Paul K. Johnson

Analex Corporation, Brook Park, Ohio

Lee S. Mason

Glenn Research Center, Cleveland, Ohio

Performance and Operational Characteristics for a

Dual Brayton Space Power System With Common

Gas Inventory

NASA/TM—2006-214393

November 2006

AIAA–2006–4167
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Paul K. Johnson

Analex Corporation, Brook Park, Ohio

Lee S. Mason

Glenn Research Center, Cleveland, Ohio

Performance and Operational Characteristics for a

Dual Brayton Space Power System With Common

Gas Inventory

NASA/TM—2006-214393

November 2006

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Fourth International Energy Conversion Engineering Conference and Exhibit (IECEC)

sponsored by the American Institute of Aeronautics and Astronautics

San Diego, California, June 26–29, 2006

AIAA–2006–4167
Acknowledgments

The work in this paper was performed for NASA Headquarters, Prometheus Nuclear Systems Program.

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Available electronically at http://gltrs.grc.nasa.gov

Level of Review: This material has been technically reviewed by technical management.

This report contains preliminary findings,

subject to revision as analysis proceeds.
This paper provides an analytical evaluation on the operation and performance of a dual

Brayton common gas system. The NASA Glenn Research Center in-house computer 

program Closed Cycle System Simulation (CCSS) was used to construct a model of two 

identical 50 kWe-class recuperated closed-Brayton-cycle (CBC) power conversion units that 

share a common gas inventory and single heat source. As operating conditions for each CBC 

change, the total gas inventory is redistributed between the two units and overall system

performance is affected. Several steady-state off-design operating points were analyzed by 

varying turbine inlet temperature and turboalternator shaft rotational speed to investigate

the interaction of the two units. 

Nomenclature 

CIT compressor inlet temperature 

CPR compressor pressure ratio 

DP/P relative pressure drop

m mass 

P pressure 

R gas constant

T temperature 

TIT turbine inlet temperature 

TPR turbine pressure ratio

V gas volume

I. Introduction 

HE closed-Brayton-cycle (CBC) is a candidate thermodynamic cycle for space power conversion applications.

The CBC can be integrated with various heat source options including solar heat receivers, radioisotope fuel 

sources, or fission reactors. Recent NASA studies have investigated CBC-based reactor power systems for the

Jupiter Icy Moons Orbiter (JIMO) mission1

 and for lunar and Mars surface power missions.2,3 The reactor heat 

source could include a separate primary coolant, such as liquid metal, and an intermediate heat exchanger or provide

direct heating of the Brayton working fluid via a gas-cooled reactor. The JIMO mission was pursuing the 

development of a 200 kWe gas-cooled reactor with direct Brayton power conversion. Some of the features of the 

direct gas-cooled reactor concept that led to its selection for JIMO were reliability, deliverability, cost, and safety. 

The gas reactor system fulfilled the mission requirements for the envisioned Nuclear Electric Propulsion (NEP) 

mission with simplified engineering development testing and fewer development hurdles than the other options

considered.

4

 For similar reasons, the CBC-based reactor power system is a leading candidate for lunar and Mars 

surface mission applications. The expected power requirements for surface missions are projected to be in the 25 to 

100 kWe range.

T 

Potential CBC designs might consist of multiple power conversion systems to allow for system redundancy. The 

JIMO mission was considering options that included as few as one Brayton unit and as many as four units with

NASA/TM—2006-214393 1

Performance and Operational Characteristics for a Dual Brayton 

Space Power System With Common Gas Inventory 

Paul K. Johnson 

Analex Corporation 

Brook Park, Ohio 44142 

Lee S. Mason 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 
variations that utilized shared heat exchangers among the units.5

 NASA also examined design tradeoffs relating to 

the number of Brayton units and the expected off-design performance.6 There are several architectures to be

considered when configuring multiple power conversion systems. One option is for the systems to share a common

gas inventory and common heat source, where the working fluid from all the CBC units combine before entering the 

heat source and split before entering each CBC unit’s respective turbine. Modeling such a power conversion system

would provide an understanding of how the gas inventory is distributed among the CBC components during offdesign operation and, ultimately, the impact on overall system performance. A recent Bettis paper explored the 

performance characteristics of a common-loop multi-Brayton power plant with emphasis on the reactor dynamic 

response to off-nominal operating conditions.

7

 This paper expands on the Bettis study and examines various 

operational scenarios including startup, imbalanced speed operation, and single-unit operation with one as standby. 

