Space Habitat Deployment Opt

8/8/2019 Space Habitat Deployment Opt

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American Institute of Aeronautics and Astronautics

1

Deployment of Inflatable Space Habitat Models

Jeremy Hill1 and Jamey Jacob2

Mechanical and Aerospace EngineeringOklahoma State University, Stillwater OK 74078

The experiments described in this paper were performed with the goal of obtaining both

qualitative and quantitative pressure and motion data for an inflatable habitat model in both

a 1-g and microgravity environments. The experiments investigated the impact of fold

method and inflation speed on the deployment dynamics of the space habitat model. The

first part of the experiment was conducted in a microgravity environment on NASAs

modified Boeing 727 aircraft in June of 2009. The second part of the experiment was a

series of ground tests conducted in the fall of 2009 aimed at developing a more

comprehensive idea of the relationship between model size and inflation time, as well as, the

model accelerations during deployment. The runs taken under microgravity conditions

were significantly less dynamic. In addition, the roll and z-fold methods were shown to

affect the deployment behavior at certain portions of the ground deployments.

Nomenclature

P = Pressure

V = Volume

n = moles

R = Universal Gas Constant

T = Temperature

LaRC = Langley Research Center

X-Hab = Expandable Lunar Habitat

I. IntroductionAUNCH of space habitats suffer from limitations of volume constraints of current launch vehicles. The habitats

must fit in a very small space, limiting the design volume of the habitat and its usefulness. One way to overcome

this limitation is to use inflatable systems that pack in the payload and deploy in space for habitation. The primary

benefit of deployable inflatable habitats is that they enable very large volumes to be packed in small volumes for

launch. Reduced volume results in lower costs and increased options for launch vehicles. It also reduces costs since

very little, if any, on-site construction is required. A deployable inflatable space habitat is ready for use immediately

after deployment. This paper investigates the deployment of inflatable space habitat models under microgravity and

1-g conditions. Data includes deployment shape, inflation pressure and reaction forces. The resulting data will be

used to develop and verify models to design space habitats and their stowage and launch parameters.

A. BackgroundIn March of 2001, students from the University of Kentucky participated in a similar reduced gravity flight

experience on board a KC-135. The students tested the deployment characteristics of a solar concentrator model

composed of several inflatable tubes. The results showed overall consistent inflation characteristics over multiple

deployments; while showing constant pressure with increasing volume and constant volume with increasing pressure

once all of the stages were released.1 In 2002, a paper was presented by Cadogan and Grahne that described several

types of inflatable structures, as well as their potential applications in space technology. 2 Results were presented

with respect to rigidization methods and volume efficiencies: both of which result in direct reductions in launch

1 Undergraduate Research Assistant, Department of Mechanical and Aerospace Engineering, Student Member

AIAA.2 Associate Professor, Department of Mechanical and Aerospace Engineering, Associate Fellow AIAA.

L


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payload volume and weight. In 2001 AIAA paper by Clem, Smith, and Main, the structural behavior of their

inflatable model was found to a function of the inflation rate of the inflatable structure. In addition, the slower

inflation rates produced more favorable deployment behavior.3 Another observation is that the external pressure has

a substantial affect on the pressurization inside the model.4

Most recently, ILC Dover (ILC), in conjunction with NASA, has been studying inflatable, deployable structures

to expand the architectural options available for exploration in lunar and Antarctic environments. The overall

objective of this combined effort was to design, construct, and test a proof-of-concept inflatable structure, focusing

on how easy it is to deploy and how durable it is in an extremely harsh environment, i.e. Antarctica. 5 In addition,

under contract with NASA Langley Research Center (LaRC), ILC has also worked to design and fabricate an

expandable lunar habitat (X-Hab). X-Hab is a deployable cylindrical habitat designed to demonstrate packing and

deployment of an inflatable habitat under expected loading scenarios.6

B. HypothesisDuring the experiment, it was expected to see significant differences in the deployment behavior of the inflatable

space habitat model under microgravity conditions from the behavior observed in Earth gravity. The impact of fold

method and inflation speed was also expected to be significant.

C. Test ObjectivesThe objectives were to observe the deployment of an inflatable space habitat model and measure the effect of

microgravity on deployment behavior. The group investigated the impact of (i) fold method and (ii) inflation speed

on the deployment dynamics of the space habitat model. With respect to the folding methods investigated, the rolland z-fold methods shown in Figure 1 were considered. Basic models for the deployment behavior of simple

inflatable beams have already been developed and these models will be adapted based on the observations. The

results of these tests will be used to develop a high fidelity model for inflatable space habitat deployment for use in

future development of deployable space habitats.

