Thesis (MScEng)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: The focus of this thesis is on the investigation of the functioning of a liquefied-gas
thruster. Such a thruster could be used to provide secondary propulsion to a microsatellite
in orbit. A general overview of the need for thrusters in micro-satellites is put
forward in the introduction. Motivation for deciding to investigate a liquefied-gas
system is presented. Recent developments in the field of micro-satellites are
discussed as well as their relevance to the project undertaken. Fundamental
background theory relevant to the engineering problems associated with the
development and analysis of such a system is also presented. Computer programs
were written to simulate such a liquefied-gas thruster system. The experimental work
carried out to analyse the system from a practical view-point is documented.
Attention is also given to the measurement and calibration techniques used to obtain
experimental data.
One-dimensional fully explicit transient mathematical models of the thruster system
were developed to model the system using both compressed air and butane as
propellants. These models were incorporated into computer programs used to
simulate the transient behaviour of the system. Although it is intended to use butane
as the propellant onboard a satellite, the reason for modelling and simulating a system
using compressed air is because air is a convenient fluid to work with from both a
theoretical and practical point of view.
An experimental model of a thruster system was designed, built and tested using air
and butane as propellants. Most of the model was built using perspex to allow for the
observation of the two-phase behaviour of the propellant inside the system. Locally
purchased components were used for the solenoid and fill valves. Readily available
butane lighter fluid was used for butane testing. Self-made heating elements were
used to provide heat input to the propellant. Testing was done at different back
pressures ranging from 100 kPa down to 20 kPa in a vacuum chamber.
Good comparison between theoretical and experimental results was obtained for air.
Theoretical results for peak thrusts tended to over predict experimental results by approximately 15 % for a system exhausting to a pressure of 100 kPa. Peak thrusts as
high as 0.2 N were obtained for vacuum tests conducted at an absolute pressure of 20
kPa.
Peak thrusts of approximately 50 mN were achieved for experimental testing III
atmospheric conditions using butane with a starting pressure of between 270 and 290
kPa. Typical average thrusts of between 20 mN and 30 mN were noted for butane
testing with initial pressure of between 200 to 300 kPa. Peak thrusts of over 0.1 N
were observed for vacuum testing at an absolute pressure of 20 kPa. An equation to
correlate the experimentally determined average thrust as a function of pulse duration
and starting pressure was developed. This correlated most of the experimental data to
within ±25 %. Theoretical results for butane testing are able to predict peak thrusts
within approximately 20 % for starting pressures in the range of 200 to 300 kPa.
Since the project was an exploratory investigation into a liquefied-gas thruster, some
additional aspects relating to such systems were also given attention. The effect of
liquid propellant motion or sloshing was considered and recommendations regarding
the design and placement of the propellant tanks were made. The use of heat pipes as
an alternative to electrical heating elements was investigated and some elementary
design aspects are presented graphically. The management of the liquid propellant
using surface tension devices was examined qualitatively.
Recommendations relating to future projects in the field of simple, low-cost
propulsion systems for micro-satellites are put forward. More specifically these
recommendations are with regard to: thermo-fluid modelling of the propellant, future
experimental work to be done, techniques to measure small thrusts and vacuum
chamber testing. / AFRIKAANSE OPSOMMING: Die tesis ondersoek die funksionering van 'n vervloeidegas stuwer. So 'n stuwer kan
gebruik word om sekondêre aandrywing aan 'n mikro-satelliet in 'n wentelbaan te
verskaf. 'n Algemene oorsig oor die behoeftes van stuwers vir mikro-satelliete word
voortgesit in die inleiding. Redes vir die gebruik van 'n vervloeidegas stuwer word
bespreek. Onlangse ontwikkelinge in die veld van mikro-satelliet aandrywing word
bespreek asook die toepaslikheid daarvan. Fundamentele teoretiese agtergrond
verbonde aan die ontwikkeling en analise van so 'n stuwer stelsel word ook gegee.
Rekenaarprogramme is geskryf om die gedrag van so 'n stuwer stelsel te simuleer.
Eksperimentele werk is gedoen om die stelsel vanuit 'n praktiese oogpunt te analiseer.