II. Method 

The approach of this study was to investigate several steady-state, off-design operating points that would provide

insight regarding the interaction between two Brayton power conversion units sharing a common gas inventory.

Although system transient behavior is of great importance, particularly with respect to a nuclear reactor heat source, 

one must first gain an appreciation for the impact that one Brayton’s operating point has on the other Brayton. 

Steady-state analysis allows one to observe scenarios that might not otherwise be apparent during transient

simulation. Some of the cases analyzed might not represent practical, long-duration operating points, but a wide

range of cases is necessary to depict fundamental trends in system performance as operating conditions are varied. 

The system can be thought of as two gas flow loops in parallel where each Brayton represents a loop. Therefore,

when both Braytons are operating nominally, their exit pressures are equal and their inlet pressures are equal. This

holds true even if the shafts are rotating at different speeds. Figure 1 is a block diagram that shows the gas flow from

each Brayton combining before entering the heat source and splitting after exiting the heat source. Gas exiting each 

Brayton is from the recuperator high-pressure exit, 

and gas entering each Brayton flows to the turbine 

inlet. This particular setup shows a check valve 

located at the exit of each Brayton to prevent 

reverse flow should one of the Braytons become

standby (i.e., it is shut off). Without check valves, 

gas would be forced to flow backwards through the 

standby loop as the other Brayton continued to

operate, because the pressure at the heat source inlet 

is slightly higher than at the heat source exit. If a

steady stream of gas was allowed to flow backwards 

through a Brayton the shaft would spin in reverse 

and possibly damage the gas-foil bearings, which 

are designed for rotation in one direction. 

The distribution of gas throughout the system depends on the gas temperature, pressure, and volume in any given 

system component. The amount (mass) of gas that resides in a given component is defined by the ideal gas equation. 

Radiator 1

Radiator 2 Brayton 2

Heat Source

Check Valves

Brayton 1

Figure 1. Dual Brayton System Block Diagram. 

m = PV RT (1)

If the Braytons are operating at two different speeds, state-points will adjust accordingly and the gas inventory 

will naturally redistribute as needed to balance the pressures at the heat source inlet. If one Brayton is standby while 

the other is operating and check valves are in place as shown in Figure 1, the standby Brayton acts as an

accumulator for stagnant gas, and the stagnant gas pressure equilibrates with the heat source exit pressure. Stagnant

gas temperature could vary from component to component, but will assume a value bounded by the heat source exit 

temperature and the environmental far-field sink temperature. After long periods of time the stagnant gas could 

become uniform throughout the standby Brayton. Regardless of the operating scenario, there is a fixed amount of

gas in the system, thus the balance of gas not residing in one Brayton is located in either the other Brayton or the

heat source. One would expect the distribution of gas to impact system performance, and the extent of that effect is

explored later in the results and discussion section.

NASA/TM—2006-214393 2
III. Computer Model 

The Closed Cycle System Simulation (CCSS) is a high-fidelity closed-Brayton-cycle design and analysis tool

written in the Numerical Propulsion System Simulation (NPSS) modeling environment.8 The source code originated

from the NASA Glenn Research Center in-house legacy program Closed Cycle Engine Program (CCEP) and has 

been used in previous studies.6,9,10,11 CCSS models all of the major CBC components (ducts, recuperator, gas cooler, 

turbine, compressor, alternator, radiator, and heat source) and accounts for details such as shaft bearing and windage

losses and bleed flow paths. A representation of a dual Brayton, common gas system was constructed in CCSS and

used to simulate steady-state off-design operating points for the purpose of further understanding the general

behavior of such a system. The following section describes the individual Brayton components as modeled by CCSS

and how they were used to construct the dual Brayton system. 

A. Brayton Components 

Gas ducts allow the working fluid to flow from one component to the next. CCSS calculates gas pressure drop

and gas mass inventory for each duct. Friction factor is calculated using a Reynolds number correlation for flow

through a duct, and inventory is calculated using the ideal gas equation. Duct lengths are input by the user and 

diameters are sized during design to achieve a specified relative pressure drop (DP/P) across each duct. 

The recuperator is a gas-to-gas, counter-flow heat exchanger with offset strip-fins in the core and plain plate fins 

in the headers. CCSS uses an approach outlined by Kays and London to calculate pressure drops and heat transfer in

the core and headers.