Figure 1: Roll Fold (Left) and Z-Fold (Right)

II. Technical ApproachA. Experiment Overview

Inflatable space habitats provide an attractive solution to the problem of launching large volume habitable

structures into space using current launch systems. The ability to pack the habitats into a small volume is their

primary advantage. The deployment of an inflatable space module is complex, however, and models of the

deployment are necessary to determine deployment system and stabilization design parameters. This experiment

measures several quantities important in developing these models, including time dependent inflation pressure,

model accelerations/forces during deployment, and the model shape during unfolding. The models are packed in

different manners, notably either rolled or z-folded. The packing volumes are different for each method. Also, the

addition of an airlock on the far side will provide a large point mass that will greatly vary the deployment behavior.

These airlocks will likely be implemented in most designs to provide access to other inflatable and non-inflatable

modules. Both the effect of point masses and distributed masses only (no airlock) were examined on the deployment

dynamics of the models, as well as the packing method.

The first part of the experiment was conducted in a microgravity environment on NASAs modified Boeing 727

aircraft in June of 2009. Figure 2 shows students performing the experiment in the microgravity environment. The


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primary objective of the microgravity experiment was to provide a comparison to the ground deployments that

would take place at a later date. The second part of the experiment was a series of ground tests conducted in the fall

of 2009 aimed at developing a more comprehensive idea of the relationship between model size and inflation time,

as well as, the model accelerations during deployment.

Figure 2: In-Flight Experiment

B. Experiment DescriptionThe experiment consists of a stowed inflatable space habitat model that is deployed once the microgravity

environment is reached or when the solenoid is triggered by the computer. Deployment is initiated by a solenoid

valve through which low pressure air (


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As stated, the experiment consisted of two phases: a microgravity portion and a ground portion. Typical g-forces

from the flight parabola during a microgravity run are shown in Figure 4. Note the microgravity conditions last for

approximately 20 seconds. The microgravity phase involved the pressurization of a 6 in diameter model with and

without an accelerometer attached to the end of the inflatable model. Two cameras were used to record high speed

video from different angles. Due to the space limitations imposed by the size of the aircraft, the cameras were

mounted on the top instead of side of the frame. Both Figures 5 and 6 show the shape of the models at distinct

points of the deployment using both the roll and z-fold methods, respectively. It can be seen that the shape of the

model proceeds to its full volume state in a reasonably uniform manner for both cases. While both models reached

full volume around the same time, it was also seen in the second frame of the sequences that the roll fold method

took longer to reach full length than did the z-fold method.

The ground based portion involved the deployments of models of varying diameters. This was done not only to

offer a comparison to the microgravity runs, but to also provide a broader view of the impact of model size on

inflation time and deployment accelerations. Because the space limitations no longer applied, one of the cameras

was moved to the ground to give a side view of the deployment. Both Figures 7 and 8 show the deployments of the

6in diameter models with the roll and z-fold, respectively. It can be seen in the second frame of the deployment

sequences that the z-fold method expands to near full length sooner than the roll fold method. This was consistent

throughout all ground deployments. The corresponding accelerations will be explored later in the paper.

Figure 4: G-Forces from Typical Flight Parabola

Figure 5: Roll Fold (Microgravity)


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Figure 6: Z-Fold (Microgravity)

Figure 7: Roll Fold (Ground)

C. Model ManufacturingModels were manufactured out a polyethylene coated nylon fabric. The material thickness is approximately

0.01inches. The material was cut to dimension and seal together by a student. The models were sealed using acommon heat sealer to bond the polyethylene sides of the material. Two types of heat sealers were used to construct

the models. The first was a large straight sealer that was used to bond the length of the model. The second was a

smaller hand heat sealer that was used to for the more intricate parts of the model, i.e., the end caps. Several models

were constructed of various diameters while keeping the lengths constant. Each model took approximately 5-6

hours to make. The manufacturing process was accurate to within the measuring capabilities of the hand rulers

used, but the quality was influenced the most by the heat sealers. The models were accurate enough for the purposes

of the project without requiring more complex manufacturing processes.


