EXTERNAL TANK UTILIZATION ON ORBIT
Feb 11, 1987
National Aeronautics and Space Administration
Washington, D.C. 20546
Reply to the Attention of: C:JJH:brb
Honorable Bill Nelson, Chairman
Subcommittee on Space Science and Applications
Committee on Science, Space and Technology
House of Representatives
Washington, DC 20515
Dear Mr. Chairman:
The House Authorization Report 99-829, requested NASA to study the technical, operational, cost and safety requirements for External Tank (ET) orbit insertion, basic station-keeping and life support to determine the feasibility of such usage of the ET. The enclosed report provides an overview of the requirements associated 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.
John F. Murphy
Assistant AdministratorFor Legislative Affairs
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 orbit and to maintain it on orbit for later scientific or engineering tasks. The engineering and operational problems involved with this objective are basically within the current state-of-the-art of Shuttle operations, support system and technology. However, in order to take the ET to orbit and to maintain it on orbit, several areas of concern must be evaluated. Of primary interest are the following areas: reduction of Shuttle payload capability, propellant requirements to prevent premature re-entry of the ET, additional propulsion and guidance equipment to maintain the ET 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, cost of ET modifications and operational costs, etc.
Analysis of these and other areas provides an overview of the requirements associated with the utilization of the ET.
To evaluate the additional propellant needed to take the ET to orbit, one should first be acquainted with the two customary Shuttle ascent profiles. One is called the “Nominal STS Mission” and the other is known as the “Direct Orbit Insertion Mission.” In the Nominal STS Mission, the Shuttle main engines burn until a specific set of “main engine cut off” target conditions is reached. The ET is separated. The Shuttle orbital maneuvering system (OMS) then provides the additional velocity needed to place the Orbiter into a transfer orbit with the apogee equal to the desired orbit. The second OMS maneuver occurs at apogee and places the Orbiter into its final orbit. In the Nominal STS Mission, the ET then impacts in the Indian Ocean or the Southern Pacific Ocean depending on launch from the Eastern Test Range (ETR) or Western Test Range (WTR) respectively. To take the ET to orbit in this mission, a payload penalty results which is shown in Figure 1. The penalty is approximately 3000 lbs. depending on the circular orbit altitude to which the ET is taken. The payload penalty is additive to the reduction of Shuttle payload weight which always occurs with increasing altitude.
Figure 1. External Tank Utilization
Shuttle Payload Penalty vs Altitude for Carrying ET to Circular Orbit
The Direct Orbit Insertion Mission eliminates the OMS 1 burn by continuously burning the main engines to a set of cut-off conditions which will provide the proper apogee. In this scenario, the ET impacts in the Southern Pacific Ocean or the Northern Pacific Ocean for either ETR or WTR launches respectively.
Taking the ET to orbit in this scenario results in a payload penalty of approximately 2200 lbs., as indicated in Figure 1 for altitudes ranging from 200 to 300 n.m. At altitudes below this range, orbital lifetime without reboosting is limited to a few days. For instance, decay and re-entry from 300 km (160 n.m.) would occur within 30 to 35 days for atmospheric density conditions during 1988 and “broad side” drag orientation, unless reboosting of the ET can be accomplished. Since the atmospheric density is a strong function of altitude and solar cycle activity, and since the drag forces acting on the ET vary extensively with the ET orientation, a comparison of these parameters must be made to determine the reboost propellant requirements. Figure 2 shows the yearly propellant requirements to periodically reboost the ET, i.e. to maintain the initial “storage” altitude. The propellant requirements are parameterized with respect to solar cycle activity and drag orientation of the ET.
Figure 2. Reboost Propellant to Maintain
ET at Constant Altitude
Since the-ET cannot be maintained at low orbital altitudes for any significant time without large propellant consumption, it is necessary to place the ET at an orbital altitude which. is relatively drag free. As can be seen from Figure 2 at altitudes above 500 km (270 n.m.), the propellant requirement for reboosting the tank is negligible for nominal atmospheric conditions and nominal solar activity.
Figure 3. OMV Propellant
Initial Placement and Orbit Maintenance vs Time
An alternative, shown in Figures 3 and 4, is to place the ET initially at a sufficient altitude to conserve reboost propellant utilizing the Orbital Maneuvering Vehicle (OMV) for transfer of the ET to an initial altitude above the 160 n.m. Shuttle insertion altitude. Boosting the ET to higher altitude via an OMV results in the lowest overall propellant requirement for lifetimes over three years. If it is desired to-use the tank within a few months from the launch date, a combination of boosting (to an intermediate altitude) and altitude maintenance results in the lowest propellant consumption.
