ECI: MTP-03-34 MICROGRAVITY ACTIVITIES FOR THE VINCI ...anne.pacros.pagesperso-orange.fr/career/Davos-MTP-03-34.pdf · phenomena in microgravity may prove that such a system is not

Preliminary Proceedings MTP-03-341

Proceedings of the Microgravity Transport Processes inFluid, Thermal, Biological and Materials Sciences III

Davos, Switzerland.September 14 to 19, 2003

ECI: MTP-03-34

MICROGRAVITY ACTIVITIESFOR THE VINCI ENGINE RE-IGNITION CAPABILITY

Anne Pacros, Jérémy FolletSnecma Moteurs

Direction Grosse Propulsion à LiquidesForêt de Vernon - 27208 Vernon Cedex - France

Bruno VieilleCentre National d'Etudes Spatiales

Rond-Point de l'Espace91023 Evry-Courcouronnes Cedex - France

ABSTRACT

The VINCI engine will power the Ariane 5 ESC-Bupper stage and contribute to providing a 12-tonpayload capability in geostationary transfer orbit tothe Ariane 5 launch system. VINCI is a cryogenicexpander cycle engine and produces a 180-kNthrust using liquid oxygen (LOX) and liquidhydrogen (LH2). It is designed to be restartable upto 4 times on orbit, which broadens the array ofpossible missions.

Before a reignition, the gravity level onboard thestage is very low and this may have an effect onseveral crucial steps in the reignition process. Themain concerns are fluids behavior (filling of theinjection cavity) and heat transfer during chilldown(pool boiling and convective boiling in pipes ofvarious dimensions). Predicting the magnitude ofthese effects is of prime importance in order to takethem into account in the design and eventuallyjustify the reignition capability. This paper describesthe expected effects of microgravity on the VINCIengine and the plan of dedicated studies that areforeseen.

INTRODUCTION

In June 1998, the European Space Agency (ESA)took the decision to develop a new cryogenic upperstage, named ESC-B, powered by a new cryogenicengine, the VINCI (figure 1). These programs arepart of the Ariane 5 Evolution program, acontinuous upgrade of performances of the Ariane 5launcher. The ESC-B and VINCI developments areprograms of the European Space Agency (ESA)whose management has been delegated to CentreNational d'Etudes Spatiales (CNES), the FrenchSpace Agency.

Figure 1: VINCI Engine CAO Model


The VINCI overall system design and integration isunder the responsibility of Snecma Moteurs inVernon, France, as well as the functional propulsivesystem of the VINCI (pressurization equipment,helium command system, etc). Major subsystemcontractors are EADS-ST (Ottobrunn, Germany)for the combustion chamber and Avio (Turin, Italy)for the oxygen turbopump.

From the beginning of the project, the requirementsfor low cost, high performance, multiple ignitions(up to 4 restarts on orbit) and high reliability haveled to the choice of an expander cycle for theVINCI (figure 2).

Figure 2: Simplified VINCI flow diagram

The VINCI uses liquid oxygen (LOX) and liquidhydrogen (LH2) to produce a 180kN thrust with464 seconds ISP. But what is more, the VINCIintroduces the concept of versatility: with itsmultiple-firing capability, it gives Ariane 5 thepossibility to perform a broad array of missions. Forexample, in the GTO/GTO+ mission, a first payloadis injected on a GTO orbit; then a 5.5-hour ballisticphase occurs before the VINCI is restarted. At theend of this second boost, a second payload isinjected on another GTO orbit (figure 3).

Figure 3: ESC-B mission with 1 restart(VINCI boosts are symbolized by )

VINCI is the first-ever European cryogenic enginedesigned to be restartable. Therefore, some specificdesign issues arise and are presented hereafter.

RE-IGNITION ISSUES

The first VINCI boost occurs just after the firststage jettisoning. Then, after the 5.5-hour ballisticphase, several operations have to be performedunder reduced gravity (around 10-4 g) before theengine can be restarted:

- Tanks reconditioning: the tanks are slowlydepressurized to bring the propellants temperaturesdown. Then they are repressurized to the nominalstart pressure.

- Engine chilldown: given the very low temperaturesof the propellants (around 90K for LOX and 20Kfor LH2), propellants must be flowed through theengine in order to cool down the subsystems (lines,valves, and especially turbopumps) to their nominaloperation temperature.