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Figure 8: Z-Fold (Ground)

D. Test MatrixThe baseline test matrix is divided into three categories: Fineness Ratio, Folding Patterns, and Inflating

Pressures. An example table for how the parameters of the test matrix were varied is shown in Table 1.

Table 1: Experiment Test Matrix

The fineness or aspect ratio was developed by maintaining a constant length of 20 inches and varying the

diameters to 3 inches, 6 inches, and 10 inches. The inflation pressures were varied between 1, 3, and 5 psi for the

3inch and 6 inch diameter models, but the pressures were increased to 5, 7, and 9 psi for the 10 inch due to the large

increase in inflation time.

III. Results and DiscussionA. Pressurization Results

The pressure history was recorded for all deployments for purposes of understanding how the model inflates by

examining any changes in the internal pressure. Figure 9 shows six plots of the pressure history for the 3in diameter

model. In addition, the plots also show the cut-off pressure setting that was used to close the solenoid. These were

the runs taken on the ground. The model was tested using both the roll and z-folding methods. The plots on theright are those using the roll fold method and the plots on the left are those using the z-fold method. The model was

also run at three different inflation pressures: 1 psi, 3 psi, 5psi. The plots at the top are the 1 psi inflation pressures.

The middle and bottom plots are the 3 psi and 5 psi inflation pressures, respectively. It was decided to not go

beyond 5 psi because the inflation time became so rapid that a usable pressure history could not be obtained.

Looking at the pressure histories for the six runs, it can be seen that there are no noticeable differences in the roll

and z-fold methods. In both methods, two distinct regions can be seen. The first part of the inflation is

characterized by a constant pressure expansion until the model reaches its full volume. The model then transitions

into the second region of constant volume with increasing pressure. Both the 1 psi and 3 psi inflation runs maintain


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their respective pressures throughout the entire run; however, the 5 psi runs shows a slight disturbance in the

pressure history. The pressure initially rises to just above 5 psi and then transitions to just below 5 psi before

reaching full volume and climbing to the cutoff pressure. Comparing the data and the video showed that at this

inflation pressure, the 3 in model inflated too quickly for the pressure to stabilize back to the regulator setting.

Figure 10 shows eight plots of the pressure history for the 6in diameter model. Examining the pressure histories

for the eight runs, it can be seen that again there are no noticeable differences in the roll and z-fold methods. In both

methods, the same two distinct regions can be seen. The first part of the inflation is characterized by a constant

pressure expansion until the model reaches its full volume. The model then transitions into the second region of

constant volume with increasing pressure. Both the 1 psi and 3 psi inflation runs maintain their respective pressures

throughout the entire run; however, the 5 psi and 7 psi runs do not return to the desired pressure right away. The

pressure initially spikes above the set pressure and then drops back to the desired setting in approximately 2 seconds.

This can be explained by the large spike in pressure experienced when the solenoid opens and releases the

compressed air.

Figure 4: (3in Model) Pressure History vs. Time; z-fold (left), roll fold (right)


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Figure 5: (6in Model) Pressure History vs. Time; z-fold (left), roll fold (right)


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Figure 11 shows six plots of the pressure history for the 10in diameter model. Examining the pressure histories

for the six runs, it can be seen that again there are no noticeable differences in the roll and z-fold methods. In both

methods, the same two distinct regions can be seen. The first part of the inflation is characterized by a constant

pressure expansion until the model reaches its full volume. The model then transitions into the second region of

constant volume with increasing pressure. In all of the runs, the pressure does not return to the desired pressure

right away. The pressure initially spikes above the set pressure and then drops back to the desired setting in

approximately 1-2 seconds due to the transients generated when the solenoid valve is opened.

Figure 6: (10 in Model) Pressure History vs. Time; z-fold (left), roll fold (right)


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Figure 7: Inflation Pressure vs. Time

The data from the grounds runs was compared to the high-speed video to determine when the models reached

full volume. Figure 12 shows the time it took to inflate each model to full volume at a given inflation pressure. This

first observation is the nonlinear nature of the relationship between the inflation pressure and the time to inflate. It

can also be seen in here why higher inflation pressures were not explored for the 3 in model. Higher inflation

pressures would inflate the model in less than 2 seconds, which was reasoned to be too rapid to be useful. Similarly,

for purpose of time savings at this time, slower inflation pressures were not explored for the 10 in model. An

interesting relationship was noticed with relation to the trend of the data. When the data was normalized with

respect to volume, the data collapsed on itself. Figure 13 shows the data once it has been normalized. The x-axis is

simply the inflation time based on the volumetric flow rate Q (in 3/s).

