Engineering 176 Meeting #9 Plats du jour 9 Thermal Principles (March 19) –Convection, Conduction,...

Engineering 176 Meeting #9

Plats du jour• 9 Thermal Principles

(March 19)

– Convection, Conduction, Radiation in the spacecraft environment

– Heat capacity and other simplifying considerations

– Minimalist’s FEA: MOST

– Hints from Heloise

• 10 (11?) -Thermal / Mechanical Design. FEA

(Joel Pedlikin - April 4 or 11)

• 11 (10?) - Project Management, Cost & Schedule (April 18)

• 12 - Design work +digital (April 25)

• 13 - Presentations (May 2 or…)

• 1 - Introduction• 2 - Propulsion & ∆V• 3 - Attitude Control

& instruments• 4 - Orbits

& Orbit Determination

• 5 - Launch Vehicles• 6 - Power

& Mechanisms• 7 - Radio & Comms• 8 & 9 Reliability

(March 14 & 19)


Last week’s note on do-ability• Orbital Rockets - barely do-able and for 10,000 years, not

do-able. 100 years from now, might be as easy as flying a Cessna to 10kft.

• Television - barely do-able in 1940s

• Flight- barely do-able: Lindberg and Earhardt

• Digital graphics - JPL IPL - famous in 1980s

• Radios: barely do-able in Marconi era

• Maybe we will say the same, 50 years from now, about… - personal satellite comms

- earth services from space (light, power)- space billboards

Critical step is finding out what’s missing


Feynman on do-ability

• There was a time we didn’t understand even how gravity behaved

• Then we modeled it (Kepler) but didn’t have a law of gravitation

• Now we have the law of gravitation, (but not the physics)

• Understand large scale (planets, stars) ( Newton and Einstein)

• Understand meso scale (atoms to planets) (Newton and Planc et al.)

• Do not understand sub-proton / sub-neutron scale• QED was the first successful attempt to describe

behavior of sub-nuclear particles (Feynman Nobel)


Circles, Ellipses and Beyond

r

a rp

vb

Hyp

erbo

lic A

sym

ptot

e

Ellipse:

Transfer, Molniya, Reconnaissance orbits

Comets, Asteroids

Real Planets, Moons, LEOs, GEOs

Kepler’s 2nd law

e = c / a

r = p / [1+ e cos(v)]

cOrbit Elements:

a (or p), e (geometry) plus ip= a(1-e2)

Ω (longitude of ascending node)

(argument of periapsis, ccw from Ω)

tp, 0… (epoch)


Vernal Equinox (‘02 March 20, 2:16 pm)

Every 4th year correction is the Julian Calendar leap year - but it’s slightly to much - the equinox slips earlier - the calendar pages turn too slowly

Julian -> Gregorian subtracts leap years on centuries (00 years) except 400, 800)

(next one to skip is in 2100 - see you there!)

Too many days

Skip a leap

Don’t Skip a leap

Skip a leap


Last week: Reliability

‘60s ‘70s ‘80s ‘90s

Government Commercial

Class S implemented

beginClass S

planning

1st Qualified

ManufacurersList

1st Automotive

apps

Transistors

JapaneseQuality

Challenge

submicrontechnology

Customer / supplier

partnerships

• MIL-HNBK-217E

Sensor Encoder Computer Tx

0.99 0.99 .98 0.97 Ps = 0.93

Connector

0.99 0.99 .98 0.97


Ps = 0.83

• Real World

Leads to

What

&

Have in common


Real World FMECA Stats.Chances of: Flipping Heads:

1x: 0.51 = 0.5

2x 0.52 = 0.25

3x: 0.53 = 0.125

4x: 0.54 = 0.0625(one out of 24 = 1:16)

Chances of: Rolling one (snake eye)

1x: (1/6)1 = 0.1667

2x: (1/6) 2 = 0.02778

3x: (1/6) 3 = 0.004630

4x: (1/6)4 = 0.000772(one out of 64 = 1: 1296)

Expected Value = P(success) x Payoff

• Bet on one roulette slot: 1/36 x 35x bet = 35/36

• Lottery: 1: 10,000,000 x $10,000,000 = $1(but tickets are $2)

• Insurance: Premium is always > EV • Betting: Jackpot is always << EV

=> Why buy insurance or bet?


