Monday, January 29, 2007

(in work) OUT-LINK: Beyond GEO

Thought Experiment (TE) considers innovative ways to leverage Space Elevator (SE) beyond Geosynchronous Equatorial Orbit (GEO).

Beyond GEO: For climber operations beyond GEO, TE names them "Out-Link" operations.
BACKGROUND: TYPICAL TETHERS
Up-Link tether stretches to GEO from Earth's ocean. Climbers use solar power to overcome Earth's gravity which ever decreases until arrival at GEO Node.
Down-Link tether uses Earth's gravity to accelerate climber back to Earth's surface. Power for a timely deceleration might come from an Earth based particle beam.
Out-Link tether enables delivery of payloads beyond GEO. Stretching from GEO Node to Apex Anchor (AA), an Out-Link tether sits atop the Up-Link; another sits atop the Down-Link; both are bounded by a GEO Node. Out-Link provides ever increasing centrifugal force to push climber further out from Earth.  Along the Out-Link tether, many points have sufficient orbital speeds for payload satellites to escape Earth's orbit and travel "interplanetary" throughout the inner Solar System.
Tether Maintenance.  To keep tether in tip top shape, TE assumes every climber continually monitors tether for anomalies. An occasional climber will be scheduled to do strictly tether repairs. 
BACKGROUND: CLIMBER PHASES
TE considers Out-Link as one of SE's 4 operational, climber phases.
① Up-Link Phase.  All climbers would travel from Marine Anchor up to GEO Node where they may either offload their payload or or retain it for continued transport.
 Cross-Link Phase.  Space tug transports climber along Geosynchronous Equatorial Orbit (GEO) from Up-Link GEO Node to Down-Link GEO Node.
③ Down-Link Phase. Typical climber returns from GEO back to Earth surface via dedicated Downlink Tether.  It might return empty or carry retrograde cargo destined for Earth.
 Out-Link Phase Climbers continue beyond GEO.  With a slight outward push from either GEO Node; centrifugal force accelerates climber further outward. Of course, climber must decelerate prior to its destination; power for deceleration would most likely come from same solar panels which powered it in Up-Link phase.  This chapter discusses Out-Link Phase in more detail.
Out-Link Gateways
TE defines "gateway" as a transition station between dissimilar links.  Of course, an Out-Link Gateway (OLG) connects an Out-Link with other type links.  The first Out-Link Gateway (OLG1) is atop the Up-Link tether which hosts climbers using solar power to overcome Earth's gravity.  OLG1 is also at the bottom end of an Out-Link tether which provides centrifugal force for climbers to travel further out from Earth.  OLG2 is atop the Down-Link tether; OLG2 interfaces an Out-Link with the "Cross-Link",  a non-tethered, space tug connection from OLG1. Upon arrival at either OLG, a typical climber off-loads and/or on-loads cargo in preparation for subsequent link.
BACKGROUND: Any climber must start its mission from the Earth's surface, most likely from one of SEE's Marine Anchors on the Equator in the Pacific Ocean. From this oceanic complex, all climbers must ascend the Up-Link tether to OLG1. Most climbers will return to Earth via OLG2; then, descend the Down-Link tether to another Marine Anchor. However, some climbers will not return; instead, they will become part of an AA atop one of the Out-Link tethers. 
Apex Anchor Atop the Outlink 
Apex Anchor (AA)  keeps the tether taunt by defying Kepler's Third Law.

