Electric Propulsion for Rockets

Transcription

1 Elctric Propulsion for Rockts Elctric Propulsion-1 Elctric Propulsion Systm Elmnts Propllant Gas (X), liquid (N H 4 ), solid (tflon) Storag Enrgy Sourc chmical, nuclar, radiation (solar) Storag Fd Systm Thrustr Thrmodynamic (nozzl), lctric, lctromagntic Convrsion to lctricity at propr V, DC/AC( frq), currnt, stady/pulsd Powr sourc sparat from propllant I sp not limitd by chmical bond nrgy Elctric Propulsion- 1

2 EP Powr Rquirmnts Jt Powr P 1 j m u Minimum possibl powr rquird 1 m u u 1 u 1 I g sp Incras in I sp (or u ) ntails incras in powr u for constant flowrat u for constant thrust For comparison, chmical rockt with thrust of just 1000 lb f (4.5 kn) and I sp = 350s, P j =7.7MW P j =66 MW for lc. rockt with I sp =3000s and sam thrust EP tnds to b powr-limitd to low thrust (and low acclration) Elctric Propulsion-3 Supply Powr Rquird Supply Powr P s Pj Ovrall convrsion fficincy has 3 main componnts nrgy convrsion - of raw nrgy sourc to lctricity ~100% for photovoltaics dirct convrsion of photons (dos NOT includ fraction of solar radiation that can b convrtd to lctricity by typical solar array, ~18-5%) T T S pp total fficincy of nrgy convrsion th powr prparation and conditioning lctronics (losss in lctronics) ~9% (lctrostatic) ~98% (stady arc jts) 10-40% for nuclar thrmal, thrmodynamic cycl limits = lots of wast hat thrustr fficincy - only part of input lc. nrgy convrtd to KE 40-70% Elctric Propulsion-4

3 Mass Rquirmnts What limits availabl powr to EP systms? mass of powr plant and associatd systms If powr plant mass is significant fraction of propllant mass, thn som advantags of highr spcific impuls opration ar lost payload may consist mostly of powr plant/lctronics for propulsion systm Elctric Propulsion-5 Mass Rquirmnts (con t) Powr sourc mass Mass powr sourc P s s narly linar rlationship spcific mass (.g., kg/kw) Not: somtims usd for spcific mass, but also usd by othrs for spcific powr (kw/kg), i.., = 1/ s ~7-5 kg/kw lc for solar arrays (dpnds on cll dsign, substrat) s ~-4 kg/kw thrmal for nuclar ractors (dpnds on shilding) to rjct wast hat, also rquir additional mass for radiators R ~ kg/kw wast hat Elctric Propulsion-6 3

4 Mass Rquirmnts (con t) Powr prparation and conditioning Mass pp pp P pp Larg variation with typ of EP dvic (spcially if nd high voltag or powr, and short puls forming lctronics/switchs) pp ~0. kg/kw lc for typical arcjts pp ~0 kg/kw lc for PPT (pulsd plasma thrustrs) Elctric Propulsion-7 Elctric Rockt Propulsion A. Background Elctric Propulsion-8 4

5 Elctric Propulsion - Acclrators Elctrothrmal Elctromagntic Elctrostatic Accl. Forc p Prssur, Elctrically hat propllant and us nozzl xpansion F m Lorntz, Magntic and lc. filds acclrat chargd particls F Elctrostatic, Static lctric fild alon acclrats chargd particls I sp (s) 300-1,500 1,000-10,000, ,000+ Thrust Wight <10-3 <10-4 < Elctric Propulsion-9 Elctrical Hating Currnt passing through conductor hats it by amount proportional to its rsistanc Currnt (A=C/s) Q J R Rsistanc (Ohms) Wir Gas (Plasma) Discharg Cathod rsistanc du to collisions E V V E Elctric Propulsion-10 J - i J + Anod 5

6 Forcs on a Chargd Particl To xamin how to us lctrical nrgy to acclrat a propllant, considr acclration of a particl with mass m and charg q Elc. Fild Mag. Fild du m qe qu B p dt Elctric Propulsion-11 q + Elctrostatic Forc E F u q Lorntz Forc + Collisional Forc (Momntum Transfr) F B m u F m B (1) Motion of Chargd Particl in E&B Filds How dos chargd particl mov in lctric and magntic filds du q Elctric fild only E dt m lctron lightr, highr accl. + - Magntic fild only du q particl gyrats u B dt m (cntripdal accl.) Elctric Propulsion-1 radius of gyration frquncy of gyration No work; B and F prpndicular r g m u B qb qb g m r g E B u 6

7 Inducd Magntic Filds In gnral, currnt flow inducs a B fild B fild inducd by linar currnt B fild inducd by coil J B B From Spac Propulsion Analysis and Dsign, Humbl, Hnry and Larson, 1995 Elctric Propulsion-13 Motion of Chargd Particl in E&B Filds Crossd E and B filds Elctric Propulsion-14 E fild acclrats particl upward B fild causs acclration prpndicular to u ovrall rsult is drift vlocity normal to E and B E B u d B E r g m m B g 1 havy u d light 7

