et utilization in orbit, nasa, feb 11, 87ocr

7/30/2019 ET Utilization in Orbit, NASA, Feb 11, 87ocr

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National Aeronautics andSpace Administration

Washington, D.C.20546

to Am ot’ C:JJH:brb

Honorable Bill NelsonChairmanSubcommittee on Space Science

and ApplicationsCommittee on Science, Space

and Technology

House of RepresentativesWashington, DC 20515

Dear Mr. Chairman:

The House Authorization Report 99-829, requested NASA to studythe technical, operational, cost and safety requirements forExternal Tank (ET) orbit insertion, basic station-keeping and

life support to determine the feasibility of such usage of theET. The enclosed report provides an overview of the requirementsassociated with.the utilization of the ET and examines several

areas of concern which must be evaluated,

Please call me if you have questions concerning this report.

Enclosure

cc: Honorable Robert S. Walker



EXTERNAL TANK UTILIZATION ON ORBIT

An analysis has been conducted to investigate the

feasibility of taking the Shuttle External Tank (ET) to orbitand to maintain it on orbit for later scientific orengineering tasks. The engineering and operational problemsinvolved with this objective are basically within the currentstate-of-the-art of Shuttle operations, support system andtechnology. However, in order to take the ET to orbit and tomaintain it on orbit, several areas of.concern must beevaluated. Of primary interest are the following areas:reduction of Shuttle payload capability, propellantrequirements to prevent premature re-entry of the ET,additional propulsion and guidance equipment to maintain theET oriented in orbit, accessibility of orbiting ET,

probability of micrometeoroid or space debris damage to the

ET or potential impact of the ET with useful satellites, costof ET modifications and operational costs, etc.

Analysis of these and other areas provides an overviewof the requirements associated with the utilization of theET.

To evaluate the additional propellant needed to take theET to orbit, one should first be acquainted with the twocustomary Shuttle ascent profiles. One is called the"Nominal STS Mission" and the other is known as the "DirectOrbit Insertion Mission."

In the Nominal STS Mission, the Shuttle main enginesburn until a specific set of "main engine cut off" targetconditions is reached. The ET is separated. The Shuttleorbital maneuvering system (OMS) then provides the additionalvelocity needed to place the Orbiter into a transfer orbitwith the apogee equal to the desired orbit. The second OMSmaneuver occurs at apogee and places the Orbiter into itsfinal orbit. In the Nominal STS Mission, the ET then impactsin the Indian Ocean or the Southern Pacific Ocean dependingon launch from the Eastern Test Range (ETR) or Western TestRange (WTR) respectively. To take the ET to orbit in thismission, a payload penalty results which is shown in Figure1. The penalty is approximately 3000 lbs. dependinzh;z thecircular orbit altitude to which the ET is taken.payload penalty is additive to the reduction of Shuttlepayload weight which always occurs with increasing altitude.

_-

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.

Figure 1. External Tank Utilization

Shuttle Payload Penalty vs Altitude for Carrying ET to

Circular Orbit

The Direct Orbit Insertion Mission eliminates the OMS 1burn by continuously burning the main engines to a set ofcut-off conditions which will provide the proper apogee. Inthis scenario, the ET impacts in the Southern Pacific Oceanor the Northern Pacific Ocean for either ETR or WTR aunchesrespectively.

Taking the ET tcorbit in this scenario results in apayload penalty of approximately 2200 lbs., as indicated inFigure 1 for altitudes ranging from 200 to 300 n.m. Ataltitudes below this range, orbital lifetime withoutreboosting is limited to a few days. For instance, decay andre-entry from 300 Ian (160 n.m.) would occur within 30 to 35days for atmospheric density conditions during 1988 and

"broad side" drag orientation, unless reboosting of the ETcan be accomplished. Since the atmospheric density is astrong function of altitude and solar cycle activity, andsince the drag forces acting on the ET vary extensively withthe ET orientation, a comparison of these parameters must bemade to determine the reboost propellant requirements.Figure 2 shows the yearly propellant requirements toperiodically reboost the ET, i.e. to maintain the initialWtorage" altitude. The propellant requirements are

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parameterized with respect to solar cycle activity and dragorientation of the ET.

Figure 2. Reboost Propellant to MaintainET at Constant Altitude

, TWOlnMmS

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e- '\

Since the-E3 cannot be maintained at low orbitalaltitudes for any significant time without large propellantconsumption, it is necessary to place the ET at an orbitalaltitude which. is relatively drag free. As can be seen fromFigure 2 at altitudes above 500 Jan (270 n-m.), the propellantrequirement for reboosting the tank is negligible for nominal

atmospheric conditions and nominal solar activity.

forFigure 3. OMV Propellant

Initial Placement and Orbit Maintenance vs Time

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An alternative, shown in Figures 3 and 4, is to placethe ET initially at a sufficient altitude to conserve reboostpropellant utilizing the Orbital Maneuvering Vehicle (OMV)for transfer of the ET to an initial altitude above the 160n.m. Shuttle insertion altitude. Boosting the ET to higheraltitude via an OMV results in the lowest overall propellantrequirement for lifetimes over three years. If it is desired

to-use the tank within a few months from the launch date, acombination of boosting (to an intermediate altitude) andaltitude maintenance results in the lowest propellantconsumption.