12 Gas mass inventory is calculated for the high- and low-pressure streams using the ideal gas

equation. Core width and length are determined during the design to achieve a specified effectiveness and DP/P

across the low pressure side of the recuperator. Off-design effectiveness and pressure drops are calculated using the

geometries determined from the design run. 

The heat source is the most basic Brayton component modeled in CCSS. Ideally, a detailed model would be 

included to better represent the heat transfer, pressure losses, etc., but the characterization of a nuclear fission reactor 

is beyond the scope of this study. The heat source instead is represented by a gas volume, fixed relative pressure

drop, and user-specified outlet temperature. This simplified heat source is sufficient because this study only 

investigates steady-state scenarios. 

The radiator is a pumped-loop configuration13 with sodium-potassium (NaK-78) coolant and an annular linear

induction pump (ALIP). CCSS calculates fluid pressure drop and heat transfer using Reynolds number correlations

appropriate for liquid metals. At design the coolant duct diameter is sized to achieve a desired pressure drop across

the radiator, and panel lengths are sized to reject the appropriate amount of heat. Off-design heat transfer and 

pressure drops are calculated using the geometries determined from the design run. The gas cooler is nearly identical 

to the recuperator with the exception that it is a liquid-to-gas heat exchanger without headers, and liquid metal 

correlations are used for the cold stream calculations. 

The compressor, turbine, and alternator rotate on a common shaft supported by gas-foil journal and thrust

bearings. The alternator converts shaft power into electric power with an electromagnetic efficiency defined as a

function of alternator power. Windage (viscous drag) and bearing losses are estimated as functions of shaft cavity 

pressure and shaft rotational speed. Generic performance maps are scaled by CCSS to estimate efficiency and 

pressure ratio for the turbine and compressor as a 

function of corrected mass flow rate and percent 

of design corrected speed. The maps were slightly

modified to accommodate the low-speed, lowpower operating points anticipated for this study. 

As is the case with most turbomachinery maps, 

these maps do not extend to low speeds and low

flow rates. Therefore, for model convergence

purposes the turbine and compressor maps were

extrapolated with the assumption that flow rate is 

zero at zero speed, and pressure ratio is unity at 

zero speed and zero flow. Figure 2 is a generic 

plot of turbine pressure ratio as a function of

corrected mass flow rate at lines of constant

speed. Dashed lines represent the extrapolated 

portion of the map. 

Corrected Mass Flow Rate

Pressure Ratio

0

0

1

Zero Speed

Extrapolation

Increasing

Speed

Figure 2. Turbine Performance Map Extrapolated to Zero 

Flow and Zero Speed. 

NASA/TM—2006-214393 3
B. Dual Brayton System 

Brayton components were arranged in CCSS to construct the dual CBC system of interest. Figure 3 is a 

schematic of the system design point providing the state point conditions and performance parameters of a dual 

Brayton system under balanced operation. Each Brayton unit was designed for 50 kW of electrical power output for

a total of 100 kW. Design input parameters such as compressor inlet temperature (CIT) and pressure (400 K and 0.5

MPa), compressor pressure ratio (CPR) (2.0), turbine inlet temperature (TIT) (1150 K), recuperator and gas cooler 

effectiveness (95 and 97 percent), helium-xenon (He-Xe) working fluid molecular weight (40 g/mol), and 

component DP/P were selected based on previous studies.3,6,9,10 The system is configured with 2 percent gas bleed 

flow for bearing cavity cooling and 5 percent liquid coolant bleed for alternator stator cooling. Radiator NaK mass 

flow rate was specified to yield the lowest combined gas cooler and radiator mass. All other cycle state points, HeNaK 393 K 5% Bleed 1150 K