Figure 8: Inflation Pressure vs. Normalized Time


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Using the ideal gas law, as shown in (1) the mass flow rate into the model was calculated for the several ground

runs. Figure 14 shows the linear relationship between mass flow rate and inflation pressure.

(1)

(2)

(3)

(4)

Figure 9: Mass Flow Rate vs. Inflation Pressure

Figure 10: Inflated Fabric Beam


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Figure 12: (3in Model) Acceleration vs. Time; z-fold (left), roll fold (right) [1 psi, 3 psi, 5 psi (top-bottom)]


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Figure 13: (6 in Model) Acceleration vs. Time; z-fold (left), roll fold (right) [1 psi, 3 psi, 5 psi, 7 psi (top-

bottom)]


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Figure 14: (10 in Model) Acceleration vs. Time; z-fold (left), roll fold (right) [5 psi, 7 psi, 9 psi (top-bottom)]


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C. Microgravity Pressure and Acceleration ResultsFinally, the reduced gravity runs were compared to the ground runs for purposes of finding differences, if any, in

the deployment dynamics. Figure 20 shows four plots of pressure history and model acceleration for the 6 in

diameter model with an inflation pressure of 3 psi. Looking at the pressure histories for both the microgravity and

ground run, there is no noticeable difference in the inflation profile. The microgravity run does have a large spike in pressure initially, but this is explained by the pressure source used on the aircraft and the time between runs.

Therefore, the time to fully inflate was observed to not change significantly between both runs. Looking at the

acceleration profiles, a noticeable difference can be seen in the deployment dynamics. During the microgravity

deployment, the model has motion during the initial break away from the strap and when the model is reaching full

volume. However, the model has zero acceleration during the rest of the deployment. This is unlike the ground run

which is observably more dynamic throughout the entire inflation. A point worth mentioning is the increasing in

acceleration at the end of the microgravity run. This coincides with the aircraft transitioning back into the high-g

portion of the flight profile.

Figure 15: Pressure History and Acceleration History [Microgravity (top), Ground (bottom)]

Differences in deployment behavior can be illustrated by tracking the deployment rate between the different g-

loading and packing arrangements. Here we simply track the maximum deployed dimensionless length, x/L. When

x/L=1, the model is completely deployed. Figure 16a shows the horizontal distance the 6 in model has deployed in a

microgravity environment. The inflation pressure was 3 psi and was kept consistent with the deployments shown in

Figure 16b. The distances were found by overlaying a calibrated grid of the deployment video and observing howfar the model had progressed over time. The microgravity deployments show a clear difference between the roll and

z folding patterns. The z-fold deploys more quickly requiring less inflation pressure to trigger the deployment

sequence. Figure 16b shows the horizontal distance the 6 in model has deployed in a earth environment. The model

was again inflated at 3 psi. Unlike the microgravity tests, the ground deployments do not show the same clear

difference between the roll and z folding patterns as seen in Figure 16a. Here the body force is greater than the

deployment forces in the initial deployment period. Only once filling has occurred and a critical pressure is reached

does the deployment extend beyond the initial fabric release.


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(a) Tip deployment rate in microgravity.

(b) Tip deployment rate in 1 g.

Figure 16: Measurements of deployment time of model tip for 6 in model at 3 psi.

D. DiscussionDuring the experiment, it was expected to see significant differences in the deployment behavior of the inflatable

space habitat model under microgravity conditions from the behavior observed in Earth gravity. The impact of foldmethod and inflation speed was also expected to be significant. The hypothesis was confirmed with respect to the

difference between deployment behavior in the microgravity and ground runs. The runs taken under microgravity

conditions were significantly less dynamic. In addition, the roll and z-fold methods produced different results at

certain portions of the ground deployments. Most notably, the roll fold was observed to reach full length later in the

deployment than did the z-fold method. However, the roll fold and z-fold methods were found to take the same

amount of time to inflate to full volume.