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Orbit Bound Velocities
MSL + Earth R Orb Vel Orb Period
km (a) km (v)km/sec (T) day
GEO 35,786 42,164 3.075 0.997
AA 100,000 106,378 1.936 3.996
μ = 398,600 km3/s2 
(μ/a) (a3/μ)
However, if AA was tether bound, it would have same period as GEO;
thus, much greater linear velocity
if we extend table to even further
Tether Bound Velocities
MSL + Earth R Orb Vel Orb PerEscape Vel
km (a) km (v)km/sec (T) day(ve) km/sec
GEO 35,786 42,164 3.066 1 day4.348
AA 100,000 106,378 7.736 1 day 2.738
Further 144,000 150,378 10,936 1 day2.302
Earth R = 6,378 km
2π × a/T Given (2μ/a)
For a spherically symmetric, massive body such as a star, or planet, the escape velocity for that body, at a given distance, is calculated by the formula[3
]
where G is the universal gravitational constant (G ≈ 6.67×10−11 m3·kg−1·s−2), M the mass of the body to be escaped, and r the distance from the center of mass of the body to the object
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3) determine equal momentum equations
both angular
and linear
apply to Out-Link Mom = Up-Link Mom
SOURCE DOCUMENT  For the GEO Node to stabilize at GEO, angular momentum of the Out-Link tether above GEO must equal angular momentum of Up-Link tether below GEO.  Without a counterweight, total tether length must be about 150,000 km.  Fortunately, AA can enable the required counterweight with a much shorter Out-Link.  With the correct amount of mass, AA can shorten total tether length to 100,000 km.  With considerable on-board tether material as well as a 1,000 rpm rotating drum, AA will have considerable reel in/out capability. Also, current plans call for AA to store considerable fuel for thrusting engines to instantly adapt to dynamic forces from tidal forces and climbers.After successful delivery of payload, Outlink climber will most likely continue on to AA to join its mass collection.  NOTE:  Expendable Outlink climbers might prove practical; more economical than returning via Out-Link tether.
TE proposes: One way only. All Out-Link climbers should go to AA, to never return. Thus, all Out-Link climbers (and some payloads) end their existence inside AA to further contribute to AA mass. TE assumes a mass manufacturing, economy of scale can create many climbers much like we now manufacture automobiles, TE further assumes an economical "end of life" for Out-Link climbers as part of the cost of business for SE Enterprise (much cheaper than disposable fuel tanks, i.e., "stages" of current chemical rocket launches.)

Total Tether Mass: 6,300 metric Tonnes (mTs)
Apex Anchor Mass: 1,900 mTs
Initial source of infrastructure mass for AA might include:
---Dead GEO satellites
---Out-Link climbers at end of life.
---Added mass from Earth as required
---Space debris, perhaps passing asteroids

Outlink still needs Solar Power operate the spacecraft beyond GEO, braking behyond GEO and cooling brakes. Cooling braking system might be a major concern. Future design of Out-Link climbers might well differ from Up-Link climbers which must overcome gravity during climb from Marine Anchor to GEO.Out-Link
Out-Link Cargo might include:
---payloads for interplanetary missions,
---mass for AA counter weight.