Figure 4. Alternative Mission Scenario OMV Boosts ET
For altitudes less than 500 km (270 n.m.), a guidance system is required for attitude control and reboost guidance. This control can be provided by either an OMV or an ET onboard system. The weight and power requirements for this system are ‘estimated to be 2500 lbs. and 125 watts respectively. A small (6.8 sgm) fixed solar array/battery system is included to provide the required power. Above an altitude of 500 km, the air drag is low enough to avoid the need for attitude control of the ET.
Another approach, based on the Martin-Michoud scavenging study, would be the use of a customized Orbital Transfer Vehicle (OTV) which would use residual propellants left in the ET after insertion. Sufficient propellant remains in the
ET to boost the ET to greater than 500 km (270 n.m.). Such a system would require a substantial new development program however.
Safety considerations would probably demand the incorporation 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 subsequently recovered by the Shuttle would be 5175 lbs. Another option is the use of solid propellant STAR motors which would have been installed prior to launch. De-orbit impulse is provided by 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 type has been defined for another application. This also requires an attitude control system to establish proper attitude for de-orbit and to provide an ET spin rate of 20 degrees per second prior to SRM firing. A baseline hydrazine system provides these capabilities. The total system weight including electrical power and communication system, a hydrazine attitude control system, and the STAR SRM’s is approximately 5411 lbs.
The ET is already equipped with the necessary hardware to dump and vent the residual propellants remaining at MECO. However, a tumble valve which is normally activated by the Orbiter after MECO’must be modified to prevent end-over-end tumbling of the ET. In addition, the-range safety system must be modified to make the ET safe for later use. Further, to assure venting of the gaseous hydrogen and oxygen from the ET for post MECO safety, a small gaseous helium bottle must be added, and a non-propulsive GH2 vent duct must also be installed. The modifications of the safing system and additional helium pressurization, etc. are estimated to weigh approximately 335 lbs. Safing of the STAR retromotors would be accomplished with a state-of-the-art range safety system similar to the system5 used on the solid rocket boosters or the Inertial Upper Stage (IUS). Arming of the system would also have to be accomplished from the Orbiter.
The probability that any of a large number of ET’s (10 to 100) would collide with another useful satellite at any altitude is very low. This is illustrated in Figure 5. However, on the average each ET will be penetrated by 1 to 3 impacts per year with micrometeoroids, as indicated in Figure 6. Furthermore, the tank walls may be penetrated by man-made space debris between 1 and 3 times per year, increasing with orbit altitude. There is negligible probability (less than 0.001 per tank per year) that a micrometeoroid or man-made debris large enough to penetrate the ET wall and then exit from the opposite wall, generating shrapnel, will occur. However, it should be noted that the current model on which these data are based has a fairly wide range of uncertainty, and the protection capability of the ET insulation is largely unknown. In any event, penetration of the ET would limit the eventual use as a man-rated pressurizable vessel unless a 10,000 lbs. micrometeoroid bumper is provided.
Figure 5. Probability of Collision per Year per ET with Useful Satellites
Figure 6. ET Collision Rates with Orbit Debris
per Year per ET
The rough costs of two options have been analyzed for taking the ET’s to higher orbit for “storage” until they can be used. Option I utilizes an OMV, as discussed above, for boost to higher orbit and eventual controlled reentry. In this option each ET is equipped with a docking probe, a minimal communications capability, and the necessary structural strengthening. The ROM recurring cost estimate for this option in 1990 dollars is $l5M (including $4.5M use charge for a ground-based OMV to deliver each ET to higher orbit and to return the 0247). In Option II each ET is maintained at the Shuttle delivered altitude, oriented for maximum on-orbit lifetime by an onboard attitude control system. 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 would be an additional $850k (recurring) for four STAR solid rocket motors.
The rough cost per day for the Orbiter to station-keep while scientists are working on the ET is estimated as $700K add-on, if performed in conjunction with a nominal Shuttle mission, based on STS reimbursement guide escalated to 1990 dollars. To rendezvous with an ET previously stored on orbit, its orbital parameters must be matched exactly by the rendezvousing Shuttle Vehicle (restricted launch window, launch inclination, unique phasing orbits, etc.). A dedicated STS mission may be required. The mission cost to dedicate an eight-day STS mission to modify a standard ET already in a 270 n-m. orbit, using a two-man NA crew for six days, is in excess of $100M. The actual figure would depend on the type of mission and other factors that cannot be predicted now.