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During the inflation process, 2 distinct regions can be identified in the pressure measurements. The first is the

deployment region indicated by constant pressure. Since pressure is constant, the volume is increasing and the

model is being filled and extended during the deployment. The second region is denoted by a constant rate of

pressure increase. At this stage, the model has filled and completely expanded, but will most likely not yet be

completely deployed since the critical pressure criterion has not been met. Complete deployment occurs sometime

during this phase once the critical pressure has been reached. After this, the pressure reaches the maximum value

and the fill process is complete.

The primary difference between the models deployment behavior is seen in the early deployment stages. Once

released, the z-fold model reaches full length more quickly than the roll-fold models. This is evident from the data in

Fig. 16, where the z-fold is an unrestricted deployment and deploys more quickly requiring less inflation pressure to

trigger the deployment sequence. Unlike the microgravity tests, the ground deployments do not show the same clear

difference between the roll and z folding patterns as seen in Figure 16a. Here the body force is greater than the

deployment forces in the initial deployment period. Only once filling has occurred and a critical pressure is reached

does the deployment extend beyond the initial push provided at the start of deployment. This is due to the symmetric

tensions seen in the accordion folding pattern of the z-fold. In the z-fold configuration, average tension around the

circumference of the model is approximately equal since the packing configuration is symmetric. However, in the

roll fold configuration, non-zero tension is first seen in the outside of the rolled model. As the model inflates, this

tension increases to a constant value as the model unrolls. Only once the model is completely filled but not fully

inflated do we expect to see tension in the inside of the rolled model as shown in Figure 17.

Figure 17: Stresses in a Rolled Model

IV. SummaryThe deployment of inflatable space structures is a complex process in which both the motion and pressurization

affect each other. This experiment was focused on microgravity deployments conducted on simple inflatable habitat

models. Deployments of the models were controlled throughout the inflation. Pressurization time histories show

generally consistent inflation characteristics over multiple deployments with varying inflation pressures, including

constant pressure increasing volume and constant-volume increasing pressure once the model reached full volume.

Variations in deployment acceleration over several runs can be most likely attributed to fold inconsistencies.

V. AcknowledgmentsThe team would like to acknowledge Dr. Andy Arena and Dr. Victoria Duca-Snowden with the Oklahoma Space

Grant Consortium, Dr. Larry Hoberock and the MAE department, Dean Karl Reid, and the OSU Student

Government Association for all their assistance. Jeremy Hill, Josh Hathaway, Eric Johnson, and Alan Larson

performed the experiment on the NASA aircraft. Johnny Chandler served as the ground crew for the experiment.

The authors and team would also like to thank Sara Malloy of NASAs Reduced Gravity Education Flight Program

and Chris Nelson of Oceaneering Space Systems who served as the NASA mentor.


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VI. References1) J. E. Campbell, S. W. Smith, J. A. Main, and J. Kearns, Staged Microgravity Deployment of a

Pressurizing Scale-Model Spacecraft, Journal of Spacecraft and Rockets, Vol. 41, No. 4,

JulyAugust 2004.

2) D. Cadogan and M. Grahne, Inflatable Space Structures: A New Paradigm for Space StructureDesign, 49th International Astronautical Congress, IAF-98-I.1.02, Sept 28-Oct 2, 1998.3) Clem, A. L., Smith, S. W. and Main, J. A., A Pressurized Deployment Model for Inflatable Space

Structures, AIAA Paper No. 2000-1808, Proceedings of the AIAA SDM Gossamer Spacecraft Forum,

Atlanta, GA, April 2000.

4) Clem, A.L., S.W. Smith, and John A. Main, Experimental Results Regarding the Inflation of UnfoldingCylindrical Tubes, AIAA Paper no. 2001-1264, Proceedings of the AIAA SDM Gossamer Spacecraft

Forum, Seattle, WA, April 2000.

5) Cadogan, D.P., Scheir, C., Expandable Habitat Technology Demonstration for Lunar and AntarcticApplications, 2008-01-2024, International Conference on Environmental Systems, San Francisco CA, 29

June-2 July 2008

6) Hinkle, J., Lin, J.K., Watson, J., Deployment Testing of an Expandable Lunar Habitat, AIAA 2009-6447,SPACE 2009 Conference & Exposition, Pasadena CA, 14-17 Sept. 2009.

7) M. Salama, C.P. Kuo, M. Lou, Simulation of Deployment Dynamics of Inflatable Structures,AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference and Exhibit,

Apr. 12-15, 1999.

Paper v. 1.1, Dec. 28, 2009

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Space Habitat Deployment Opt