1.38 kg/s 0.138 MPa 0.073 kg/s 0.963 MPa

1.45 kg/s 232 kWt He-Xe

 513 K 2% 1.82 kg/s

1145 K 40.0 g/mol

82.405% 87.357% 72% He

1.86 kg/s 28% Xe

400 K 924 K

0.500 MPa 0.515 MPa

CPR 2.00 Speed 45 krpm TPR 1.866

150 kW EM loss 3.3 kWt 213 kW

Brg Loss 4.252 kWt

175 kWt Wnd loss 4.513 kWt

DP/P = 0.0100 TIT/CIT 2.86

184 kWt

97.0% Cyc Eff 21.51%

Gas Inv. 0.6486 kg

93.6 m2 905 K

555 K 332 kWt 0.988 MPa

0.998 MPa

532 K 580 K 95.0%

Tsink 0.276 MPa 0.506 MPa 923 K

200 K

DP/Pcold = 0.0086

DP/Phot = 0.0150

System Power 100.0 kWe

System Cycle Eff 21.51%

System Gas Inv. 1.5565 kg Gas Inv. 0.2588 kg

DP/P = 0.0250

NaK 393 K 5% Bleed 1150 K

1.38 kg/s 0.138 MPa 0.073 kg/s 0.963 MPa

1.45 kg/s 232 kWt

 513 K 2% He-Xe

1145 K 1.82 kg/s

82.405% 87.357% 40.0 g/mol

1.86 kg/s 72% He

400 K 924 K 28% Xe

0.500 MPa 0.515 MPa

CPR 2.00 Speed 45 krpm TPR 1.866

150 kW EM loss 3.3 kWt 213 kW

Brg Loss 4.252 kWt

175 kWt Wnd loss 4.513 kWt

DP/P = 0.0100 TIT/CIT 2.86

184 kWt

97.0% Cyc Eff 21.51%

Gas Inv. 0.6491 kg

93.6 m2 905 K

555 K 332 kWt 0.988 MPa

0.998 MPa

532 K 580 K 95.0%

Tsink 0.276 MPa 0.506 MPa 923 K

200 K

DP/Pcold = 0.0085

DP/Phot = 0.0150

1.39 kWe

DP 138 kPa

27.0 gpm 94%

16.9%

50.0 kWe

DP 138 kPa

1.39 kWe

16.9%

27.0 gpm

465 kWt

50.0 kWe

94%

Recuperator

TurboAlternator

Gas Cooler

Main Radiator

ALIP Pump

Recuperator

TurboAlternator

Gas Cooler

Heat Source

Main Radiator

ALIP Pump

Joiner

Com

Turb

p Com

Turb

p

Duct

Duct

Duct

Duct

Duct

Duct

Duct

Duct

Duct

Duct

Duct

Duct

Figure 3. Dual Brayton System Design Point. 

NASA/TM—2006-214393 4
Xe mass flow rate, gas inventory, and system performance are determined by CCSS. Only one design run was

required to initialize the system and determine the component design parameters, all subsequent runs were offdesign analysis based on the design point parameters. 

IV. Results and Discussion 

The following section presents analytical results of four scenarios. The first three scenarios investigate the

effects of (1) imbalance speed operation, (2) single-unit operation with idle unit standby, and (3) motoring power for 

single- and double-unit startups. The fourth scenario looks at a possible system startup sequence. The variables

controlled by the user during these off-design analyses are heat source exit temperature, shaft speed, and stagnant

gas temperature when one unit is standby. All other system temperatures, pressures, and inventory distribution

adjust accordingly. Radiator NaK mass flow rate remains constant and the potential for coolant freezing is not 

explored. Freezing can be avoided through flow rate and volume control to prevent NaK coolant temperatures

below 262 K. 

A. First Scenario: Imbalance Speed Operation 

The first off-design, steady-state scenario investigated the effect of varying the shaft speed of one Brayton

(Brayton1) while holding the other Brayton (Brayton2) shaft speed at 45 krpm. This scenario could represent a 

system startup situation where one Brayton is at design speed and the other is slowly increased to design speed. The

analysis is also applicable to the graceful shut-down of one Brayton that could occur during an unexpected speed 

control anomaly or during a potential part-power

operating mode. 

Figure 4 is a plot of output power for each 

Brayton as a function of the Brayton1’s shaft

speed. As shaft speed is reduced from 45 to 15

krpm, Brayton2 power decreases linearly from 50

to 47 kW and Brayton1 power drops to just 10

kW. The dramatic decrease in Brayton1 power is 

expected because it is operating at one third of its 

design speed, but the decrease in Brayton2 power 

must be explained by the common gas interaction.

Figure 5 shows how mass flow rate and gas 

inventory of each Brayton varies with Brayton1

shaft speed. As Brayton1 shaft speed decreases, 

Brayton2 mass flow rate drops from 1.8 to 1.6

kg/s despite a constant Brayton2 speed of 45

krpm. This occurs because the Brayton1

compressor produces a smaller pressure rise,

resulting in a higher cold end pressure in 

Brayton1 than in Brayton2. The higher gas 

density in Brayton1 deprives Brayton2 of gas and 

leaves it with only 40 percent of the shared

inventory.

