NASA / TM--2002-211722 IE CE C-2002-20023

The Evaluation and Implementation

of a Water Containment System to

Support Aerospace Flywheel Testing

Larry M. _I_:ase

Glenn Research Ce:n-ter, CleveI.and, Ohio

July 2002
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NASA / TM--2002-21_ 1.722 IE CE C-2002-20023

The Evaluation and Implementation

of a Water Containment System to

Support Aerospace Flywheel Testing

Larry M. _Ii:ase

Glenn Research Ce:nter, Cleveland, Ohio

Prepared for the

37fl_ In.tersociety Energy Corwersion Engin.eerir G Corfference

sponsored by the Institute of Electrical and Electronics Engineers,

Electron Devices Society

Washington, DC, July 28-A ugust 2, 2002

National Aeronautics and

Spa ce Adminis tration

Glelm Research Center

July 2002
NASA Center for Aerospace Information

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Hanover, MD 211076

Available frorn

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Available electronical] y at http://gltrs.grc.nasa.gov/GL,TRS
The Evaluation and Implementation of a Water Containment

System to Support Aerospace Flywheel Testing

Larry M. Trase

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Ph.: 216-433-5347

Fax: 216-433-8410

E-mail: la__sa, c.v

ABSTRACT

High-energy flywheel systems for aerospace

power storage and attitude control applications are

being developed because of the potential for

increasing the energy density and reducing

operational costs. A significant challenge facing the

development of the test hardware is containment of

the rotating elements in the event of a failure during

the development and qualification stages of testing.

This containment is critical in order to ensure the

safety of the test personnel and the facility. A

containment system utilizing water as the containment

media is presented. Water containment was found to

be a low cost, flexible, and highly effective

containment system. Ballistic test results and

analytical results are discussed along with a

description of a flywheel test facility that was designed

and built utilizing the water containment system at the

NASA Glenn Research Center at Lewis Field in

Cleveland, Ohio.

INTRODUCTION

The High Energy Flywheel Facility (HEFF)

located in the Power Systems Facility (PSF) at the

NASA Glenn Research Center in Cleveland, Ohio will

be used to demonstrate a single axis attitude control

and energy storage system utilizing flywheels. The

facility will also be used to test prototype electronics

and control algorithms for proposed flywheel energy

storage systems for the International Space Station

(ISS) and other satellite systems.

The hardware configuration for the experiment

will consist of two flywheel modules with parallel spin

axis vertically mounted on an air-bearing table. The

electronics and controls to support the magnetic

bearing and motor/generator systems and provide

overall system control are a combination of

commercial of the shelf and brass board hardware

with rapid prototype software.

Each flywheel module stores 350 W-hrs of

electrical energy. The rotor is suspended with active

magnetic bearings and has a rolling element

touchdown bearing for off nominal conditions. The

flywheel module housing provides the vacuum

enclosure and mounting locations for the stator

components.

The flywheel testing has three objectives:

demonstrate two flywheel modules operating on a

lightly constrained mounting system (air table),

demonstrate a momentum and energy control in

charge mode with the flywheels on separate power

buses, and finally demonstrate full momentum and

energy control in charge and discharge modes on a

single power bus. (Kascak, 2002)

The operational demonstration of a magnetically

suspended flywheel module functioning on an air

table will be used to verify an analytical modeling

effort which is exploring the effect of mount stiffness

on magnetic bearing stability.

In the second phase of the testing the combined

charge rate of and net torque of the two flywheel

modules will be controlled. This demonstration will be

limited to charge mode only, since the flywheels are

operated on separate power buses. Step response,

overshoot and regulation band will be compared to

analytical models.

Both flywheel modules will be run on one power

bus in the final phase of the testing. Charge mode

will operate in the same manner as in the second

phase. In discharge the controller will regulate the

bus voltage and the net torque of the two-wheel

system.

All of this testing will be conducted using first

generation flywheel modules. The flywheel modules

do not have any containment, the housing only

provides a vacuum environment for the rotating

NASA/TM--2002-211722 1
elements. To insure the safety of personnel and

facilities these flywheels will require containment in

the event of a rotor failure at high speed. This paper

summarizes the work that has been completed to

provide effective containment for flywheel systems.

CONTAINMENT

The primary hazard in testing flywheels is a rotor

burst at high speed. The rotor is constructed with

rings of carbon fiber filaments bound with epoxy. The

rotors are wound in layers. These layers form rings

and provide a method of reducing the energy release

in the event of a rotor failure. In an overstressed

condition, the outer ring will separate first. If a failure

would occur, only the outer ring would fail and the

energy contained in the outer ring, while significant, is

not nearly the energy available in the entire rotor. If

an overspeed event were to occur, just the outer ring

would potentially fail and stop the test.