GEO

6000 km

~200 km

Apogee35468 km


- Finally, the VINCI ignition sequence itself: thisincludes filling the LOX dome (figure 4), a cavitylocated just upstream the injectors which atomizethe LOX in the combustion chamber.

Figure 4: LOX dome

Problems that may occur during these phases inmicrogravity are:

- Propellant behavior in the tanks (sloshing,ingestion of liquid in pressurization lines);- Fluid flows in pipes and systems with a complexgeometry;- Boiling and heat transfer during chilldown inmicrogravity.

The most common way of dealing with theseproblems is to produce a thrust using accelerationdevices, usually hydrazine thrusters. They create anartificial gravity level which settles the propellants.However, such an additional propulsion system isheavy: it requires specific propellant tanks(hydrazine bottles for example), feed lines, a controlsystem, and the thrusters themselves. This affectsthe performance of the whole upper stage of thelauncher in terms of payload capability. It also addsto the recurring cost.

A better knowledge of fluids and heat transferphenomena in microgravity may prove that such asystem is not necessary. Specific microgravitystudies are therefore needed and will lead to a betteroptimized design of the engine and its propulsivesystem.

In the following paragraphs, details about themicrogravity issues placed under the responsibilityof Snecma Moteurs, the engine and propulsivesystem prime contractor, are provided. Activitiesand studies that are currently under considerationare also described.

SLOSHING AND INGESTION

It is a very old concern (some references are datingback to the 1960's). Several causes can explainpropellant sloshing:

- Shut down of propulsion;- Structures relaxation;- Attitude control, spin;- Inertia, viscosity and surface tension forces.

Consequently, the location of the liquid phase in thetank changes. Various configurations are possible:vapor at the center of the tank and liquid around it,liquid in the center and vapor around it, liquid onone side of the tank or the other… The free surfacemay even break up into drops of various sizes.

Risks linked to these phenomena are:

- Unexpected loads perturbing the stage attitude andtrajectory;

- Impossible venting if liquid is present at theventing line connection: liquid propellant dropscould be ingested in the lines and eventually gothrough the attitude control system and to thepressurization plates, which are not rated for such atwo-phase flow and may be damaged. Moreover,the pressurization loop and the stage attitudecontrol rely on predictions that were made using agas-only flow, and having a two-phase flow insteadwould compromise the accuracy of suchpredictions. An unstable behavior of thepressurization loop could also occur.


Foreseen activities/studies: the tank contractor isresponsible for sloshing limitation and non-ingestionof liquid propellant in the pressurization lines. Thecurrent baseline solution is to use a specific diffuserat vent lines inlets (under the responsibility ofEADS-ST, Bremen, Germany), and to settle thepropellants at the tank bottom before reignition bycreating artificial gravity and/or using phase-separation devices. Moreover, a study group calledCOMPERE and led by CNES works on propellantbehavior in tanks.

As the propulsion system and engine designer,Snecma Moteurs is concerned by these issues inorder to be able to contribute to the definition of theappropriate sequence of operations before restartingthe VINCI: if artificial gravity is needed forpropellant retention and settling, what would be thelevel of thrust to be produced? Could the propulsivesystem be designed in some way to produce thisthrust without the need for a specific system likehydrazine thrusters?

More knowledge on the behavior of fluids inmicrogravity will help answer these questions.

INJECTION CAVITY FILLING

When the LOX chamber valve (VCO) is openedduring the reignition sequence, the LOX enters the"LOX dome", which is a cavity just above the thrustchamber (figure 5). At the bottom of this cavity,injectors lead the propellant into the thrust chamber.While LH2 injection is almost unaffected by gravityand acceleration due to its low density, the LOXdome cavity filling may be affected: injection massflow and repartition of the flow among the LOXinjectors may be different from what happens on theground. Moreover, helium injection justdownstream the VCO plays an important role inatomizing the LOX.