This scenario is a good example of the

interdependent nature of a dual Brayton system. 

Cycle pressures and temperatures change with 

operating conditions and inventory distribution is

governed by the gas density in each component. 

This is the underlying theme for understanding

system behavior for the scenarios in the following 

sections. 

0

10

20

30

40

50

60

0 10000 20000 30000 40000 50000

Brayton1 Speed, rpm

Power, kW

B1

B2

Design Point

Figure 4. Brayton Output Power as a Function of

Brayton1 Shaft Speed. 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 10000 20000 30000 40000 50000

Brayton1 Speed, rpm

Mass Flow Rate, kg/s

Gas Inventory Fraction

B1

B2

Flow Rate

Inventory

Design Point

Figure 5. Brayton Gas Inventory and Mass Flow Rate as a

Function of Brayton1 Shaft Speed. 

B. Second Scenario: Single-Unit Operation with Standby Unit 

The second scenario investigated the effect of gas temperature of a standby Brayton (Brayton1) while the other

Brayton (Brayton2) is operated at design speed. A scenario with one Brayton running and the other standby might

occur during startup or if one Brayton has to be shut off. A large temperature range (100 to 1000 K) was chosen

NASA/TM—2006-214393 5
because the stagnant gas temperature could vary 

greatly depending on the situation. For this 

analysis the gas temperature in Brayton1 is 

assumed to be uniform throughout (i.e., after 

reaching steady-state equilibrium). 

Figure 6 is a plot of Brayton2 power as a 

function of Brayton1 stagnant gas temperature,

and Figure 7 is a plot of gas inventory division

between the two Braytons. For Brayton2 to be

able to produce 50 kW of power a Brayton1 gas 

temperature of 1000 K is required; power output

drops to just 18 kW when the gas temperature is 

100 K. At 1000 K inventory is divided equally

between the two Braytons, but at 100 K, 90

percent of the inventory resides in Brayton1. A

temperature of 100 K might be excessively low,

but Figures 6 and 7 illustrate that the idle gas 

temperature has a significant effect on Brayton2’s 

performance; the colder the idle gas gets the more 

the gas inventory will want to reside in the 

standby Brayton.

It might be feasible to manage the stagnant

gas temperature to alleviate the degradation on 

performance. For this system, 1000 K would

allow Brayton2 to produce its design power of 50

kW. External heaters could be used to raise the 

stagnant gas to this temperature, but heater power 

would have to be considered. A second option is

to suppose that the check valves (shown in Figure 

1) were not in place and the working fluid is free

to flow backwards through Brayton1. A portion of

the hot gas from the Brayton2 recuperator high pressure exit (around 900 K in Figure 3) would circulate through

Brayton1, keeping its inventory relatively warm. However, Brayton2 TIT would be lower than the 1150 K design

point because mixing of the two gas streams would occur at the heat source exit. This option is feasible as long as

the standby shaft is locked in place, either electrically or mechanically, to avoid damaging the bearings. A third 

option is to have check valves in place and periodically change Brayton2 shaft speed. This would briefly alter

system pressure, which would circulate the gas in Brayton1 and cause some hot gas from Brayton2 to mix with the 

cold stagnant gas in Brayton1. 

All three options are simply possibilities at this point. It may be the case that the stagnant gas cools at a very

slow rate and the system experiences enough natural circulation to maintain an acceptable standby Brayton gas

temperature. Further analysis and system testing are required to determine the possible issues associated with this

operating scenario. 

0

10

20

30

40

50

60

0 200 400 600 800 1000 1200

Standby Brayton Gas Temperature, K

Power, kW

B1

B2

Figure 6. Brayton2 Output Power as a Function of

Brayton1 Gas Temperature. 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 200 400 600 800 1000 1200

Standby Brayton Gas Temperature, K

Gas Inventory Fraction

B1

B2

Figure 7. Brayton gas inventory as a function of Brayton1 

gas temperature. 

C. Third Scenario: Estimate Motoring Power for System Startup 

The power conversion system will have to undergo at least one startup during its mission. This scenario

investigates the estimated power required to motor the Brayton units during startup. Motoring power is an important

issue because a power source (batteries, solar arrays, etc.) is required to start the system before the system can

become power-producing. An obvious question when considering startup scenarios is whether the Braytons should 

be started one at a time or both simultaneously. 