The flywheel rotor is suspended via active

magnetic bearings. The feedback sensors constantly

monitor the position of the rotating shaft. Currently,

the test operator monitors the shaft position for any

long-term deviations or trends. In the future this

capability will be added to the magnetic bearing

controls to enable the rotor performance and status to

be monitored automatically.

The traditional method of containment for

rotating hardware is several inches of steel. Spin pits

are large, extremely heavy steel chambers typically

with several rings of steel surrounding an inner burst

liner. This inner burst liner is sometimes lined with

lead bricks to preserve fragments. Also the burst liner

has a lip around the top to redirect any fragments that

are directed vertically up after a failure. This failure

mode is of particular concern for the composite

flywheels. The composite can be redirected by the

burst liner and introduce significant vertical forces.

The challenges of containing the total energy in

one flywheel are considerable, however when faced

with the requirement to test multiple flywheels as a

system, the use of traditional containment becomes

impractical. This fact lead to the investigation of other

containment methods and water was selected as an

effective alternative.

Water containment for explosives safety has

been investigated by the military and has been found

to be an effective means of suppressing ordnance

detonations and reducing the stay clear zones for the

storage of explosives. (Malver, 1998)

The effectiveness of water for explosive

suppression has been demonstrated by military

testing. A number of factors have been presented

regarding the effectiveness of water for suppression.

One factor is the drag forces are much higher than

that of air since the density of water is 800 times that

of air. (Salter, 1994; Karplus,1964)

A second factor may be the bulk modulus of

water being so much above that of air that just after

an event the water pressure on the outside of a

fragment can rise very quickly leaving a smaller

pressure gradient across it. In addition due to the

latent heat of water, a considerable amount of energy

is absorbed as the water is turned into steam at the

time of impact. (Salter, 1994; Karplus, 1964)

ANALYSIS

The flywheel consists of a titanium inner hub

wrapped by several separate layers, or rings. Each

ring consists of a composite made up of carbon fibers

wound in an epoxy matrix. The outer ring is the most

highly stressed, and in the event of a failure is

designed to separate from the next ring. For this

analysis, the kinetic energy contained in the outer ring

is given by:

Flywheel operational speed:

or 6283 rad/sec

= 60,000 rpm

Composite material density:

(0.0595 Ib/in 3)

p = 1647 kg/m 3,

Composite outer ring dimensions:

0.2358m, (9.283") OD, radius (ro) = 0.1179m

0.2256m, (8.883") ID, radius (ri) = 0.1077m

Height (h) = 0.133m, (5.25")

Polar moment of inertia:

I = Y2/_ h p ((ro)4 - (ri)4))

I = _/2(3.1416) (0.133) (1647) (0.1179) 4 - (0.1077) 4)

I = 0.0202m 4

Kinetic energy of outer ring:

KE = _/2I Q2

KE = _/2(0.0202) (6283) 2

KE = 3.99 x 105 J

For comparison purposes, 0.454 kg (1 Ib) of

TNT generates 2.1 x 106 J of energy, i.e., the outer

ring contains approximately the same energy as 0.1

kg (0.2 Ib) of TNT, with a maximum fragment velocity

due to rotation of 740 m/sec (2430 ft/sec).

An analysis of the motion of a fragment in water

was completed determining that the energy vs.

distance curve for a fragment of size L can be

represented by

E = Eo_-d/L (Sandier, 1998)

Where E0 = the initial kinetic energy of the fragment,

E is the remaining energy, and d is the distance

traveled in water.

Plots of this equation with several fragment

lengths are shown in Figure 1.

NASA/TM--2002-211722 2
"s

0 2 4 6 8 10 12 14 16 18 20 22 24

Distance (in)

FIGURE 1 : ENERGY ABSORPTION IN WATER

By graphing the particle energy vs. distance

traveled in water it may be noted that particles that

can reasonably be expected to be generated by an

outer ring failure lose at least 95% of their energy by

traveling through 0.3 m (12 in) of water, and virtually

all their energy in less than 0.6 m (24 in) of water.

TESTING

Limited fragment impact testing was performed

in the Ballistics Impact Lab located at the NASA

Glenn Research Center. Several fragments were shot

into pools of water and filmed with high-speed

photography. The fragment size was approximately

2.5 x 2.5 x 1.0 cm (1.0 x 1.0 x 0.325 in) and fired with

a velocity of approximately 730 m/s (2400 fps). Two

fragments were fired into water and the fragments

were stopped within 30.5 cm (12 in).