This whole sequence of operations is critical toensure a correct ignition in the thrust chamber.There is consequently a need to know better thenature of the flow downstream the VCO and tomake sure all injectors in the LOX dome cavity are

fed properly. Given the high velocities of the LOX(5 to 20 m/s) and of the Helium (flow rate of 10 to15 g/s), the engineering feeling is that the LOXdome filling is driven more by fluid momentum thangravity, so the process should not be affected a lotby microgravity. But the injection process is socritical that a verification test is mandatory forqualification purposes.

Figure 5: LOX dome cavity filling

Possible activities/studies: the goal is tocharacterize the flow downstream the VCO ("flashvaporization" of the LOX, influence of heliuminjection and of the specific geometry of the VCO,impact of the flow on the LOX dome walls, …)

Experimental work is proposed by EADS-STOttobrunn (thrust chamber contractor) andnumerical studies by Snecma Moteurs. Theproposed activities are to:

Ø conduct simulations with a 3D, two-phase flowsimulation tool taking into account microgravityand both the complex physical phenomena andthe complex geometry;

Ø verify experimentally the injection flow patternin microgravity, and especially point out thedifferences, if any, with what happens in 1g;propose an adaptation of the filling sequence ifneeded.

Experiments could be conducted in a drop towerand a very preliminary concept was defined to thatend. It is shown on figure 6; the idea is to feed thedome with pressurized LOX and visualize the flowprofile downstream of the injectors during the droptower free-fall time.


Dischargetank

to feed valve

Figure 6: LOX dome experiment concept

CHILLDOWN

Context

In order to chilldown the turbopumps beforereignition, liquid propellants must be run throughthem. The problem is to limit the mass ofpropellants used to this end while still ensuring agood chilldown in all parts of the turbopump (in themain circuit but also in "dead ends"). Moreover,there should be only liquid in the turbopumps (no"trapped bubbles"). Once the turbopumpstemperature has reached predefined criteria, the restof the reignition sequence can be launched.

Using very preliminary assumptions, the currentbaseline sequence (in GTO/GTO+) aims at limitingthe propellants consumption to 125 kg of LOX and35 kg of LH2. The scenario is to open the feed andpurge valves, let propellants flow (heat transfer isthen mainly done through forced convection, exceptin the dead ends) and close the purge valves whenconsumption reaches respectively 100 kg and 20 kg.

Chilldown continues by percolation (pool boiling inmicrogravity), and purge valves are reopened justbefore reignition, the remaining 25 kg of LOX and 5kg of LH2 being consumed at that moment.

It is important to be able to confirm that thischilldown scenario will effectively enable theturbopumps to reach their operating temperature. Inorder to do so, current codes must be updated withthermal correlations on the heat transfer coefficientsin microgravity, as shown on figure 7. Indeed,available codes currently use 1-g correlations and itis suggested in the literature that heat transfer inmicrogravity may be less efficient than on theground. In that case, chilldown in microgravity mayrequire more propellant consumption thandemonstrated in 1 g and last longer than expected.

Physics analysis: poolboiling in dead ends(1g and µg)

Integration inchilldown code

Validationof physicsanalysis

µg tests

Literaturereview

Experimentconception

Chilldown inballistic

phase code

Figure 7: Taking microgravity into account in the current simulation tools

Consequently, the need associated with this problemis to conduct dedicated activities to answer thefollowing questions:

- how (quantitatively) does convective heat transferin microgravity (forced convection in the maincanal, natural convection in the dead ends, poolboiling) compare to? What is the importance of theconvective speed: high speed flows are probably notvery affected by microgravity, but is there athreshold effect?


- what is the motion of gas bubbles in theturbopumps, especially during the percolationphase? Do they go up to the tank, or do they staytrapped in the turbopumps, in the dead ends? Whatcould cause them to disappear? How do theybehave in the presence of zones of various flowvelocities?

- from the system point of view, how can thechilldown scenario be optimized for both lowpropellant consumption and good chilldown?

Literature review

A literature review was conducted on pool andconvective boiling in microgravity.

On pool boiling, the review showed that despite thelack of a unified theory, some tendencies could bedefined, as in articles of Straub [1], Merte & Lee[2,3], DiMarco & Grassi [4]. The geometries inliterature are small wires and plates ; the differenceconcerning boiling between these two geometries ispointed out.

Generally speaking, the heat flux is found to be afunction of R' = R / L, where R is a characteristiclength and L is the capillary length :

( )gLgl ρρ

σ−

= .