For the startup of one Brayton (Brayton2) while the other is standby (Brayton1), a uniform system initial 

temperature is assumed. Upon startup, Brayon2 is motored at 20 krpm‡

 while Brayton1 stagnant gas remains at the

assumed system initial temperature. Reactor thermal power is increased until the heat source exit temperature

reaches 1150 K. Figure 8 is a plot of power as a function of heat source exit temperature; values are positive when

 ‡ The speed of 20 kRPM was selected to allow a large margin for gas-foil bearing liftoff. No rotor-dynamic issues

were taken into account. Break-away torque and acceleration from 0 to 20 krpm were not considered. 

NASA/TM—2006-214393 6
the system is power-producing and negative

when motoring is required. Results are given 

for system initial temperatures of 200 and 400 

K. The power trends are nearly linear and the

system power breakeven point occurs at a heat

source exit temperature of 600 K for the 400 K 

stagnant gas temperature case and 560 K for the 

200 K case. CCSS had difficulty generating

data below the breakeven point, requiring

extrapolation of the positive power data points. 

Extrapolating (dashed line in Figure 8) the 200

K stagnant gas temperature case down to a heat 

source exit temperature of 200 K shows that

when one of the Braytons is operated at 20 

krpm, the maximum required motoring power is

approximately 6 kW. When the 400 K case is

extrapolated down to a heat source exit 

temperature of 400 K, the maximum motoring

power is approximately 4 kW. 

For the simultaneous startup of both 

Braytons, both shaft speeds were held at 20 

krpm while the heat source exit temperature 

was increased from 200 K to 1150 K. Figure 9

is a plot of power as a function of heat source 

exit temperature. Brayton1 and Brayton2 power

are identical, which is why the plot appears to

have only one curve. The breakeven point

occurs around 600 K, as it did in Figure 8, but

extrapolation suggests that 8 kW is required to

motor each Brayton at 20 krpm for a system at

200 K. Thus, the total power required to motor 

both Braytons is approximately 16 kW. For a system startup at 400 K, approximately 5 kW of power is required to

motor each Brayton, for a total of 10 kW. 

Motoring power for the simultaneous startup was more than twice that of the single Brayton startup. One

explanation for this is because there is more gas in the standby Brayton than in the motored Brayton, less power is 

required to pump the lower density fluid. When the units are motored simultaneously, the Braytons contain equal

amounts of gas. If the Braytons were not sharing a common gas inventory one would expect the single Brayton 

startup power to be exactly half of the simultaneous startup. Therefore, it appears as though motoring a single 

Brayton in a common gas system requires less power than a single Brayton in an independent gas loop

configuration. 

This scenario looked at a specific case of 20 krpm and cold system temperatures of 200 and 400 K. Results can

be expected to vary with different speeds and cold system temperatures, but the motoring power for a single Brayton

startup should be no more than half the total power required for a simultaneous startup. The colder system required

more motoring power. Therefore, the use of strip heaters to elevate the stagnant gas temperature before startup could 

potentially reduce the startup power demands. 

-8

-6

-4

-2

0

2

4

6

8

10

12

0 200 400 600 800 1000 1200

Heat Source Exit Temperature, K

Power, kW

B1

B2 (B1 at 400 K)

Extrap B2

B2 (B1 at 200 K)

Extrap B2 B1 at 200 K

B1 at 400 K

Startup

Figure 8. Power as a Function of Heat Source Exit 

Temperature for One Brayton at 20 krpm. 

-10

-5

0

5

10

15

20

0 200 400 600 800 1000 1200

Heat Source Exit Temperature, K

Power, kW

B1

B2

Extrap B1

Extrap B2 B1 & B2

Startup at 400 K

Startup at 200 K

Figure 9. Power as a Function of Heat Source Exit 

Temperature for Both Braytons at 20 krpm. 

D. Fourth Scenario: Possible System Startup Sequence 

This scenario looks at a possible system startup sequence. Several steady-state operating points were combined 

to create this sequence and time scales were not explored because system transients were not modeled. Results are

plotted against non-linear time, which simply means that the pace of operations is yet to be determined. According 

to McCann, a typical staggered startup profile would take approximately 7 hr.

7

 

The previous scenario revealed that motoring one Brayton requires less than half the power of motoring both 

Braytons simultaneously. With that in mind, the sequence outlined here brings one Brayton (Brayton2) up to design 

speed before starting the second Brayton (Brayton1). Figure 10 depicts the system startup sequence in five

increments. Prior to startup, the entire system gas inventory is assumed to be at 200 K. The sequence starts

(indicated by label A in Figure 10) when Brayton2 is motored at 20 krpm while Brayton1 remains off at 200 K.