Several methods of holding the water were

considered and after an examination of commercially

available water containers, a water bladder within a

strong cardboard carton was selected. See Figure 3.

The water bladder size is approximately .28 x .28 x

.28 m (11 x 11 x 11 in) and the overall size is a cube

.29 m (11.5 in) on each side. Each container holds 5

gallons of water.

Using these containers allows a containment of

any suitable size to be easily constructed using the

containers as "building blocks". Two rows of

containers will be utilized, providing a minimum of

0.56 m (22 in) of water in the path of any fragments.

The containers in the outer wall of the containment

will be horizontally and vertically offset from the

containers in the inner wall by one half the width of

the containers. A cross section of the stacking is

shown in Figure 2.

A series of tests was conducted to investigate

the ability of the water-filled containers to absorb the

energy of high-speed projectiles. For the first three

tests, the setup was the same. Two containers were

filled with water and placed together. A series of

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FIGURE 2: CONTAINER STACKING PROFILE

witness plates was placed behind the containers as in

figure 3. Each witness plate was a sheet of 3/16" luan

plywood, and the sheets were placed approximately 1

inch apart. Any particles that were not stopped by the

water would impact these witness plates, and a rough

evaluation of their remaining energy could be made.

FIGURE 3: CONTAINER TEST CONFIGURATION

TEST #1

A 30-caliber rifle was used to fire a 150-grain

(9.7 g) bullet at approximately 2600 ft/sec (790 m/sec)

into the first container. Bullets consist of very dense

materials, and are designed to penetrate, being

streamlined and spin-stabilized, and thus represent a

rather severe test of the absorption capability of any

medium.

For this test, a blunt bullet was fired, to more

closely approximate an irregularly shaped fragment

that would be generated by a flywheel failure. The

bullet used a lead core surrounded by a copper

jacket.

NASA/TM--2002-211722 3
In this test the bullet was completely stopped by

the first container, i.e., in less than .28 m (11 in) of

water. The bullet was intact, but deformed and

expanded to approximately twice its original diameter.

The container was completely split open and the

water was widely dispersed.

TESTS #2 and #3

A 12-gauge shotgun was used to fire a magnum

00 buckshot cartridge into the first container. This

cartridge was comprised of 15 lead balls, each

weighing 3.2 g (50 grains), for a total projectile weight

of 48 g (750 grains). Velocity was approximately 430

m/sec (1400 ft/sec). Shot patterns tend to 'string out'

into a fairly long patterns, thus many projectiles will

travel in the wake of the leading projectiles as they

impact the water. The first projectiles that hit will begin

to disperse the water, and later projectiles will

encounter less resistance as they will be traveling in a

liquid/vapor mix, rather than the all-liquid state the

leading projectiles will encounter.

As a result, most projectiles (13 in test #2, 14 in

test #3) completely penetrated the first container, and

entered the second. The container was simply

pierced, and was not split open like the much higher

velocity rifle bullet in test #1.

In test #2, 8 of 13 projectiles reached the back of

the second container and made marks on the first

witness plate, which had become pressed against the

container by the dynamics of the impact. Several of

these projectiles were actually stopped by the rear

cardboard wall of the second container as it pressed

on the witness plate, and thus possessed essentially

zero energy. A single projectile penetrated the first

witness plate, and was stopped by the second

witness plate barely leaving a mark.

In test #3, 10 of 14 projectiles reached the rear

wall of the second container. Two penetrated the first

witness plate. None damaged the second witness

plate.

TEST #4

For this test, actual samples of carbon fiber

matrix from a scrap flywheel were fired from GRC's

Ballistic Impact Facility into an array of containers.

Two samples approximately 12 x 19 x 25 mm (W' x

3⁄4" x 1") weighing 12 g and one sample 12 x 19 x

50mm (W' x 3⁄4" x 2") weighing 25 g were placed such

that their long axis was aligned with the direction of

firing. Velocity obtained was 700 m/sec (2300 ft/sec).

One container was placed forward for the samples to

impact, and 4 others were placed behind it, offset

horizontally and vertically by one half the width of a

container.

Recovered samples indicate that the carbon

fiber matrix broke up into small fragments upon

impact with the water. The largest recovered piece

was approximately 3 g while most were much smaller.

Total mass of the recovered fragments that were

significant in size was 6.7 g. The remainder was

comprised of small pieces of fiber strewn throughout

the first container and surrounding area.

