Generator Warm-Up Characteristics

Transcription

1 NO. REV. NO. : ; ~ Generator Warm-Up Characterstcs PAGE OF Ths document descrbes the warm-up process of the SNAP-27 Generator Assembly after the sotope capsule s nserted. Several nqures have recently been receved regardng ths process, and deployment sequence studes are mpacted. Because of the several varables assocated wth the turn-on of ALSEP on the lunar surface, there s no smple plot of avalable power as a functon of tme. However, some reasonable estmates can be made of avalable power for specfc turn-on cases. Prepared By D~ A. Courtos Approved By

2 . ;. NO. MEV. NO Generator Warm-Up Characterstcs PAGE 2 Of Summary Ths ATM descrbes the generator (wth emphass on the warmup characterstcs}, estmates short-crcut current as a functon of tme, and estmates the avalable power for two turn-on sequences. Data are avalable for estmatng power avalable for other turn-on sequences. Ths analyss shows that for the o'le-ma.n deployment there wll be about watts avalable at the tme the staton s turned on. Ths s more than adequate to provde regulaton for the ntal loadng of to watts, dependng on Array. On the other hand, the two-man deployment sequences do not allow enough warm-up tme to provde adequate power at the tme the staton s turned on. As a result, the output voltages of the Power Condtonng Unt wll be approxmately 75o/o of ther normal values ' when transmtter turn-on s attempted. "Generator Descrpton Fgure 1 shows a smple thermal schematc of the generator. Heat from the fuel capsule radates to the hot frame, flows through the thermople to the cold frame by conducton, thence to the fnned radator. Durng steady-state operaton, some 5o/o of ths thermal energy s converted to electrcal energy and s dsspated n ALSEP. There are other losses, such as end effects, whch are not sgnfcant for ths descrpton. A voltage s produced at the generator termnals as a result of the thermoelectrc converson (Seebeck Effect). A current determned by nternal and external resstance wll flow f the termnals are connected to a load, and as a result of the current flow there s an nverse converson effect (Pelter Effect). The voltage produced by the Seebeck effect s proportonal to the temperature drop across the thermople; however, the Pelter effect tends to decrease ths temperature drop. A further effect, whch s not consdered here, s the effcency varatons of the thermoelectrc materal as a functon of temperature.

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4 . : NO. REV. NO ~ 'ystems Dvson Generator Warm-Up Characterstcs PAGE 4 OF The generator performance can be further descrbed by a plot such as shown n Fgure 2. Here, steady-state performance s llustrated by curves of voltage, power, hot frame temperature, and cold frame temperature as functons of the current flowng. t s emphaszed that these curves, except for the lne marked "Dynamc Resstance, are steady-state curves. The ponts on the curve were obtaned by adjustng the load and then watng several hours for the thermal transent to decay. The dynamc resstance on ths fgure llustrates the generator nternal resstance for an nstantaneous load change, n the fgure the dynamc resstance s llustrated for the ALSEP operatng pont of 16 volts. The nstantaneous nternal resstance s also approxmately the same for other ponts on the steady-state curve. Ths means that when the operatng pont s changed, the new pont wll be on a lne through the old pont wth a slope of the dynamc resstance. As the thermal transent decays, the ope ratng pont wll move to the steady- state lne along a path determned by the varable whch s held constant. The generator desgn was optmzed to gve maxmum power out whle keepng weght and volume to a mnmum. Other varables whch were consdered were thermal nput, temperature lmts of structural materals, radator sze, radator operatng temperature, and the effcency and temperature lmts of the thermoelectrc materal, lead tellurde. As a result, the thermople hot juncton temperature was set near the upper materal lmt. As shown n Fgure 2, the hot juncton temperature goes up when the load current s decreased and would be very hgh for steady-state, open-crcut operaton, exceedng the lead tellurde lmts. Operaton at temperatures n the regon of 1200 F would degrade the thermople. At the other extreme, operaton shorted s conservatve. The hot juncton temperature s low, and the slght rse n cold juncton temperature accounts for the addtonal electrcal power beng dsspated wthn the thermople due to the Pelter Effect. Warm~Up After the capsule s nserted nto the generator on the lunar surface, the shortng plug wll keep the generator shorted untl the button s pushed by the astronaut. There are two reasons for ths provson: ( 1) to keep the generator hot juncton temperature down n case the deployment sequence s nterrupted before the Central Staton s connected, and (Z.; to mnmze the voltages present on the connector when the Central Staton s connected.

