PARAMETERS AND DESIGN

TURBOPUMP PARAMETERS AND DESIGN Pump Specific Speed

• For a pump with M stages: !H_t = pt 2 " pt1

M#g head per stage !S =

! !V g"H_t

(

)

where:

! is the rotor (angular) speed !V is the volumetric flow rate

!g is the fluid specific weight • !_S is a key parameter characterizing:

– the pump type:

centrifugal (!_S << 1) mixed-flow ( !S ! 1 )

axial (!_S >> 1) – the pump performance:

efficiency !_p (! max for !_S " 1) flow coefficient !

TURBOPUMP PARAMETERS AND DESIGN Pump Speed and Diameter

• For a single pump stage ( M = 1): – stage flow coefficient:

! = !V A U =

!V "r_Tb

(

)

( )

! = g"Ht U2 = "p_t #$2_r T2 where:

b is the discharge blade height A =!r_Tb is the discharge cross-section !H_t = !p_t "g is the total head rise

U =!r_T is the rotor tip speed and:

! ! 0.4 ÷ 0.7 near design conditions • Pump shaft power:

P = !V!pt

TURBOPUMP PARAMETERS AND DESIGN Pump Selection (based on requirements)

• Requirements derived from engine system: – fluid (hence density ! and viscosity µ) – inlet total pressure p_t1

– inlet total temperature T_t1

– pump discharge total pressure p_{t 2} – pump flowrate !V

– operating range !_min , !_max

• Head coefficient is limited for pump type: !p_t "#2_r T 2 $%max & #rT = !p_t %max"

Hence the required !p_t constrains rotor speed ! and tip radius r_T • Given the required pressure rise, the designer:

– maximizes ! within constraints:

generally advantageous to maximize speed many limits restrict max speed

– selects the rotor tip radius r_T and therefore the pump size (scales with r_T )

TURBOPUMP PARAMETERS AND DESIGN Cavitation and Suction Performance

• Inlet flow pressure strongly affects pump design • Cavitation can develop when:

p < p_V (vapor pressure) • Effects of cavitation:

– performance:

loss of head, efficiency, flow rate – machine life:

structural damage (erosion)

flow instabilities and dynamic loads rotordynamic unbalance (instabilities) • Suction performance specified by:

NPSH = p! pV "g +

u2

2g (Net Positive Suction Head) or by the nondimensional parameter:

!_SS = ! !V g NPSH

(

)

TURBOPUMP PARAMETERS AND DESIGN Turbopump Inducers

• Low NPSH requires the use of an inducer: – inducer normally necessary if:

!SS > 20 ( SS > 10000 )

(limit should be lower for long pump life) • High suction specific speed inducers:

– permit minimum pump inlet pressure – result in lower tank pressure and weight

TURBOPUMP PARAMETERS AND DESIGN Turbine Performance Parameters

• Isentropic velocity ratio: U c₀ = !r_T 2"#p (single stage) U_i2 i

!

( )

!

• Design trade-offs for increased efficiency: – increase the turbine tip speed U =!r_T :

speed limitations encountered due to stress and bearing limits increasing diameter increases weight, envelope, cost

– decrease the spouting velocity c₀ = 2!"p :

decreases turbine power, generally unacceptable: could be traded-off with higher flow rate

TURBOPUMP PARAMETERS AND DESIGN Rotordynamics

• Trends influencing space T/Ms rotordynamics: – higher engine power densities (high p_c):

higher turbopump pressures – higher rotational speeds:

smaller, less rigid turbopumps • Dynamic problems encountered:

– LOX pump explosions

– unsteady cavitation problems – seal reliability

– bearing loads and durability – turbine blade failures

– turbine wheel vibration

– balancing, synchronous vibrations – critical speeds

– start transient problems – synchronous whirl

SSME TURBOMACHINERY Vehicle/Engine Requirements

• Two turbopumps required for each propellant: – engine must accept low inlet pressures:

minimize vehicle propellant tank weight dictates low turbopump speed

– main combustor must operate at high pressure: maximize available energy

requires high pump discharge pressures dictates high pump speed

• Engine must throttle:

– dictates operation over wide flow range • Other design considerations:

– efficiency and weight

– dynamic seal life and efficiency – rotor axial thrust balance

– bearing life

– rotor balancing capability

– rotor critical speeds and dynamic stability – service in high pressure hydrogen or oxygen

SSME TURBOMACHINERY SSME Propellant Flow Schematic

SSME TURBOMACHINERY SSME Powerhead Arrangement

LOW PRESSURE OXIDIZER TURBOPUMP (LPOTP) Design Description

• Pump:

– four blade axial flow inducer • Turbine:

– full admission six stage impulse turbine – LOX powered, driven by HPOTP

• Pump/turbine rotor axial forces balance: – residual carried by thrust bearings • Operates below first rotor critical speed Design for Oxygen Hazards

• Close clearances for performance, no rubbing: – heat generation, rapid oxidation of metal • Protection provided by material selection:

– K-Monel rotor elements & inducer tunnel liner – silver wear rings

LOW PRESSURE FUEL TURBOPUMP (LPFTP) Design Description

• Pump:

– four blade axial flow inducer four partial blades at exit • Turbine:

– partial admission 2 stage impulse turbine – LH2 powered, driven by HPFTP

• Pump/turbine rotor axial forces balance: – residual carried by thrust bearing pair • Operates below first rotor critical speed • Utilizes a lift-off seal:

– prevents hydrogen leakage external to engine – pressure actuated by internal pressures

LOW PRESSURE TURBOPUMPS Development Problems

• LPOTP:

– early thrust bearing ball wear:

reduced loads by force balance change reduced labyrinth seals

increased turbine load in aft direction • LPFTP:

– low pump performance:

reduce no. of diffuser vanes eliminate partial blades

– reduced energy absorption from turbine: switch to partial admission

– turbine labyrinth seal degradation:

caused by non-symmetrical admission: resultant radial load

HIGH PRESSURE OXIDIZER TURBOPUMP (HPOTP) Design Description

• Pump section: – main pump:

double entry centrifugal impeller two inducers

volute with vaned diffusers discharges to:

LPOTP turbine

preburner (boost) pump main combustor

– preburner (boost) pump:

single entry centrifugal impeller boosts ~11% of engine flow • Two stage reaction turbine

• Axial thrust balance:

– preburner impeller balances turbine thrust – balance piston controls residual

HIGH PRESSURE OXIDIZER TURBOPUMP (HPOTP) Main Pump Design

• High speed desired for turbine efficiency • High suction performance:

– low Net Positive Suction Head (NPSH): NPSH= pt1 ! pV

"g

– high suction specific speed: !_SS = ! !V

g NPSH

(

)

• Main impeller sets shaft speed !:

– determined by suction performance capability: ! !V " !SS

(

)

3 4

= k # constant

• Hence, for single/double entry impellers ( !V1 = 2 !V2) with equal suction performance:

!1 !V1 = !2 !V2 " !2 = !1

!V₁

!V₂ =!1 2

HIGH PRESSURE OXIDIZER TURBOPUMP (HPOTP) Balance Piston

• Axial motion of the rotor used to generate:

– differential leakage of discharge flow on rotor sides – restoring axial force on the rotor

• Main features:

– effective only during pump operation

– decreasing effectiveness at reduced pump loads Bearings

• Spring preloaded, angular contact pairs • Propellant cooled

• Turbine end – 57 mm, axially restrained • Pump end – 45 mm, axially unrestrained • Dynamic seal package:

– controlled gap, intermediate seal with He purge: separates hot gas and oxidizer leakage flows

HPFTP DESIGN DEVELOPMENT Preliminary Design Selection

• HPFTP design requirements:

– LH2 flow: 1.08 m3_{/s @ 23.9 K}

– inlet pressure: 14.8 bar – discharge pressure: 485.4 bar – operational range: !" "_max = 0.2 • Comparative design features:

– no. of stages: 2 3 (3 stage design selected) – rotational speed (rpm): 40874 37245