NASA/TM—2006-214393 7
Brayton2 is held at 20 krpm to circulate the 

working fluid while the heat source temperature is 

increased to 1150 K (label B). Motoring is only

required until the breakeven point is reached. 

After the system becomes power-producing, 

motoring is no longer required and the shaft speed

is actively controlled to maintain a set speed. 

Figure 11 is a plot of Brayton output power

according to the sequence in Figure 10. At 1150 

K and 20 krpm Brayton2 produces 8 kW of

power. At this point Brayton1 could be motored 

using the power produced by Brayton2, or

Brayton2 speed could be increased to its design 

point of 45 krpm. Although 8 kW at 20 krpm is

likely sufficient power, at 45 krpm Brayton2 

would supply 28 kW, perhaps a safer margin of

available power. However, one reason it might be

desirable to start Brayton1 before increasing 

Brayton2 speed is to diminish the thermal shock 

seen by the heat source from the sudden increase 

in mass flow rate. This particular sequence 

increases Brayton2 speed to 45 krpm before

starting Brayton1. 

Once a heat source exit temperature of 1150 K

is reached, Brayton2 speed is increased to 45 

krpm at a rate slow enough for the heat source to 

maintain a steady 1150 K (label C). Brayton1 is 

then motored and will become power-producing 

almost immediately because a high temperature is

already established in the heat source (label D). 

Figure 11 shows a jump in Brayton2 power output

once Brayton1 begins to generate power. As noted earlier, the sudden increase in mass flow rate upon Brayton1

startup will likely cause a brief temperature drop in the heat source. Finally, Brayton1 speed is slowly increased until 

its design speed of 45 krpm is reached (label E). 

Many factors need to be taken into consideration for a system startup procedure. This study presents one possible

scenario and only considers a few main factors. A transient model with appropriate system controls is necessary to

further define the startup sequence. 

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

Time, non-linear

Shaft Speed, rpm

0

150

300

450

600

750

900

1050

1200

1350

1500

Heat Source Exit Temp, K

B1 RPM

B2 RPM

Exit Temp

A

B C

D

E

Figure 10. Shaft Speeds and Heat Source Exit 

Temperature for System Startup Sequence. 

-10

0

10

20

30

40

50

60

Time, non-linear

Power, kW

B1

B2

Motor B2

Increase 

TIT to

1150 K

Increase 

B2 RPM

Increase

B1 RPM

A Start B1

B

C

D

E

Figure 11. Brayton Input/Output Power During System

Startup Sequence. 

V. Conclusion 

The CBC-based reactor power system is a leading candidate for various nuclear electric propulsion (NEP) and 

surface power applications. Systems that utilize multiple Brayton converters with a common gas inventory may be

pursued in order to provide power conversion redundancy, especially if gas-cooled reactors are considered. This

study explored the performance and operational characteristics of a dual Brayton system with shared gas inventory.

The study revealed several important system attributes associated with startup, imbalanced speed operation, and 

single-unit operation with idle standby. The startup power requirements can be minimized by staggering the startup

of the two units and utilizing the first unit to start the second. Reducing the operating speed of one of the units 

causes a major reduction in system output power and results in a deleterious redistribution of working fluid. The 

operation of a single unit with an idle standby also results in a power reduction, which is exacerbated by the cooldown of the idle unit’s gas inventory. 

NASA/TM—2006-214393 8
References 1

Mason, L.S., “A Power Conversion Concept for the Jupiter Icy Moons Orbiter,” AIAA Journal of Propulsion and Power, 

Vol. 20, No. 5, 2004, pp. 902–910; also NASA/TM—2003-212596, September 2003. 2

Elliot, J.O., Reh, K., and MacPherson, D., “Lunar Fission Surface Power System Design and Implementation Concept,”

Proceedings of the Space Technology and Applications International Forum, edited by M. El-Genk, American Institute of 

Physics, Vol. 813, New York, 2006, pp. 942–952. 3

Mason, L.S., “A Comparison of Fission Power System Options for Lunar and Mars Surface Applications,” Proceedings of

the Space Technology and Applications International Forum, edited by M. El-Genk, American Institute of Physics, Vol. 813, 