8 u Elctric Propulsion-15 Plasmas Gas composd of qual amount of ngativly and positivly chargd particls lctrically nutral ngativ particls usually - positiv particls ar positiv ions typically most of gas molculs rmain nutral Momntum quation for plasma u u u p j B t u, u u, 1 3 Currnt Dnsity (A/m ) Elctric Propulsion-16 Inducd E Filds Hall Effct Elctron acclration lightr - acclrat mor quickly and accommodat to flow fild (most of j) collisional coupling (momntum) of lctrons and havis (ions and nutrals) is wak B p E j u B j () n n plasma rsistivity (m) m v c n collision frq. lctrons with havis Inducd E fild du Hall Effct to plasma acclrats havy motion particls in charg nutral plasma lctron prssur trm 8

9 Elctric Rockt Propulsion B. Exampls Elctric Propulsion-17 Elctrothrmal Rockts Rsistojt (1W-10kW) walls hottr than gas, lowr T max can combin with chmical hating,.g., hydrazin monopropllant Arcjt (1 kw- 10 MW) high T arc >T wall ( K) must mix with rst of gas vry high T o, frozn flow losss in nozzl arc unstabl Insulation Spac Propulsion Analysis and Dsign, Humbl, Hnry and Larson, 1995 Elctric Propulsion-18 9

10 Rsistojts Hating mthods flow ovr coils of wir flow through hollow tubs flow ovr hatd cylindr or plat Matrial limits tmpratur/prformanc conducting matrials < 700 K rhnium, rfractory mtals/alloys (tungstn, tantalum, molybdnum), platinum, crmts max I sp ~ 300 sc Powr supply AC or DC Powr Q V R mc T p Elctric Propulsion-19 Elctric Propulsion-0 Arcjts Gas hatd by lctrical discharg (arc) lctrically conducting gas: plasma Joul hating Q j from E mc () B p T E j u B j p n n j j u B Hall Effct normal to j j u j B Rsistiv Hating j p n j p n Work from EM forc Nozzl and lctrod rosion high tmpratur, currnt arc I sp for H ~ sc Work from prssur grad. no work 10

11 Arcjts for Spac Missions Application dsigns from station kping of modrat-sizd spaccraft (fw kw, hydrazin) 1 st satllit station kping application - Dcmbr Lockhd Martin Astro Spac Sris 7000 Tlstar 401 satllit (ownd by AT&T) to pilotd Mars mission (100 kw, hydrogn) Advantags high spcific impuls, approximatly 600 s (hydrazin), 800 s (ammonia, TRW/AFRL ESEX projct) to,000 s (hydrogn) compard to that usd hydrazin dcomposition thrustrs ( Isp ~0 s) and rsistojts (Isp ~300 s) Improvs propllant fficincy and allows satllit to carry largr payload instad of th additional propllant normally rquird for long duration missions similarity btwn hydrazin arcjts and hydrazin thrustrs and rsistojts alrady intgratd into spaccraft for station kping Drawbacks xcssiv hating limitd lif and/or advancd matrials highr frozn flow losss in nozzl (high T), quivalntly low thrust fficincis (~35% of input powr is convrtd to usful thrust) possibl plum contamination Elctric Propulsion-1 Elctric Propulsion- 1.8 kw Hydrazin Arcjt Tlstar IV application stationkping Hot gas from catalytically dcomposd liquid N H 4 Hatd valv prvnts N H 4 from frzing I sp ~570 sc ~30 mn (~40mg/s) Spac Propulsion Analysis and Dsign, Humbl, Hnry and Larson,

12 Hydrazin Arcjt 1999 Tst Firing of a Primx (now Arojt) MR-510 Hydrazin Arcjt Thrust -58 mn. Input 4340 W (into Powr Supply [PCU] for Powr arcjts); 000 W input to ach arcjt from PCU) Isp > s Elctric Propulsion-3 A shipst of Arojt MR-510 Hydrazin Arcjts and a Powr Conditioning Unit 16 Lockhd Martin A100 spaccraft (satllits) with MR-510 systms hav bn launchd Each spaccraft has 4 thrustrs and 1 PCU; at a tim usd for N-S station kping orbit manuvrs Elctric Propulsion-4 Arcjt with Applid Magntic Fild High currnt dnsity in arc tnds to dstroy lctrods Add solniod B Fild adds azimuthal drift vlocity to arc improvs azimuthal symmtry of gas tmpratur rducs local lctrod hating, rducs rosion ndd for high powrs (100 kw) Microwav discharg altrnat to arc hating Spac Propulsion Analysis and Dsign, Humbl, Hnry and Larson,

