated to be 14 to 37 s, while those for PVC targets were
230 to 320 s. In addition, the coupling coefficients (thrust-toradiant flux ratios) were 29 to 83 μN/kW for nylon target and
176 to 245 μN/kW for PVC target.

On the other hand, a thrust, Fphoton, induced only byphoton pressure of light of power P (= 2040m W) is expressed
as,𝐹photon = 𝑃/𝑐 = 6.80 nN

The thrusts measured in this experiment were larger at
least in two orders of magnitude than the photon pressure of
the uv-photons.
4. Conclusion
In this study, a fundamental investigation on use ofinteraction of uv-LEDs and polymer targets for
micropropulsion thruster was conducted. The following
conclusions were obtained;
1) By irradiating uv-photons to nylon and PVC targets
continuously, mass reduction primarily caused byphotochemical reactions were observed on each target surface.
2) By measuring the thrust by using the torsional
pendulum thrust stand, it was confirmed that nylon and PVC
targets can both generate thrust in durations of irradiation ofuv-photons.
3) In the case of using PVC as a propellant, the massconsumption rate and the thrust were about one third and 4
times of that of nylon, respectively.
From these results, specific impulse and coupling
coefficient of PVC were considered to be superior to those ofnylon. In addition, the thrusts produced by photo-dissociation
were larger in two orders of magnitude than the photonpressure of the uv-photons.
The differences between thrust, or specific impulses, ofnylon and PVC are supposed to be related to the gas species
exhausted from target surfaces and its exhaust speed, or vapor
pressure. One of our future works will include aninvestigation on correlations of thrust performances andtarget properties through photochemical reactions for various
polymers.

Figure 6. Calibration curve of thrust stand.
R2 = 0.97.

Table 4. Calibrated thrusts of polymer targets.
Number ofmeasurement12345678910

Thrust of nylon
[μN]
0.162
0.166
0.136
0.116
0.079
0.083
0.084
0.075
0.067
0.063

(3)

Thrust of PVC
[μN]
0.504
0.463
0.445
0.415
0.392
0.387
0.404
0.358
0.358
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