Burglar Alarm ParadoxBurglar Alarm Reliability: 99.9%

• False alarm happens 1:1,000 days (3 years)

Chance of being robbed: 1: 100,000 houses (or cars)

P(alarm goes off due to robbery):

Assume alarm sounds:

P(Robbery) = 0.00001

P (False) = 0.001

=> P(False) / P(Robbery) = 0.001 / 0.00001 = 100 : 1

->100 false alarms for every real robbery <-

If Alarm lives 10 years and false alarm costs $100:

Cost = $100 x 0.001 x 365 x 10 + $(buy and keep alarm) = $365 + ($250 + $10 x 12 x 10) = $1815 = Cost

Expected Value = 0.00001 x 365 x 10 x uninsured deductible (maybe $25k) = $912.50 = EV


A World of Burglar Alarms

Any test performed a large number of times looking for an unlikely result: - Engineers’ warnings about unsafe vehicles, bridges…

- Corporate whistle blowers

- Mammography & other cancer screenings

- Pregnancy & AIDS home tests

- The latest advice on Butter, Margarine, blue-green algae, wheat grass, 8 liters of water per day…

- Self test (eg in BMWs & VWs)

- Owning a gun and keeping it at home, in car, in pocket

- S - Class parts: screening for defects

- X-ray screening of parts

- Twin - engine aircraft (depends on pilot)

- Terrorist Alerts (high-res burglar alarm analogy)

- Uninteruptable PS and 9V batteries in clock radios


Real World FMECA Stats.• MIL-HNBK-217E


0.99 0.99 .98 0.97 Ps = 0.93

Connector

0.99 0.99 .98 0.97


Ps = 0.83

• Real World

• Interconnections and interactions (some unknown), dominated by human

factors, dominate risks

• Same principles apply inside each black box

Ie - if we knew how to do this, automobile

and drug recalls would be unknown


Real World ReliabilityHow others do it

– Systems Redundancy, subsystems degrade gracefully (reliability of species, not individual survival)

“In three words I can sum up everything I've learned about life: it goes on. - Robert Frost (1874-1963)

– Balance: • too much defense vs. too little • run & fight vs. reliability• Longevity vs. reproduction • Think vs. do• Trial & error in real world • learning/adaptation vs. Q

• Consumer Products– Redundancy is rare - Repair / Replace easier– Protect from user - Routine Maintenance– Product Evolution, not revolution

• No user-serviceable parts • Limited control / access• Safety interlocks • Field xperience / statistics• Manufacturing process investment (automated test & cal)

– Define “reliability” (e.g. “don’t kill people”)


• Avoid poor design:• Highest quality engineering team• People (not parts) who have done it before• Buddy system• real world testing based on engineering,

not specs/politics

• Redundancy for known problem components (batteries)

• Special treatment for special parts (DC/DC converters, electrolytic capacitors):

– Select / deselect vendors based on experience– Subject all to discrete component tests– Careful visual inspection

• All Compoments: verify environment specs + test

• Remove hardware (use software):– Packet creation / disassembly - Attitude Determination– Charge control - Fine pointing of optics– Antenna pointing - Is this trip necessary?

(use computers, drop towers,balloons, aircraft)

Real World Reliability: how we (should) do it


Weakest Link? Small Satellite Historical Survey results: 1956 - 1996

• Ground Rules:– Small defined as < 150 kg. Exceptions for a few (<5) larger payloads which had

dedicated Scout / Pegasus launches– 456 Missions counted - all flown between 1956 and 1996. Multiple deployments

of identical satellites were counted as 1 mission– Note: Some countries may not have reported all launch and/or spacecraft

failures

• The Statistics:– 310 Spacecraft inserted successfully in orbit: 69% insertion reliability

• Failures include separation systems, upper stage engines, launch failures.

– Of 310 inserted successfully, 293 (95%) performed mission successfully– If launches are historically 85% to 90% reliable, then mechanisms other than

launch and the satellite are only about 80% reliable (the least reliable link in the chain).

• My Conclusions:– This weakest link is separation and deployment mechanisms– Launch reliability: 90%. Separation reliability: 80% Spacecraft reliability: 95%:– Are we overspending on spacecraft reliability?– Are small spacecraft more reliable than conventional ones despite decreased

attention to traditional space product assurance methods?


REAL THREATS• Funding

– Optimize what? P(corporate survival) vs. P(Program Survival)– Promote, utilize foreign partners– Discipline: don’t blow the budget - cut requirements

• Team Performance– Provide lots of feedback, + and -. Don’t skimp on tools, provide goodies– Enforce buddy system, don’t hire stars (see King’s Funnel)– Don’t demand paper– Supply food

• Requirements Creep– Accept no free features!– Beware of, in fact, avoid at all costs, features justified on the basis of “easy”,

“automatic”, “built in” or “useful next program” (even if the adjective “really” is appended)

– Don’t improve the design, extend test specs and duration


JAS-1 underestimated power budget - survived with limited operations.

Replaced by JAS-2

Software design and operator errors caused Clementine to accidentally

exhaust all its propellant, ending its Mission.

HETE was stranded alive inside rocket launch envelope

TRW’s Lewis (left) failed within a few days on orbit

due to design and operator errors. Orbital’s Clark

(right), Lewis’ “twin”, was cancelled mid-program due

to budget overruns

A poorly designed fuel system destroyed mars observer just upon

reaching its destination.

Ghosts of Programs Past


Due Tonight (Tuesday, March 19)

• Reading on Reliability:– SMAD 19.2 (15 Pages worth reading / skimming)– TLOM 15 (clean rooms etc.)

