r bulletin 94 — may 1998

High-Performance Heat Shields forPlanetary Entry Systems

R. RouméasStructures and Mechanisms Division, ESA Directorate for Technical and OperationalSupport, ESTEC, Noordwijk, The Netherlands

T. Pichon & A. LacombeSEP Division, SNECMA, Bordeaux, France

The Sepcore® conceptThe Sepcore® concept has a three-layersandwich structure which can be adapted to aspecific mission profile. The core of the systemis a high-temperature-resistant ceramic matrixcomposite structure onto which a thin ablativelayer is deposited. The payload is protectedfrom the hot structure by a lightweight insulator.This layering approach allows one to design theheat shield with just the strictly necessarythickness of ablative material, which will becharred. The additional thickness of ablativematerial used in conventional heat shields to

with the high-energy reentry requirements ofthe proposed Comet Nucleus Sample Return(CNSR) mission (Fig. 3). The mock-up itself ismade up of several components:– a 15 mm-thick carbon/phenolic ablative heat

shield– a 1 mm-thick C/SiC hot structure stiffened

by stitched and co-infiltrated 15 mm-highstiffeners

– an RTV bonding layer between theablative layer and the hot structure, toprevent voids during assembly

– a high-temperature fastening systemincluding carbon/carbon (C/C) screws, onecentral insert and three lateral inserts

– an isostatic six-strut payload-supportstructure

– an internal lightweight multi-layer insulationmade of radiative barriers held in place byalumina felt pads.

As the mock-up was practically manufacturedto test standards, it was decided to upgradeand configure it for testing under conditionsrepresentative of a re-entry in the IRS plasmatest facility in Germany, with the pre- and post-test analysis carried out by FGE in the UnitedKingdom.

Testing the mock-upThe fully equipped mock-up is a cylinder 400 mm in diameter and 220 mm long andweighs approximately 13.8 kg, the Sepcore®

heat shield weighing just 2.3 kg. It was linked tothe test facility’s sample support system viaspecial test equipment, as shown in Figure 4.

A plasma generator was used to apply a heatflux to the front face of the mock-up. The testgas used as the plasma source propellant was80% nitrogen and 20% oxygen. The incomingflux was limited to the ablative layer, i.e. acircular area 320 mm in diameter, with theradial distribution indicated in Figure 5.

Given that the mission scenarios of several potential future Europeanspace projects will require spacecraft entries into the atmospheres ofvarious planetary bodies, including Earth, a number of advanced heat-shield technologies are currently under investigation. Besides thecold-structure option using materials such as beryllium, SEP has beendeveloping a very promising concept using a hot structure ofcarbon/carbon or carbon/silicon-carbide. Depending on the particularmission requirements, this hot structure can be protected fromextreme heat fluxes by an additional ablative layer. This modularapproach, which allows the thermal protection system’s performanceto be tailored to the exact needs of the mission, forms the basis of theSepcore® concept (Fig. 1).

soak up the extra heat is replaced in theSepcore® concept by lightweight internalinsulation.

The main advantages of this concept are apotential mass saving of up to 50% (dependingon the mission), its adaptability to particularmission requirements, and an increased failuretolerance due to the high temperature capacityof the carbon/silicon-carbide (C/SiC) structure.

The Sepcore® mock-upIn order to investigate the manufacturingfeasibility of the Sepcore® concept, a reducedscale mock-up was designed and manufac-tured (Fig. 2). The mock-up’s design complied

Figure 3. Re-entry heat fluxdata for the CNSR mission

high-performance heat shields

Preliminary testing on a graphite plate to checkthe possibility of achieving 10 MW/m

2without

interaction between test sample and plasmagenerator was carried out successfully. Toavoid transient-phase heating, the plasma flowwas ignited and stabilised with the mock-up instowed condition. The mock-up was thenmoved into the test position as rapidly aspossible to limit the transition phase from lowheat flux to the nominal value of 10 MW/m

2at

an ambient pressure of less than 50 Pa.