The forward container had been rotated

clockwise during the test. This was a consequence of

not being firmly tied in place, and the large air blast

that results from the operation of this facility. This

allowed some particles to exit the left side of the

container (which was now facing backwards) after

traveling only a short distance through the water,

perhaps 150mm (6 in). The large recovered fragment

penetrated one rear container. Most of the smaller

fragments stuck in the cardboard of the rear

containers and did not penetrate into the water.

FACILITY DESCRIPTION

The requirements for HEFF were to have the

capability to test two flywheel modules

simultaneously. The room in which the HEFF is

located is constructed of 12" thick, reinforced concrete

walls on 5 sides. The outside wall is designed as a

blow out wall in the event of an overpressure in the

room. This room is being used as secondary

containment for the test. The primary containment

media being used is water. Approximately 250

cardboard boxes containing plastic bladders that hold

approximately 1 ft 3 of water each will surround the airbearing platform. Figure 4 shows a plan view showing

the containers placement around the test articles.

FIGURE 4: TEST FACILITY PLAN VIEW

As a redundant layer of protection, the entire

containment assembly will be covered on 5 faces (the

4 sides and the top) by a commercial bomb blanket

designed to suppress both blast and fragments.

CONCLUSIONS

This paper has described a method to provide

containment for the testing of high-speed energy

storing flywheels. This method utilizes inexpensive

cardboard water containers filled with water and

stacked around the flywheels to be tested. The

method has been shown to be flexible yet effective.

Analysis has shown that the majority of fragment

energy will be absorbed by the initial 0.3 m (12 in) of

water. Performing testing that included firing

fragments and bullets into containers of water has

supported the analysis. Covering the entire

NASA/TM--2002-211722 4
configuration with a fragment blanket and locating the

facility inside a blast room provides secondary and

tertiary levels of containment.

REFERENCES

Kascak, P.E., and Jansen, R.H., "Single Axis Attitude

Control and DC Bus Regulation With Two Flywheels,"

Paper #20078,To be Published IECEC, Washington,

DC, July 2002.

Malver, L.J., and Tancreto, J.E., 28 th DDESB

Seminar, Orlando, FL, August 1998.

Salter, S.H., and Parkes, J.H., "The Use of Water

Filled Bags to Reduce the Effect of Explosions," DoD

Explosives Safety Seminar, Miami, August 1994.

Karplus, H.B., and Clinch, J.M., "Sound Propagation

in Two-phase Fluids," Journal of the Acoustical

Society of America, Vol. 36, p. 1040, 1964.

Sandier, I.S., "Analysis of the Effects of Flywheel

Breakup for Determining Design Specifications for a

Water-Filled Test Facility," Final Report for NASA

Lewis Research Center, Contract C-75128-1,

December 1998.

NASA/TM--2002-211722 5
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The Evaluation and Implementation of a Water Containment System to

Support Aerospace Flywheel Testing

6. AUTHOR(S)

Larry M. Trase

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John It. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135 - 3191

WU-755-1A-09-00

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National Aeronautics and Space Administration

Washington, DC 20546-0(101 NASA TM------.2002-211722

IECEC.---2002----20023

11. SUPPLEMENTARY NOTES

Prepared for the 37th Intersociety Energy Conversion Engineering Conference sponsored by the Institute of Electrical

and Electronics Engineers, Electron Devices Society, Washington, DC, July 28-August 2, 2002. Responsible person,

Larry M_ Trase, organization code 7550, 216-433-5347.

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Subject Category: 44 Distribution: Nonstandard

Available electronically at Ntp://gltrs.grc.n;_sa.gov/GIiI'RS

"l-his publication is available from the NASA Center for AeroSpace Im%rmation, 301-621-0390.

12b. DISTRNBUTION CODE

13. ABSTRACT (Maximum 200 words)

High-energy flywheel systems for aerospace power storage and attitude control applications are being developed because

of the potenti_ for increasing the energy density and reducing operational costs. A significant challenge facing the

development of the test hardware is containment of the rotating elements in the event of a failure during the development

and qualification stages of testing. This containment is critical in order to ensure the safety of the test personnel and the

facility. A containmem system utilizing water as the containment media is presented. Water containment was timnd to be

a low cost, flexible, mad highly effective containment system. Ballistic test results and mmlytical results are discussed

along with a description of a flywheel test facility that was designed and built utilizing the water containment system at

the NASA Glenn Research Center at Lewis Field in Cleveland, Ohio.

14. SUBJECT TERMS

Test fucilities; Safety; Flywheels; Containment

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