Concerning small wires, the influence ofmicrogravity is quite well known and a Nukiyamacurve can be plotted (figure 8). However there areno explicit correlations of heat flux with respect togravity.

•Tsat

q 1g

µg

Conduction

Naturalconvection

Nucleation

µg - depends on R'

1g

Figure 8 : Preliminary pool boiling curves on a wire

According to Straub [1], maximum heat flux is notso decreased by microgravity ; it can even beincreased in some cases. Stable film boiling existsand heat transfer in this regime is highly affected bymicrogravity. Correlations exist but depend on therange of gravity : influence of gravity is not thesame at 10-4 g or 10-2 g. DiMarco & Grassi suggestto rely on the wavelength of the oscillations duringfilm boiling and its dependence on R'. This methodshould lead to a global correlation for film boiling inmicrogravity on wires.

Concerning plates, only nucleate boiling and boilingcrisis are treated. Work of Merte [2,3] or Kim [5]enables to plot a boiling curve up to the maximumheat flux. The results of Merte are shown on figure 9.

Figure 9 : Pool boiling curve on a plate for R113[reference 3]


One particularity of subcooled boiling on plates inmicrogravity is the heat pipe phenomenon (figure10): a stable, huge bubble forms upon the plate;evaporation takes place at the bottom of this bubbleand condensation at the top. This enables toincrease heat transfer in nucleate boiling comparedto normal gravity.

Vapor

Evaporation

Condensation

Coalescence

Figure 10 : Heat pipe phenomenon

On the contrary, maximum heat flux is highlyreduced; but the relation between this flux andgravity level is not established. Boiling for highsuperheat is not studied but it seems that no stablefilm boiling exists on plates.

We have plotted a preliminary pool boiling curvewhich could be used in chilldown codes (figure 11).

Pool boiling curves on a plate for LOx at P=1,013 bar and T=90,18 K in normal gravity and microgravtiy (10 -4 g)

1

10

100

1000

10000

100000

1000000

0,1 1 10 100 1000

Superheat (K)

Hea

t flu

x (W

/m2 )

Normal GravityMicrogravity

Nucleation

Max. heat flux reduced by 80 %

13 K

5 K

Figure 11 : Preliminary pool boiling curves for LOX

As regards convective boiling, it is obvious that theeffect of microgravity decreases as the fluid velocityincreases. However, the range of velocities in whichmicrogravity has on effect on convective boiling isnot easy to determine, especially for cryogens.

Westheimer & Peterson [6] show the transitionsbetween regimes of flow boiling of water aredependent of gravity level. According to Ma &Chung [7], heat transfer on a wire in a crossflow ofFC72 can decrease in microgravity. Work ofKawanami et al. [8] shows that microgravity canincrease heat flux in case of an upstream flow ofLN2. But only one value of mass flow rate is tested.It should be the contrary in the case of adownstream flow as it is the case on the VINCIengine.

Specific needs

Experiments in the literature are dedicated to studyheat transfers in satellites devices.

The result is that none of the studies that werefound in the literature are directly applicable to theVINCI engine chilldown because:- they deal with simple geometries (heated wires orplates), while in the turbopumps the problem is aninternal flow in a complex geometry (figure 12);- they usually study "common" fluids (water orCFCs), while VINCI uses cryogenic LOX and LH2;- they are not focused on boiling for high superheatwhile this is the main regime during the VINCIchilldown.

Figure 12 : Longitudinal section of the VINCIhydrogen turbopump


Quantitative results on convective boiling areavailable for ducts but data for oxygen / hydrogenand for various velocities lack.

Consequently, there is a need to perform specificexperiments and simulations. Assuming an internalflow of a cryogen in cylinders of various diametersand inclinations, the objectives are the following:

Ø to determine the effect of microgravity on poolboiling with high superheats;

Ø to determine the effect of a shape factor on heatflux in all the boiling regimes (compare withboiling on a plate : for example does the sameheat pipe phenomenon exist?);

Ø to determine the effect of microgravity onconvective boiling with respect to the flowvelocity.

What is finally needed is the heat flux coefficienth(fluid, T, P, g, geometry: characteristic length,diameter, shape factor, orientation …), with:- fluid: hydrogen, oxygen- Tp - Tsat = [0, 300 K]- P = [1 bar, 5 bar]- g = [10-3 m/s² ; 10 m/s²]

Possible activities/studies

?