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6 . :. NO. REV. NO Generator Warm-Up Characterstcs PAGE 6 OF Operaton durng a shorted warm-up wll be along a load lne of ohm whch s the shortng plug resstance as shown on Fgure 3. Ths short-crcut current, whch can be drectly read on the ammeter n the shortng plug, gves an ndcaton that the generator s operatng properly. Fgure 4 shows the expected shortcrcut current as a functon of tme for a contnuous warm-up n the shorted condton. n the normal deployment sequence, after the short-crcut current s read q,nd the button s depressed, the generator wll "see" /'~'', \ a load of about 1 32,/ ohms consstng of the fxed or "ON" loads on the 29-volt PCU oufput lne through the autowndng of the PCU output transformer. Ths s a near-open-crcut load for the generator. A thermal transent wll be ntated nsde the generator (mostly at the hot frame) due to the decrease n Pelter coolng, whch s superposed on the warmup transent occurrng. The generator voltage wll move along the dynamc resstance lne to the ntersecton wth the 32-ohm load lne. The open-crcut voltage wll then ncrease along the 32- ohm lne untl the # 1 Astronaut Swtch s closed or the automatc turnon pont of 24 volts s reached. At the tme ether of these events occurs, the generator voltage wll agan move along the dynamc lne to 16 volts or a power balance condton at reduced voltage. f the ntersecton of the dynamc lne and 16 volts results n more than the ntal load of watts (Array A), the PCU regulator wll keep the generator output at 16 volts; otherwse, the PCU output voltages wll be lower than normal, approxmately on a 7. 5 ohm load lne. The PCU turn-on then ntroduces another thermal transent nto the generator as the current flow s sgnfcantly ncreased. Fgure 3 llustrates the expected turn-on sequence for a one -man Array A deployment. 1 At 13 mnutes and 28 seconds after the capsule s nserted n the generator, the astronaut reads the short-crcut current. The expected value s 6 amperes or pont A on Fgure 3. At 14:54 after nserton, he pushes the button causng the generator voltage to move nstantly from pont B to C. As warm-up contnues, the generator voltage moves from C to D. After 20 mnutes and 46 seconds, the Astronaut Swtch # 1 s closed, movng the gene rat or voltage nstantly from pont D toe at 16 volts. t has been assumed n estmatng the pont 1 Deployment sequence based on ATM-739, dated 27 May 1968.

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9 . ;. NO. REV. NO Generator Warm- Up Characterstcs PAGE 9 OF D voltage that the thermal transent assocated n movng from pont B to C has a small effect compared to the schedule for a warm-up completely along a 32-ohm load lne. The power at pont E of 42.5 watts s adequate for the ntal load of watts, and all PCU output voltages wll be regulated. The operatng pont wll probably reman near pont E for several mnutes as the Pelter Effect tends to decrease the thermal drop across the thermople. The operatng pont wll then move to pont F as steadystate thermal condtons are reached durng the next hour. n the two-man deployment sequences, 2 the shortng plug button s pushed approxmately 12.5 mnutes after fuelng and then approxmately 10 mnutes later the staton s turned on by closng Astronaut Swtch # 1. Ths sequence s shown by ponts G through M on Fgure 5. At staton turn-on, at pont K determned by a ohm equvalent system load, approxmately 19 watts wll be avalable, but all the PCU voltages wll be reduced to 75% of ther regulated values. As the warm-up contnues, pont K to pont L, the voltages wll rse to ther normal values and then wll reman constant for the remanng warm-up. Ths analyss s based on a seres of three tests performed at General Electrc3 n whch a hot electrc fuel capsule was nserted remotely nto the MOD 8B generator assembly. n these tests, the capsule was allowed to reach steady-state temperatures n vacuum wth a 0 F snk. t was then drven nto the generator assem~ly n 35 seconds. Generator thermal and electrcal measurements were made untl the generator reached equlbrum. n the frst two tests, the generator was shorted electrcally except that nstantaneous open-crcut voltage measurements were made on a perodc bass. n the thrd test, the generator was open-crcuted except for nstantaneous short-crcut measurements on a perodc bass. The results of these tests are a warm-up voltage verss current plot for a 0. 6-ohm load (test console m~_nmum resstance) and an open-crcut voltage versus tme plot. n addton, the generator nternal resstance can be estmated by assumng that the nstantaneous measurements at a gven tme can be connected to the steady-state ponts by a straght lne and that other nstantaneous load ponts would le along ths straght lne. 2 Deployment sequences based on ATM's 803, 804 and 805, all dated t..v September PR No. U-ANSO-SNAP-12, Ma.y 25, 1967, Mssle and Space Dvson, General Electrc Company

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11 . ; NO. REV. NO Generator Warm-Up Characterstcs PAGE OF These tests also show that the dynamc resstance changes somewhat durng warm-up. Early n the warm-up sequence, the dynamc resstance s about one ohm, whle the fnal value s about 2. 7 ohms. The equaton: R = t ohms, where ts n mnutes after capsule nserton, can be used to determne the dynamc resstance n ths range. The effect of ths change was consdered n constructng the plots n Fgures 3 and 5.