The test end-criteria were :

– C/SiC structure back face temperature exceeds 1000°C

– aluminium struts reach 170°C– testing time reaches 600 s.

Testing was stopped when the first criterionwas fulfilled, after 270 s of firing.

Figure 1. Current andadvanced thermal-protection-system concepts

Figure 2. The Sepcore®

mock-up

Figure 4. Mock-up testequipment

r bulletin 94 — may 1998 bull

Test results and analysisThe thermal performance of the Sepcore®

mock-up was better than first predicted byanalysis, which shows good design potentialfor further mass reduction in the ablator. Afterreadjusting the material properties in theanalytical model, good matching was achievedbetween the test results and calculatedtemperatures.

Satisfactory mechanical prediction was alsoachieved for the stiffeners of the C/SiCstructure. Results on the skin were not asprecise, due to insufficiently refined modellingof the link between the ablator and thestructure. Escape of the pyrolysis gassesthrough the venting holes proved to be veryefficient, contributing to the lower temperaturesobserved. On the other hand, the ablatordelaminated from the C/SiC hot structure at theend of the test, in the region where the ablatorplies were not drilled. This proved that it isnecessary to vent the entire ablator.

A characterisation and morphological analysiswas performed which confirmed the tensilefailure mode of the C/C screws, due to thepressure build-up of pyrolysis gasses in theunvented part of the ablator. It also showedthat the ablative material was fully pyrolised, butwithout any surface recession, which indicatesa higher than expected surface enthalpy duringthe test. The C/SiC structure was undamaged.

Technology testingTwo sets of technology test samples were alsodesigned and tested in SEP’s Bordeaux (F)laboratory and ETCA’s Odeillo (F) solar furnaceFigure 6. Test/analysis temperature comparison

Figure 5. Heat-flux distribution applied in the mock-up test

Figure 7. The technologicaltest samples

Direct moulding of the ablator onto the C/SiChot structure was foreseen as an alternativemeans of bonding the ablator. However, thisproved not to be feasible and the RTV bondingwas retained as the baseline solution.

Finally, although the C/SiC sandwich hotstructure is interesting due to its high specificstiffness, it is not sufficiently well developed tobe incorporated into the Sepcore® concept. Inparticular, the bonding between the C/SiChoneycomb structure and the two skins willhave to be improved, so as to preventpremature delamination.

ConclusionThe tests described above demonstrated thatthe thermal performance of the Sepcore®

mock-up is better than first predicted byanalysis, providing the potential for furthermass reduction in the ablator whilst maintainingthe same level of performance. Both theconcept and its architecture have been proven,and a modular, low-mass, high-performanceheat shield is now available for the Europeanreentry vehicles currently under developmentand for future missions with similarlydemanding requirements. r

in support of the experimental validation of theSepcore® concept (Fig. 7). The objectiveswere twofold:– to represent those zones of the Sepcore®

mock-up, and hence also of the heatshields, that exhibit very specific behavioursunder thermal and mechanical loads, inorder to improve our overall understanding

– to evaluate candidate improvements to theSepcore® concept, with a view toimplementing them in future developments ifthey proved interesting.

The sets of test samples were manufactured tobe representative of:

– the link between the skin and the stiffener ofthe C/SiC hot structure

– assembly technologies between the ablativelayer and the C/SiC hot structure

– a C/SiC sandwich to be used for the hotstructure.

The skin/stiffener link test specimen showedthat the concept of sewn and co-infiltratedparts could withstand the thermal andmechanical loads encountered during a re-entry. The assembly technology specimendemonstrated that the concept is indeedsuitable for the planned application, but thatspecial care will have to be taken to dimensionthe insulation beneath the bolts correctly, as theheat fluxes are higher in this area due to thehigher conductivity of the screw. Nodelamination between the ablator and theC/SiC hot structure was observed.

high-performance heat shields