Mainflow

Deadend

Figure 13 : Experimental configuration to be tested

Snecma Moteurs proposes to conduct a dedicatedexperiment about heat transfer in microgravity. Atypical situation in which correlations are needed isthe one shown on figure 13, where the main flowpresents the classical regimes of convective boiling.The boiling regime in the dead end is not defined.

The experiment should allow to find out thenecessary correlations in order to determine thetemperature evolution of such a system.

The main difficulty is that the boiling curve must bedetermined for a wide range of superheats,especially for high superheats (> 100 K). Thismeans that a constant heat flux source cannot beused because of burnout.

The only way is to initially heat the sample and tomeasure the temperature drop as the fluid flowsthrough it (convective boiling) or is in contact withthe walls without flowing (pool boiling) : this is thequenching technique.

This method has already been employed by Merte[2] for pool boiling but the surface was very simple.It was also used by Kawanami et al. [8] forconvective boiling but the influence of velocity hasnot been studied.

Problems to address

A lot of questions ought to be solved in order todesign an experiment :Ø Is it possible to treat pool and convective boiling

in microgravity on the same device?Ø What is the best experimental means for this

purpose: Zero-G plane or drop tower?Ø Similitude: as experiments are difficult to do

with LH2, a method to extrapolate results toother fluids is needed.

Ø How can we predict the value of heat flux inorder to design the experiment :

Is there a heat pipe phenomenon?What is the expected regime with highsuperheats?What is the effect of subcooling?


CONCLUSION

In this paper, the main problems concerning thereignition process of a cryogenic upper stage enginein microgravity were presented. When theseproblems are solved, an auxiliary propulsion system(usually required to create artificial gravity) may notbe needed anymore, thus saving mass and cost.

A dedicated work logic with theoretical analyses,experiments and simulations is therefore being setup. In 2002, an extensive literature review wasconducted; it is summarized in this paper. In 2003,contacts with specialized laboratories are beingestablished, in order to refine the needs and theexperimental concepts. Finally, synergies andpartnerships with these laboratories are underconsideration.

REFERENCES

[1] Johannes Straub, Boiling Heat Transfer andBubble Dynamics in Microgravity, Advances inHeat Transfer, Vol. 35, pp 57-172.

[2] Herman Merte Jr., Nucleate Pool Boiling:High Gravity to Reduced Gravity; Liquid Metals toCryogens.

[3] H. S. Lee, H. Merte & F. Chiaramonte, PoolBoiling Curve in Microgravity, Journal ofThermophysics and Heat Transfer, Vol.11, No. 2,April-June 1997.

[4] P. DiMarco, W. Grassi, Pool Film BoilingExperiments on a Wire in Low gravity: PreliminaryResults, Proceedings of the Microgravity TransportProcesses in Fluids, Thermal, Biological andMaterial Sciences, Banff, Alberta, Canada,September 30 to October 5, 2001.

[5] J. Kim, J. F. Benton, D. Wisniewski, PoolBoiling Heat Transfer on Small Heaters: Effect ofGravity and Subcooling, IJHMT, 2002.

[6] D. T. Westheimer & G. P. Peterson,Visualization of Flow Boiling in an Annular HeatExchanger Under Microgravity Conditions, Journalof Thermophysics and Heat Transfer, Vol.15, No. 3,July-September 2001.

[7] Y. Ma, J.N. Chung, An Experimental Studyof Critical Heat Flux in Microgravity Forced-Convection Boiling, Int. Journal of MultiphaseFlow 27, pp. 1753-1767, 2001.

[8] O. Kawanami, T. Hiejima, H. Azuma, H.Ohta, Experimental Study of Boiling Phenomenonfor the Transfer of Cryogenic Liquid underDifferent Gravity, Jpn. Soc. Microgravity Appl.Vol. 18, Supplement, 2001.

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ECI: MTP-03-34 MICROGRAVITY ACTIVITIES FOR THE VINCI ...anne.pacros.pagesperso-orange.fr/career/Davos-MTP-03-34.pdf · phenomena in microgravity may prove that such a system is not