Aandag word ook gegee aan die metings- en kalibrasietegnieke soos toegepas vir die
eksperimentele werk.
Eendimensionele volle eksplisiete wiskundige modelle is ontwikkelom die
oorgangsgedrag van die stuwer-stelsel te simuleer met beide lug en butaan as
dryfmiddel. Hierdie modelle is geïnkorporeer in die rekenaar programme om die
stuwer stelsel te simuleer. Alhoewel dit beoog word om butaan as die dryfmiddel aan
boord die satelliet te gebruik, is lug ook gebruik vir simulasie weens sy gerieflikheid
as 'n vloeier uit beide 'n teoretiese en 'n praktiese oogpunt.
'n Eksperimentele model van die stuwer stelsel is ontwerp, gebou en getoets met beide
lug en butaan as dryfmiddels. Die model is hoofsaaklik uit perspex gebou sodat die
twee-fase gedrag van die butaan uitgebeeld kon word. Vrylik beskikbare butaan
aansteker vloeistof IS gebruik VIr butaan toetsing. Selfvervaardigde
verhittingselemente is gebruik om hitte aan die dryfmiddel te verskaf. Toetse is
gedoen deur verskeie omgewingsdrukke varieërend van 100 kPa af tot 20 kPa in 'n
vakuumtenk te gebruik.
Goeie ooreenstemming tussen die teoretiese en eksperimentele resultate vir die
toetsing van lug is verkry. Die teoretiese resultate neig om die piek stukrag 15 % hoër
te voorspel as die eksperimentele resultate vir 'n stelsel wat tot 'n omgewingsdruk van
100 kPa by die uitlaat. Piek stukragte van meer as 0.2 N is gekry vir vakuum toetse
wat gedoen is by 'n omgewingsdruk van 20 kPa. Tydens eksperimentele toetsing met butaan teen 'n aanvanklike druk tussen 270 en
290 kPa, in atmosferiese toestande, is piek stukragte van ongeveer 50 mN behaal.
Tipiese gemiddelde stukragte van tussen 20 en 30 mN is waargeneem vir butaan
toetsing teen 'n aanvanklike druk tussen 200 en 300 kPa. Piek stukragte van meer as
0.1 N is behaal vir vakuum toetse met 'n absolute druk van 20 kPa. 'n Vergelyking
om die gemiddelde stukrag, wat eksperimenteel bepaal is, as 'n funksie van puls
tydsduur en aanvanklike druk te korreleer, is ontwikkel. Die meeste eksperimentele
data se afwyking van die korrelasie-vergelyking was minder as 25 %. Teoretiese
resultate vir butaantoetse het piek stukragte binne 20 % van die eksperimenteel
metings korrek voorspel vir aanvanklike drukke tussen 200 tot 300 kPa.
Weens die feit dat die projek 'n oorhoofse ondersoek in In vervloeidegas stuwer
behels, is aandag ook gegee aan addisionele aspekte wat verband hou met sulke
stelsels. Die effek van die vloeistof-dryfmiddel se onstabiele beweging in sy tenke is
in ag geneem en voorstelle vir die ontwerp en plasing van die dryfmiddel tenke is
gemaak. Die gebruik van hitte pype as 'n alternatief vir elektriese verhittingselemente
is ondersoek. Verskeie ontwerp aspekte word grafies voorgestel. Die bestuur van die
vloeistof-dryfmiddel deur van oppervlak spannings apparaat gebruik te maak, is
kwalitatief ondersoek.
Voorstelle vir verdere navorsing in die veld van eenvoudige, lae-koste stuwer stelsels
vir mikro-satelliete is gemaak. Meer spesifiek is hierdie voorstelle gerig op die
termo-vloeidinamiese modellering van die dryfmiddel, verdere eksperimentele
navorsing, tegnieke om klein stukragte te meet en vakuumtenk toetse.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/49765 |
| Date | 12 1900 |
| Creators | Weyer, Robert Bernhard |
| Contributors | Dobson, R. T., Van der Westhuizen, K., Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 1 v (various foliations) : ill. |
| Rights | Stellenbosch University |
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