• Reading on Thermal Design– SMAD 11.5 (31 pages worth reading + good ref. Data)– TLOM 10

• Mission Success / Reliability plan– Designing in Reliability - Mission Definition– Insurance - Risk mitigation – Estimate lifetime, P(Success) - Test Plan


Due Thursday, April 4

• Reading on Project Management:– SMAD Chapter 23 (9 easy pages)– TLOM ?

• Reading on Structural Design– SMAD 18.3 (10 easy pages on structural requirements)– Review/use SMAD 11.6 (36 pages on Structural analysis)

• Budgets– Link - Bits (how many you need)– Power - Mass– $ (for key components +?) - ∆V (station keeping / ACS…)– Thermal - schedule and labor (ROM)


Conductionthe primary heat transfer mechanism

within a small satellite

Q = Joules / sec = WattsA = area in m2

conductivity: J / s K m²T = temperature differenceL = length between temp extremes

o

Q = A ²T / L o

Big vs. Small Satellite Conduction ExamplesBig Small

²T = Q L / A L=4m; A=10-2m2; Q solar area = 16 m2

²T = Q L / A L=.5m; A=10-3m2; Q solar area = .25 m2


Convectionthe least significant heat conduction mechanism within a small satellite

Q = A c ²T V o

Q: Why do we care about convection?

A: We don’t - there is no flowing medium to conduct heat - but note that in an atmosphere + g-field, there is.

A’: Putting a terrestrial device in a pressurized container may not be enough - you need a fan too. Even then, some part won’t get fanned and will overheat.

A’’: Convective heat flux - without even a fan - is typically 10x to 100x conductive heat flux.


Radiationthe only way to lose or gain thermal energy

Q = sA(T14 -T24) v o

s = Stephan Boltzmann constant, = 5.67 x 10-8 W/m 2K4

= emissivity (0 to 1)

A = radiating surface area v = view factor

Interesting observation #2:Solar Flux 1350 W/m2 => net insolation is 1 kW

IO #3: Solar Flux a 1/r2=> @ 1.4 AU, flux = 51% of near earth flux; ² of 688 W

Other interesting topics: • View factors of various orbits • On making a cool spot


Heat Capacity / Thermal Inertia

²T= Q t / c m o ²T = temperature change in

time t c = heat capacity (J / kg K) m = mass (Kg)

For T in equilibrium in umbra @ 10C, in sun additional heat flux could be ~ 130 W => new equilibrium temp is 50C! But:

• Q = 130W; • t (in sun) 3600s • C (typical) 1000 J / kg C and • m 100 kg

=> ²T = 130 x 3600 / 1000 x 100 = 4.7C

Moral of the story: Don’t isolate satellite parts and then thermostat them

- or you’ll need to.


µS/C Simplifications: thermal model

31 2 4

5

Small Thermal Model

1) 1000 W/m2

2) A = 1 m2 = o 3) = x, = y y 4) Tair, Vair 5) T, sand, Tlake

__________________

Solve a few balance equations;

- intuitive and understandable - easy ¦ for resonable results - straightforward verification of

individual box models

Large Satellite: - 400 to 4000 node model: Do you believe it? How to test it? Time and $ to create it and test facilities to verify it.

Small Satellite: - 10 to 20 node model: runs on Excel - use large models for fine tuning and extra precision


MOST Heat Transfer Solver

Major Face Area = 0.6 m2 # of structural members 3

Solar Panel abs & emm = 0.85 Length of structure 0.77 m

Is = 1350 x abs. x A = 688.5 Watts Thickness of structure 0.003 m

Width (front to back) = 0.1 meter Thermal Conductance = A / L =

Material: 6061-T6 Aluminum 167 W / m K 11.642 w / K

Back side thermal emm = 0.5

T (s) Qr (s) Qr (b) T (b) ²T Qc(s->b) ²T reqd. comment

(Guess) radiate to sun Abs - (r-t-s) to radiate to radiate (conductive) Conduct->radiate

373 560 129 295 78 909 11 front way hot

323 315 374 385 -62 -722 32 front way cold

365 513 175 319 46 540 15 Front smidge hot

355 459 229 341 14 166 20 front bit cold

356 464 224 339 17 201 19 front bit cold

357 470 219 337 20 236 19 Ahhhhh!

MOST (Microvariability and Oscillations of STars) in development at University of Toronto.MOST is about the size of a briefcase and points one major face at the sun. (back side shown including marmon ring)

Case Study: MOST thermal model


More µSC Simplifications


Thermal Tools & Tactics

• Avoid insulated, stand-alone and isolated thermal members • Find payloads that don’t need extreme or extremely stable

temperature

• Coatings can give you a factor 10 control over radiation flux in and out

• Louvres offer about the same control, but can be modulatedon orbit

• Heaters don’t weigh much and make clients happy. (make sure they are fused)

• Duty cycle instruments to adjust temperature

• Modify attitude (e.g. look away from sun) to adjust temperature

• Use thermal tests for model verification, not space environment simulation

Engineering 176 Meeting #9 Plats du jour 9 Thermal Principles (March 19) –Convection, Conduction,...

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