1. Itcanbeseenhattheg-limitconstraintexpands with increasing UDuntilanUDof 1.0-1.5.This suggeststhat avehicledesignwithanL/D muchgreater than 1.0doesnotsignificantlybenefitthecomdor range. Thiscorridorisanidealconidorinthesensethat the exitconditionsoftheaembrakingvehicleare neglected. Itispossiblethatthevehiclemay successfullyaembrake andmeetitsg-loadconstraintbut deplete somuchofitsenergythatitisincapableof achievingitstargetapoapsis.Thisisespeciallytruefor thehigherorbitsin contention.
2. T h eorbitshavebeenselectedascandidateorbits afwaerocapture.Theyarea 500kmx 500km (270 nm x270nm)circularorbit,a I/3 Solorbit(periodof orbitis 1/3theMartianday) of 250km x 12,622km (135nm x6811nm) anda 1Solorbit@nod equals v i, Martianday) of 250km x 33,800km (135nm x 18,270nm). Therelativesizeof theseorbitsareshown in figure 2. Figure 1alsoshowsthethreelines repsenting themaximum energydissipation (maximum orsteepestflightpathangle)allowableto obtaineachofthethreeorbits.ItcanbeSeenthatthis constraintgreatlydecreasesthecomdorsatthelower L/D'sandrestricttheflightpathanglerangemuchmore thantheg-limiw.Theconidoristhedifferencebetween theskip-outlimitand eithertheg-limitorthe minimumenergylimit,whichever isshallower.For example,them econidorwidth forthe 1Solorbitis shownin figure 3.c2.LoaaThemaximumg-loadsencounteredbyan aerobrakingvehicleareshownin figure 4.Hereg-loads (in earthg's) areplottedvcrsusL/Dforvariousballistic coefficientsataflightpathangleof -13degreesinan open-loop,fulllift-upsimulation.It isinterestingto notethattheg-loadsdecreasewith increasingL/D until anL/D ofabout 1.0isreached.If theL/D isincreased funher,theg-loadslevel outandwillthenincrease.For various ballisticcoefficients,theminimumg-loadsare atanL/Dof 1.0-1.5.ThisagainshouldbeaL/Ddesign considerationforthevehicle.
3. From thisrelation. itcan beseenchatanunstableCnp canbeovercomeby astabilizing(negative)Cis.This effectisespecially noticeableforconfigurationslikethe shuttle orbiconicsbecausetheinertiain theyawaxisis 4-6Onestheinertiain the roll axis. Foravehicle withoutaerodynamicsurfacessuch asawingortail,the rollingmomenthastobegeneratedby vehicle asymmehiesaboutthecg.Figure 15showsC I forthe cgonthevehiclecenterlineandwiththecgdisplaced 6 inchesbelow thecenterline. Figure 16showsCnp-dynamic vsalphaforthecglocationsyieldingthe nominal 27degreetrim angle.Inthisgraph,positive valuesindicatestability,whilenegativevaluesindicate instability. With theZcgoffset,the instability boundaryispushedback toabut 17degrees,which is 10d e wbelowthedesiredtrimangle. Duetothis large spread,thisconfigurationisfelttobelaterally stablein the trimangleofattackrangechosen.The
4. atleast 0.8. G-loadswerealsoshowntobeminimum atan LDof 1.0-1.5. Duetothesetrends,avehiclewas desiredwithanUDof 1.0andwasselectedasthe baselineconfiguration.