13 Elctric Propulsion-5 Elctrostatic Thrustrs Ion ngin xampl Nar vacuum rquird Ion sourc usually lctron bombardmnt plasma also ion contact: propllant flowing through hot porous tungstn fild mission: chargd spray droplts/particls Elctrons addd to nutraliz xhaust thrmionic mittrs A Acclrator L Spac Propulsion Analysis and Dsign, Humbl, Hnry and Larson, 1995 Elctric Propulsion-6 Elctrostatic Thrustr Prformanc Maximum xhaust vlocity (Isp) thortically limitd by voltag diffrnc across acclrator High thrust rquirs high mass flowrats and thrfor currnt (and numbr, n) dnsitis Ion currnt limitd by spac-charg fild from lots of ions ovrcoms applid E fild Child-Langmuir Law j max u q m j nqu 13,800 for singly ionizd V xit V inlt 4 q V o j max 9 m L V (volts) MW 3 m s acclrator lctrod spacing prmittivity 1 Farad o of fr spac mtr 3 8 V (volts) Amps MW L m m for singly ionizd 13

14 Elctrostatic Thrustr Thrust m u jam qu Maximum thrust 3 m 4 q V m q o max j max A u A q 9 m L q m 8 A V o L for circular cross-sction 9 max 9 D L V max o D L V High rquirs high V and aspct ratio spac chargd/l ~1 us many small ion bams to gt mor thrust A=cross-sctional ara flow V of diamtr, D in Nwtons L D Elctric Propulsion-7 Elctrostatic Thrustr Thrust For fixd spcific impuls max max A 4 q V 9 m L m bst propllant for high thrust havy molculs (particls) o I u sp xnon (X) good choic (MW=131.3) q 3 accl m A q I sp Elctric Propulsion-8 14

15 Elctric Propulsion-9 Elctrostatic Thrustr Powr Elctrical nrgy convrtd to kintic nrgy with som fficincy 1 I V m u conv Must also account for nrgy usd to crat ions ionization losss minimum loss givn by ionization potntial for atom tims currnt m P I q ionloss I I m Powr I V jav Elctron Bombardmnt X + Engin Exampl Oprating conditions Elctric Propulsion-30 V=700 V, L=.5 mm, 00 hols (grids) ach with D=.0 mm MW(X)=131.3, I (X)=1.08 V Dtrmin, u, Isp. m minimum powr rquird (including ionization loss) 15

16 max P Elctric Propulsion-31 6 V grids Solution D L 6 / N / grid N / grid 4. mn 1 3 max, total u 13,800 V MW m s 13, m u 31, 860m m u s I sp 348 s 4 Pxhaust Pion loss m 67.9W 1. 18W min u g s q m m I flown ngins up to fw kw P maximum ffic. =67.9/69.1 min 69. 1W = 98.3% practically < 60-70% 7 m MW kg kg 19 q C s for singly ionizd molc. flown ngins up to ~100-00mN Hall Thrustr Hall ffct most noticabl at low particl dnsitis Stationary Plasma Thrustr (SPT) dvlopd in Russia 10 s kw axial currnt flow across radial B gnrats azimuthal lctron flow inducd (axial) Hall E acclrats ions also calld gridlss (nutral) ion thrustr s Isp with >50% fficincy using X and no oscillations lik MPD Elctric Propulsion-3 Spac Propulsion Analysis and Dsign, Humbl, Hnry and Larson,

17 Elctromagntic Propulsion Systms Us applid or inducd magntic filds to produc acclration of propllant Elctric Propulsion-33 high currnts/ powrs rquird to produc significant inducd filds high powr availabl only (normally) in pulsd opration Spac Propulsion Analysis and Dsign, Humbl, Hnry and Larson, 1995 Pulsd Plasma Thrustr Propllant producd by vaporizing solid matrial with discharg B fild inducd by discharg also acts to acclrat vaporizd propllant Advantag of simplicity Acclration forc ~ j (discharg currnt) Elctric Propulsion-34 Spac Propulsion Analysis and Dsign, Humbl, Hnry and Larson,

18 Magntoplasmadynamic (MPD) Thrustrs Rsmbl arcjts Lowr flow dnsitis to attain highr xhaust vlocity Diffus discharg, low rosion Slf fild rquirs high J Applid B fild allows highr V at lowr discharg currnts incras accl. largr Hall ffct Elctric Propulsion-35 Slf-Fild MPD Thrustr Applid-Fild MPD Thrustr Spac Propulsion Analysis and Dsign, Humbl, Hnry and Larson, 1995 MPD Thrustrs (con t) Most fforts focusd on applications with xhaust vlocitis (Isp) gratr than arc jts Typically rquir highr powrs than currntly availabl on in-spac vhicls Exhaust spd u limitd by rosion and oscillations at high j j m Elctric Propulsion-36 18