– tip diameter (m) 0.305 0.305 (same in both cases)

– tip speed (m/s): 652 393 (compatible with Titanium rotor) – efficiency 0.78 0.83 (significant efficiency benefits) – stage head coefficient: 0.70 0.56 (reasonable head coefficient) – specific speed: 0.055 0.068 (compatible with radial design) • Additional features to be checked for speed compatibility:

– turbine design

– bearing and seal capability – suction performance

HIGH PRESSURE FUEL TURBOPUMP (HPFTP) Design Description

• Three stage centrifugal pump: – two high efficiency cross-overs – diffuser and volute at 3rd stage • Two stage reaction turbine:

– 700 HP per blade

– hydrogen embrittlement protection • Angular contact 45 mm duplex bearings:

– thrust bearing for transient loads • Pressure actuated lift-off seal

• Axial thrust balance:

– pump impeller balances turbine thrust – balance piston is single acting

Development Problems • Turbine end bearing life • Rotor subsynchronous whirl • Turbine blade life

HPFTP DESIGN DEVELOPMENT Turbine Design Selection

• Turbine design requirements:

– working fluid: LOX-Hydrogen:

selected by engine system based on mission studies – low pressure ratio, high flow turbine:

engine systems selected stage combustion cycle limits pressure ratio across turbine

requires flow increase to achieve power – required power for driving the pump:

P_p = !V!p_t "_p = 57.8 #107

MW

• Turbine power output (total-to-static, perfect gas): P_t =!t !mcp

(

)

– mass flow rate through the turbine !m – the energy content of the fluid c_p

(

)

HPFTP DESIGN DEVELOPMENT Selection of Turbine Inlet Conditions

• Inlet conditions established through iterations with engine balance • Inlet pressure:

– increased pressure desirable to minimize flow and temperature results in increased pump pressures

requires higher power and torque • Inlet temperature:

– higher values desirable for power – lower values desirable for life

– values ≤ 1111 K (2000 °R) evaluated

• Parameters selected after iterative engine system studies: – inlet pressure p_t1: 38.96 MPa (5650 psi)

– pressure ratio p_t1 p₂ : 1.56

HPFTP DESIGN DEVELOPMENT Turbine Design Trade-Off and Preliminary Selection

• Selected pump rotor speed and diameter: 37000 rpm and 0.305 m: – turbine diameter range: 0.203 – 0.305 m

• Available energy determined from fluid conditions (spouting velocity c₀ ) • Comparative design features (3 stage impulse design selected):

– no. of stages: 1 2 2 3

– tip diameter (m): 0.305 0.203 0.305 0.203 – stage isentropic velocity ratio: 0.42 0.49 0.59 0.60 – turbine type: impulse reaction reaction impulse

– efficiency 0.79 0.79 0.85 0.84

• Constraints:

– diameter has a significant weight impact

– added stages adds length/weight/complexity/cost – turbine stress limits diameter

• Final selection optimized within above constraints: – 2 stage reaction blading

HPFTP DESIGN DEVELOPMENT Potential Bearing Arrangements

1. Bearing outboard, pump inlet in the middle: – requires high pressure hot gas seal

– permits hot gas flow into pump inlet – impacts suction performance

2. Turbine overhung, pump inlet outboard:

– turbine bearing too large (excessive DN value) – shaft sized for carrying torque

– bearing diameter set by shaft diameter

3. Pump 1st stage overhung, turbine bearing outboard: – requires special tooling for 1st cross-over stage – no significant critical speed gain

4. Bearings outboard, pump inlet outboard:

– small bearings (45 mm) yield low DN (1.66!106 in rpm ) – hydrogen bearing state-of-the-art (DN ! 1.66 "106 in rpm ) – critical speed control

– favorable pump-turbine sealing arrangement Selected as optimum

HPFTP DESIGN DEVELOPMENT Seal Selection

• All dynamic seals are labyrinth seals – operate with clearance

– maximize seal life – no rubbing velocities – non impact on shaft speed

• Pump pressure exceeds turbine pressure: – no need for mechanical dynamic seal – H2 leak into turbine exhaust used in MCC • Seals designed for minimum clearance @ FPL:

– highest pressure

– most critical performance

• No high pressure external flange seals required: – piston rings seals used internally

HPFTP DESIGN DEVELOPMENT Typical Seal Operating Characteristics

No. Fluid Δp bar Clearance mm Flow kg/s 1 LH2 31 0.177 0.50 2 LH2 0.7 0.254 0.09 3 LH2 61 0.127 0.23 4 Hot Gas 48 * 0.77 5 Hot Gas 27 0.508 0.38 6 Hot Gas 54 0.203 4.08 7 LH2 3.4 * 0.0002 8 LH2 408 * 0.045 9 LH2 143 0.127 0.68 10 LH2 37 0.127 1.31 11 LH2 82 0.127 0.95 12 GH2 143 * 0.045 * Static seals designed for zero clearance

HPFTP DESIGN DEVELOPMENT Suction Performance

• Suction performance requirements:

– engine Net Positive Suction Pressure: NPSP = 36.7 bar @ FPL

– suction specific speed without LPFTP: !_SS = 361 (way too high)

– current technology limit (optimistic): !SS = 162

(!_SS = 20 corresponds to S_S ! 10000 ) – speed required to get !_SS = 162 :

! = 1737 rad/s = 16600 rpm (too low for HPFTP) Hence a separate low pressure pump is required

• LPFTP supplies sufficient head for HPFTP: – HPFTP minimum NPSH ! 1900 m @ FPL – maximum !_SS = 13.8 (affordable value)

– !_SS = 13.8 is low enough to design HPFTP without an inducer – significant advantage in terms of rotor length and critical speeds

HPFTP DESIGN DEVELOPMENT Critical Speeds

• Critical speeds evaluated for compatibility: – operating speed range:

19000 – 38000 rpm – required margin:

20% from critical speeds

– optimum bearing support stiffness required – pump operates between 2nd and 3rd critical Design Speed Selection

• Selected speed satisfies all requirements:

– provides high efficiency with 3-stage pump configuration – provides high efficiency with 2-stage turbine configuration – impeller and turbine tip speeds within structural limits – bearing DN value within state-of-the-art limitation for life – no high seal rubbing velocities

– suction performance capability of pump provided – critical speed margins met

HPFTP DESIGN DEVELOPMENT Axial Thrust Balance

• HPFTP axial thrust requirements: – design for axial thrust control

– minimize axial loads carried by bearings

– bearings usually required to take start-up load • Design options available:

– arrange turbine thrust to oppose pump thrust – locate pump wear rings to minimize pump thrust – use self-compensating balance piston

HPFTP DESIGN DEVELOPMENT Diffuser-Crossover System

• Critical design feature:

– must provide efficient diffusion (8340 m/stage): from 9780 m (impeller discharge)

to 1350 m (crossover discharge) – space/weight must be minimized:

requires tight flow turns difficult fabrication

• Three concepts have been proposed and tested: – radial diffuser and crossover vane system – axial diffuser and crossover vane system – continuous channel diffuser and crossover Tested in air rig using existing impeller:

– axial diffuser showed stall, low efficiency – radial diffuser had highest efficiency (75.3%) – continuous channel diffuser (selected):

comparable efficiency (74.0%)

HPFTP DESIGN DEVELOPMENT Fluid Properties Impact on Design

• Oxidizer generally highly explosive: – must avoid rubbing contact

– results in larger clearances

– results in lower pump efficiency

– requires in-depth analyses throughout operation • Hydrogen compressibility results in loss of head:

– LH2 thermodynamic properties more sensitive – LH2 results not following normal scaling laws

– predicted performance dependent on thermodynamic modeling • High vapor pressure fluids provide thermodynamic benefits:

– cavitation is delayed due to local thermodynamic effects – referred to as thermodynamic suppression head

– required NSPH is reduced

– effect can be very significant in LH2:

HPFTP DESIGN DEVELOPMENT Rotordynamic Design

• HPFTP rotor architecture:

– single shaft on 2 supporting end bearings – 5 rotors (3 pump stages, 2 turbine disks) – 12 labyrinth seals

• Elastic supports:

– radial flexure for shaft/bearing/carrier – Belleville damper of flexure

– Belleville spring axial preload on bearings • Operation:

– operating speed range: 19000 – 38000 rpm – required margin:

20% from critical speeds

– optimum bearing support stiffness required – pump operates between 2nd and 3rd critical

HPFTP DESIGN DEVELOPMENT Subsynchronous Whirl Problem

• Prior experience:

– large steam turbines, blowers, compressors – bearing whip

– textile spindles

• High incidence in LH2 space T/P: – early MK 15F (J-2 Program) – MK 9 (Rover Program)

– MK 25 (Phoebus Program) – 350K P&W (USAF)

• Eliminated by stiffening of shaft and bearings • Possible contributors to rotor instability:

– internal hysteresis

– turbine (aerodynamic) cross coupling – seal forces

– nonlinear effects

HPFTP DESIGN DEVELOPMENT Designs for Increased Stiffness and Damping

• Bearing stiffness (on the right):

– increases from configuration 1 to 3 • Seals (below):

– stiffness increases with thicker splined collar (4 & 5) – Cxx element of damping matrix increases with:

HPFTP DESIGN DEVELOPMENT HPFTP Rotordynamic Development

• Effects of H/W modifications on shaft stability and frequency: – attempts to add damping and stiffness at supports insufficient – attempts to reduce cross-coupling of seals insufficient

HPFTP DESIGN DEVELOPMENT HPFTP Rotordynamic Development

HPFTP DESIGN DEVELOPMENT HPFTP Rotordynamic Development

• Conclusions:

– computer models strong indicators of trends:

showed inadequacy of solving problem solely at bearing support showed desirability of using smooth seals

showed sensitivity to nonlinearities

could be fitted to engine runs, but never reliable in prediction of tests – major destabilizers:

seals and turbine cross-coupling

other effects (deadband, impellers, internal hysteresis, etc.) secondary or unknown – major stabilizers:

increase of bearing stiffness

increase of viscous damping of bearings (considerable increase necessary) bearing support stiffness asymmetry a stabilizer, high ratios needed (∼2.5:1)

increase of stiffness and damping of smooth seals (effective only above 25000 rpm) friction damping at supports either ineffectual or even destabilizing

– stiffness data reliable, seal dynamics more uncertain More work needed

HPFTP DESIGN DEVELOPMENT HPFTP Turbine Gas Dynamics and Cooling

• Turbine cooling required for high speeds:

– strength of material dependent on temperature – turbine gas path temperatures: ~ 815 °C (1500 °F) – high pitchline velocity: ~ 480 m/s (1600 fps)

– turbine disc, bearings & housing require cooling • Liquid hydrogen selected for coolant:

– excellent cooling capability – available in pump

– pressures sufficient to provide positive flow: turbine inlet pressure: ~ 391 atm (5750 psi)

1st nozzle discharge pressure: ~ 344 atm (5050 psi) • Flow circuit designed to achieve cooling:

– operational at ~ 1.2 seconds after start: at ~ 7000 rpm lift-off seal opens ignition in preburner ~ 1,0 second

HPFTP DESIGN DEVELOPMENT HPFTP Turbine Cooling Problems

• Inadequate LH2 coolant flow to bearings: – results in hot gas backflow

– turbine bearings failed

HPFTP DESIGN DEVELOPMENT HPFTP Turbine Housing Problems

• Housing cracks and bulging experienced: – manifested in numerous areas of turbine • Differential rubbing noted on dynamic seals:

– alignment similar to that of housing failures • Evidence of anomalously high temperatures:

– indication of unaccounted for hot gas flows • Problems related to transverse Δp:

– generated by hot gas manifold design

• Turbine discharge-hot gas manifold (HGM) problems: – flow exits turbine at high velocity

– exit guide vane flow separation

– tight turnaround duct leads to inner wall separation – transverse Δp and flow velocity due to HGM design – flow separation on some axial turbine struts