Apex Anchor (AA) Mission  Stabilize Out-Link Tether.  At the end of the Out-Link, AA's centrifugal force ensures a taunt tether. Also, AA can reel the tether in/out as required for various tasks such as space debris avoidance, damping tether end vibrations, and reacting to emergencies.  Also, AA will store and burn fuel to provide thrust as required; thus, AA is more than mere mass at end of tether. With continuous thrust capability, AA can greatly reduce impact of dynamic change; EXAMPLE:  AA can stay in designated location even after tether severs in the lower Up-Link portion (most likely location of severance).  With continuous communication with HQ-POC and Tether Ops Center can direct quick and decisive actions.  With reel in/out capability, AA can quickly adjust total tether's Center of Gravity (CG) to ensure GEO Node remains at GEO Node.
Sources of Mass. Out-Link climbers (6-20 tons each) can contribute significant mass for the AA. In fact, mass manufacturing of many "expendable" climbers might yield an extremely useful economy of scale. Ending their lives on Out-Link missions would certainly be much cheaper than disposable "stages" for current chemical rocket launches, each rocket stage now costs many millions of dollars. With no need for any Out-link climber to return back to GEO Node, this greatly eases scheduling of subsequent Out-Link missions. For more AA mass, we might collect dead satellites from deprecated GEO missions and raise them to the AA on Out-Link climbers destined to stay.  
Exact GEO.  The GEO Node must maintain an exact orbit radius; thus, AA must balance Out-Link's tether mass with Up-Link's tether mass to ensure total tether's Center of Gravity (CG) stays precisely at GEO. Such mass management will require the tether to reel in or out at the AA. Also, AA could use thrusters to move entire tether to avoid space objects.
EXAMPLE:  As stakeholder’s package travels the Out-Link, task might be one of following: 
   • Travel all the way to AA to assemble parts for a larger spacecraft; then, release it.  Out-Link phase could end with climber entering AA to further increase AA mass.
SE Climber launching into deep space
An object attached to a space elevator at a radius of approximately 53,100 km would be at escape velocity when released. Transfer orbits to the L1 and L2 Lagrangian points could be attained by release at 50,630 and 51,240 km, respectively, and transfer to lunar orbit from 50,960 km.[51]
At 144,000 km, the tangential velocity is 10.93 kilometers per second (24,450 mph). That is fast enough for probes to escape Earth's gravitational field and go as far as Jupiter.
2.2.7. Coast to Beyond GEO
Outlink payloads "coast" to higher altitudes for release into non-Earth orbits or to AA.  Various release altitudes initiate flights to the Moon, Mars and beyond.  Outlink phase also includes payload which continues on to the AA to become part of the counter-weight.
2.3.3  Delivery to Beyond GEO 
Space Elevator (SE) will host many Outlink missions to release spacecraft with significant energy to reach Solar System targets.  At various altitudes, the energy is sufficient to reach Moon, Mars and beyond. Outlink climber picks up cargo at GEO Node to go to proper drop location.



page 42.
7 Apex Anchor (AA) Road Map
7.1 Introduction
Without a counter-weight, SE dynamics indicate total tether length of 150,000 km would enable a proper balance between Up-Link tether's angular momentum (about 36,000 km × quick rotation) with that of Out-Link tether (about 114,000 km  ×  slower rotation) .   With a properly sized counter-weight,  recent architecture have standardized a total tether length of 100,000 km; thus, AA needs sufficient mass and/or thrust capability to maintain balance for GEO Node to maintain zero CG.
Tether mass: 6,300,000 kg
AA Mass:  1,900,000 kg
AA provides sufficient tension for tether to adapt to various forces, which include:
• Earth's gravity
• centripetal force
• lunar gravity
• solar gravity
• tether climbers
• others, such as electromagnetic interactions.
Thus, AA ensures proper tension in tether to create sufficient rigidity and stiffness along path of tether climbers.


Mass for AA might come from:

• asteroids
• tether climbers
• dead GEO satellites
• more mass from Earth as needed.


AA's initial mass might come from:

• from vehicle used to place initial tether fibers into orbit
• from small tether climbers used in tether construction.


Use this material as counter-weight lowers total mass to be lifted into orbit.



Capturing an asteroid as mass for AA might happen someday when SE travel becomes routine; however, near time solution must dominate.



Engineering requirements for AA mass:

• easily transported on tether
• not explosive.


7.2 AA Segment Definition and Mission.

AA is a major SE element which stabilizes tether during SE operations via thrust forces to counter mission impacting forces (either planned or unplanned).
• be refueled
• coordinate with Tether Operations Center (TOC) to damp harmonic motions
• stimulate desired movement of SE tether.
Also, smart AA will work with customer to
• deploy mission payloads
• capture returning payload
• refuel and assemble customer's system
In fact, AA will do far more than “just be a mass at the end of the space elevator.” Due to all these demands, the AA mission is:
Stabilize the SE, and Support  the Stakeholders.
Current Design Approach grows the AA from the initial space elevator deployment satellite. The process has three simple steps;
1) Deploy initial SE satellite (SE-Sat), a massive space system, assembled in LEO
2) Move SE-Sat from LEO to GEO by efficient rockets,
3) Deploy initial tether: SE-Sat will deploy one end of the seed tether in the downward direction towards the surface of the ocean while raising the massive deployment satellite in the opposite direction, keeping the whole system center of mass at the allocated GEO node. [thrusting required to compensate for angular momentum losses].