New York, 2006, pp. 270–280; also NASA/TM—2006-214120, February, 2006. 4

Ashcroft, J., and Eshelman, C., “Summary of NR Program Prometheus Efforts,” Proceedings of the Space Technology and

Applications International Forum, edited by M. El-Genk, American Institute of Physics, New York, 2007 (to be published). 5

Ashcroft, J., Belanger, S., Burge, W., Clementoni, E., Jensen, K., Proctor, N. B., and Zemo-Fulkerson, A., “Key Factors 

Influencing the Decision on the Number of Brayton Units for the Prometheus Space Reactor,” Proceedings of the Space 

Technology and Applications International Forum, edited by M. El-Genk, American Institute of Physics, New York, 2007 (to be

published). 6

Johnson, P.K., and Mason, L.S. “Design and Off-Design Performance of 100 kWe-Class Brayton Power Conversion

Systems,” Proceedings of the Space Technology and Applications International Forum, edited by M. El-Genk, American 

Institute of Physics, Vol. 746, New York, 2005, pp. 711–718; also NASA/TM—2005-213626, June 2005. 7

McCann, L.D., “Use of RELAP5–3D for Dynamic Analysis of a Closed-Loop Brayton Cycle Coupled to a Nuclear

Reactor,” Proceedings of the Space Technology and Applications International Forum, edited by M. El-Genk, American Institute

of Physics, New York, 2007 (to be published). 8

Lavelle, T.M., Khandelwal, S.C., and Owen, A.K., “Intermediate Fidelity Closed Brayton Cycle Power Conversion Model,”

NASA/TM—2006-213993. 9

Barrett, M.J., and Johnson, P.K., “Performance and Mass Modeling Subtleties in Closed-Brayton-Cycle Space Power

Systems,” Proceedings of the 3rd International Energy Conversion Engineering Conference, Vol. 3, AIAA, Washington, DC, 

2005, pp. 1596–1607; also NASA/TM—2005-213985, November 2005. 10Barrett, M.J., and Reid, B.M., “System Mass Variation and Entropy Generation in 100-kWe Closed-Brayton-Cycle Space 

Power Systems,” Proceedings of the Space Technology and Applications International Forum, edited by M. El-Genk, American 

Institute of Physics, Vol. 699, New York, 2004, pp. 445–452; also NASA/TM—2004-212741, January, 2004. 11Johnson, P.K., and Hervol, D.S., “Experimental Validation of a Closed Brayton Cycle System Transient Simulation,”

Proceedings of the Space Technology and Applications International Forum, edited by M. El-Genk, American Institute of 

Physics, Vol. 813, New York, 2006, pp. 673–681; also NASA/CR—2006-214239, March 2006. 12Kays, W.M., and London, A.L., Compact Heat Exchangers, 2nd ed., McGraw-Hill, New York, NY, 1964. 13Solar Dynamic Power Systems Branch, Solar Dynamic Power System for Space Station Freedom, NASA Reference 

Publication 1310, NASA Lewis Research Center, Cleveland, OH, 1993. 

NASA/TM—2006-214393 9
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AIAA–2006–4167

E–15671

WBS 463169.04.03

15

Performance and Operational Characteristics for a Dual Brayton Space Power

System With Common Gas Inventory

Paul K. Johnson and Lee S. Mason

Brayton cycle; Closed cycle; Simulation

Unclassified -Unlimited

Subject Category: 20

This paper provides an analytical evaluation on the operation and performance of a dual Brayton common gas system. The

NASA Glenn Research Center in-house computer program Closed Cycle System Simulation (CCSS) was used to construct

a model of two identical 50 kWe-class recuperated closed-Brayton-cycle (CBC) power conversion units that share a

common gas inventory and single heat source. As operating conditions for each CBC change, the total gas inventory is

redistributed between the two units and overall system performance is affected. Several steady-state off-design operating

points were analyzed by varying turbine inlet temperature and turbo-alternator shaft rotational speed to investigate the

interaction of the two units.

Prepared for the Fourth International Energy Conversion Engineering Conference and Exhibit (IECEC) sponsored by the

American Institute of Aeronautics and Astronautics, San Diego, California, June 26–29, 2006. Paul K. Johnson, Analex

Corporation, 1100 Apollo Drive, Brook Park, Ohio 44142; Lee S. Mason, NASA Glenn Research Center. Responsible

person, Paul K. Johnson, organization code RPT, 216–433–3814.