Figure 4. Alternative Mission ScenarioOMV Boosts ET

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For altitudes less than 500 km (270 n.r.), a guidancesystem is required for attitude control and reboostguidance. This control can be provided by either an OMV oran ET onboard system. The weight and power requirements forthis system are 'estimated to be 2500 lbs. aa3 125 watts

respectively. A small (6.8 sgm) fixed solar array/batterysystem is included to provide the required power. Above analtitude of 500 km, the air drag is low enough to avoid theneed for attitude control of the ET.

Another approach, based on the Martin-Michoud scavengingstudy, would be the use of a customized Orbital TransferVehicle (OTV) which would use residual propellants left inthe ET after insertion. Sufficient propellant remains in the

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.

ET to boost the ET to greater than 500 km (270 n.m.). Such asystem would require a substantial new development programhowever.

Safety considerations would probably demand theincorporation of means for controlled de-orbit of the ET.The ET could be de-orbited by ground control using an OMV.

The propellant required by an OMV which is subsequentlyrecovered by the Shuttle would be 5175 lbs. Another optionis the use of solid propellant STAR motors which would havebeen installed prior to launch. De-orbit impulse is providedby four Thiokol STAR 26B (or STAR 27) solid rocket motors(SRWS). Each SRM provides an average thrust of 7970 lbs.for approximately 18 seconds. A de-orbit system of this typehas been defined for another application. This also requiresan attitude control system to establish proper attitude forde-orbit and to provide an ET spin rate of 20 degrees persecond prior to SRM firing. A baseline hydrazine systemprovides these capabilities. The total system weight

including electrical power and communication system, ahydrazine attitude control system, and the STAR SRM's isapproximately 5411 lbs.

The ET is already equipped with the necessary hardwareto dump and vent the residual propellants remaining at MECO.However, a tumble valve which is noqnaIly activated by theOrbiter after KECO'must be modified to prevent end-over-endtumbling of the ET. In addition, the-range safety systemmust be modified to make the ET safe for later use. Further,to assure venting of the gaseous hydrogen and oxygen from theET for post MECO safety, a small gaseous helium bottle mustbe added, and a non-propulsive GH2 vent duct must also be

installed. The modifications of the safing system andadditional helium pressurization, etc. are estimated+0 weighapproximately 335 lbs. Safing of the STAB retromotors wouldbe accomplished with a state-of-the-art range safety systemsimilar to the system5 used on the solid rocket boosters orthe Inertial Upper Stage (IUS). Arming of the system wouldalso have to be accomplished from the Orbiter.

The probability that any of a large number of ET's (10to 100) would collide with another useful satellite at anyaltitude is very low. This is illustrated in Figure 5.However, on the pverage each ET will be penetrated by 1 to 3impacts per year with micrometeoroids, as indicated in Figure

6. Furthermore, the tank walls may be penetrated by man-madespace debris between 1 and 3 times per year, increasing withorbit altitude. There is negligible probability (less than0.001 per tank per year) that a micrometeoroid or man-madedebris large enough to penetrate the ET wall and then exitfrom the opposite wall, generating shrapnel, will occur.However, it should be noted that the current model on whichthese data are based has a fairly wide range of uncertainty,and the protection capability of the ET insulation is largely

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unknown. In any event, penetration of the ET would limit theeventual use as a man-rated pressurizable vessel unless a10,000 lbs. micrometeoroid bumper is provided.

Fiuure 5. Probability of Collision per Year per ET

with Useful Satellite&

OADIT ALpnor OF ET pml

Figure 6. ET Collision Rates with Orbit Debrisper Year per ET

The rough costs of two options have been analyzed fortaking the ET's to higher orbit for Wtoragen until they can

be used. Option I utilizes an OMV, as discussed above, forboost to higher orbit and eventual controlled reentry. Inthis option each ET is equipped with a docking probe, aminimal c&mnunications capability, and the necessarystructural strengthening. The ROM recurring cost esttitcfor this option in 1990 dollars is $lSM (including $4.5X usecharge for a ground-based OMV to deliver each ET to higherorbit and to return the 0247). In Option II each ET ismaintained at the Shuttle delivered altitude, oriented for

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c ’

- maximum on-orbit lifetime by an onboard attitude ControlSySt8lU. The ROM recurring cost estimate for this option is$30M. The deboost for Option I would be an additional $4.5M

for an OMV service charge. The deboost for Option II wouldbe an additional $850X (recurring) for four STAR solid rocketmotors.

The rough cost per day for the Orbiter to station-keepwhile scientists are Working on the ET is estimated as $7OOK

add-on, if performed in conjunction with a nominal Shuttlemission, based on STS reimbursement guide esi=alated to 1990dollars. To rendezvous with an ET previously stored onorbit, its orbital parameters must be matched exactly by therendezvousing Shuttle Vehicle (restricted launch window,launch inclination, unique phasing orbits, etc.). Adedicated STS mission may be required. The mission cost to

dedicate an eight-day STS mission to modify a standard ETalready in a 270 n-m. orbit, using a two-man NA crew for sixdays, is in excess of $lOOM. The actual figure would depend

on the type of mission and other factors that cannot bepredicted now.

P

et utilization in orbit, nasa, feb 11, 87ocr

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