Since initial "seed" tether is not nearly strong enough to support operational climbers with payloads, subsequent treatments are needed as follows:

Build up the tether as several subsequent small climbers raise and enhance the tether from
one that is a seed tether to one that is operationally capable.
• Eventually, the initial SE-Sat becomes the AA with
---computational capability,
---thrust capability,
---fuel storage [with refueling ability],
---communications links to HQ-POC, Marine Node, and satellites on the tether.
---support customer needs for release of payloads.


To deploy a long magnificent tether, we need a Space Elevator Satellite (SE-Sat), an 80 ton deployment satellite with a large deployment spool and many fuel tanks.  SE-Sat components will likely include:

Tether Payload: Tether must initially go to the GEO node to begin a smooth reel out. The initial SE-Sat will eventually become the AA.  As the tether deploys downward, the SE-Sat moves upward; this balances angular momentum between initial Up-Link and initial Out-Link; it also maintains tether's Center of Gravity (CG) at the GEO.  Once the tether lowers all the way to the surface and secures at Marine Node, the deployment satellite would reel out even more tether. This allows the total tether's heavy mass to reach further outward for even more tension; thus, the tether stabilizes the tether and adds required structural features.
On-Board Tether Spool: Anticipate rigorous testing of multiple prototypes during the development of this critical component.  The reel out speed should be high enough for a reasonable deployment time, but slow enough to control the tether tension [likely less than 1,000 rpm], DIMENSIONS:  On board tether spool should be about 6 m long with 2 m diameter would provide sufficient mechanical leverage to hold the required initial tether length of 80,000 km.
Structural Elements: include following:1) Series of propellant tanks 2) huge spool of tether material 3) tether release mechanism.  After a rocket launch, these components must survive the turbulence of the first 500 kilometers of altitude; then, they must endure long periods of low temperatures and zero-g at LEO. They must be assembled in Low Earth Orbit (LEO) to become the initial SE-Sat, which will use large ion engines to accelerate-decelerate to GEO
Power Subsystem: SE-Sat will use solar cells with efficiencies now approaching 52% with extremely light weight for weight savings.
Attitude Control with Propellant: Attitude control will be essential during assembly and transportation to GEO. While SE-Sat is in LEO (on its way to GEO), as well as early on at GEO, the attitude control will probably be achieved by spinning masses [e.g., control moment gyros (CMGs] and torque rods [more effective in LEO]. Once the tether deploys a sizable distance, the gravity gradient factor will help stabilize the deployment satellite.
Command and Control Communications: will use relay satellites to maintain constant connectivity. A geosynchronous communications satellite antenna will 1) track the LEO grouping of sub-satellites, 2) monitor the assembly, 3) track SE-Sat as it goes from LEO to GEO. Once in GEO, the communications architecture will kick in with connection to Headquarters from GEO and constant monitoring of this link.
•  Thermal and S/C Support: Overall support of the spacecraft will involve many disciplines including thermal, radiation, electromagnetic, orbital location knowledge and projection, pointing and stability. During the 14 orbits per day while in LEO, the thermal stresses will be great and will impact all aspects of a spacecraft.
Thruster Elements: Thrusters will efficiently raise a very large mass (SE-Sat) from LEO to GEO. The cluster of large [1 meter diameter] ion engines will provide continuous thrusting to raise the altitude in a spiral orbit. The specifics of the thrust, efficiency of the engine, and the time to move to GEO will be modeled in greater detail as the mission approaches.


Eventually, the initial SE-Sat will  transform into the initial AA.  Then, the actual challenges and testing requirements will be addressed. Major challenges include:

System Mass Management: AA’s principle task is to stabilize the entire SE and continuously monitor each element of the infrastructure. Critical functions include 1) manage SE's center of mass of the system  2)  understand any and all changes in mass center or motion as the tether climbers are added and moved.
Reel-in and Reel-out: As the AA will have released 100,000 km of tether during deployment, additional reel-in/out should be routine. Upper end of the space elevator (AA) must assist the Marine Node (at lower end) in closely monitoring every element of the infrastructure. These two components will work together by reeling-in and reeling-out as needed to ensure SE system stability.
Thruster Operations: During SE operations, the AA motion must be controlled. This will require thrusters with the ability to point them in almost any direction with a variable thrust.
Customer Payload: Once the SE Enterprise (SEE) initiates customer support, the desires will vary. Three approaches now seem to be in demand by potential customers: 1) non-GEO stable location for mission [stay attached to tether]  2) release to mission trajectory  3) capture for placement on a return tether.
SPECIALIZED TETHER.  Designated tether might combine Out-Link with Up-Link to perform following roles:
    • SweeperAnchored at equator, tether will eventually be in the path of any non-GEO orbital debris and will impact it.  Most (hopefully, all) such debris will stay with tether and usefully disposed, most likely Out-Linked to AA to increase mass.
    • Catcher. Recover payloads directly from Earth orbit or even interplanetary sources.  Gracefully receive incoming payloads from space; then, space tug shuttle them to a Down-Link GEO Node; then, continue them on to the surface of the Earth.
    • Host.  Tether could host ports at several different altitudes, and they would all maintain same nadir on Earth's surface.  Release satellite from the tether at a designated points for velocity needed for certain destinations,


AA Summary:
AA is a very dynamic component of the space elevator. The ability to understand the natural motion and then ensure stability will be its most significant task; however, support to space elevator customers will be paramount in ensuring commercial success.
This would include monitoring whole arena of dynamic stimuli such as
tether climbers,
reeling-­‐in/out of tether,
forces from winds,
movement of the Marine Node,
stability supplied by a large mass at the GEO Node.
SOURCE DOCUMENT: BIG PROBLEM!!! Lower portion of severed tether might wrap around earth. When severed close to AA, Earth's coriolis causes tether to fall sideways falling thus wrap around Equator,  Thus, entire Outlink portion of tether (100,000 km - GEO Node height (35,789 km) =  64,211 km) might wrap completely around Earth's equator, about 40,000 km.
SOURCE DOCUMENTperhaps at very high speeds.  Lower severed tether could hit Earth as high as 5.7 km/sec (12,759 mph); at this enormous speed, tether's considerable mass could cause huge impact on both sea and land.
SOURCE DOCUMENTAt high tensions, severed tether tends to recoil at very high speeds;
however, for AA side of tether it is desirable to reattach tether to AA ASAP.
SOURCE DOCUMENTUnfortunately, once tether severs near AA, it might well move into an escape Earth.orbit and go interplanetary.  To mitigate this risk, one might use AA's thrust capability to prevent movement while waiting for tether to reattach.
Another option:  give Outlink climbers thrust capability to aid in straightening severed tether.
AA Conclusion:  It must be an intelligent component to the SE infrastructure.
Beyond GEO, centrifugal force increases as climber as it goes further out from the Earth.  Thus, the climber's acceleration and speed ever increases as it continues toward the Apex Anchor (AA) at 100,000 km from Earth. Therefore, Out-Link climber must rigorously control its ascent speed as it thrusts further outward.
Total Tether Mass: 6,300 metric Tonnes (mTs)
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SOURCE DOCUMENTFrom Marine Anchor, daily SE climbers will routinely launch daily with typical gross weight of 20 tons: 6 tons of structure, and 14 tons of payload.
NOTE: Space launches must become routine, but daily capacity must greatly exceed 14 tons of payload. EXAMPLE:  Assume one habitat will require 3,000 tons of water and another 3,000 tons of Terran topsoil and another 4,000 tons of structure.  To satisfy only this requirement would take 
10,000 tons / 14 tons/day = 714 days = 1.96 years.
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