A Thought Experiment: SKYHOOK

VOLUME 0: ELEVATIONAL
VOLUME I: ASTEROIDAL
VOLUME II: INTERPLANETARY
VOLUME III: INTERSTELLAR

PREVIEW OF SKYHOOK CONCEPTS

as a possible alternative

to the esteemed Space Elevator.

0) VERTICAL SKYHOOK, NON-ROTATING	BACKGROUND: Like Space Elevator (SE), the initial Vertical Skyhook (VSH) was long and straight with an up-tether and a down-tether. Unlike SE, VSH does not touch the ground, and it is not restricted to GEO or any other orbit.
1) ROTATING SKYHOOK, ROTAVATOR	A GREAT IMPROVEMENT OVER VSH, Rotovator uses shorter tether more effectively by hooking an elevated payload at tether’s low end; then, vertically rotating 180° to “sling shot” the accelerated payload from the high end toward space, efficiently saving fuel.
2) TETHER MATERIALS	CHIEF CHALLENGE FOR TETHER structures (SE, VSH, Rotovator) is developing the right material(s) then finding how to best implement them (interweaving, tapering, etc.).
3) AIRSHIP TO ORBIT (ATO)	CHIEF CHALLENGE FOR ROTAVATOR is elevating the payload from the ground up to the hook. Instead of risky, rockets; Thought Experiment (TE) suggests an old tech solution (lighter than air, blimps) now used by JP ATO for many years.
4) HYBRID SOLUTION:	COMBINE ROTOVATOR WITH ATO. We end with the proposal of combining the Rotavator with the ATO.

0) VERTICAL SKYHOOK, NON-ROTATING

Non-rotating Skyhook was proposed by E. Sarmont in 1990.

MAJOR ELEMENTS OF VERTICAL SKYHOOK (VSH):

1. Low Altitude Docking Platform may be a bare truss platform to hold shuttles while cargo transfers to elevators. Local gravity is due to Terra’s natural gravity

2. Zero-G Core. The mass of the core is irrelevant due to its position at or near the natural orbital radius. As Center of Mass (CM), equal forces from the below tether and the above tether tug it both toward and away from Terra.

3. Apex, high altitude counterweight (CW) platform, stores mass to maintain the core's orbital altitude. If asteroids are captured and held at Apex for ballast, they could be mined for valuable materials. As asteroidal mass is removed, ‘above’ tether could be reeled out to maintain required centripetal force.

4. Tethers bind the platforms together; they can be a few 1,000 kms in length (much shorter than SE's 36,000 km ground to GEO tether). These optimal “high tech” materials must have enormous tensile strength and incredibly light weight. Some such materials (i.e., nanotubes) can even superconduct electricity.

5. Elevator Cars travel between the platforms. Perhaps they will travel at same speed as proposed for SE climbers, about 200 km per hour. Since VSH tethers will be much shorter than SE tethers, one can expect the climb from VSH docking platform up to the core platform to be much quicker than for SE.

Encyclopedia Galactica: Vertical Skyhooks

SFIA: Ladder to Space

VERTICAL SKYHOOK PROs and CONs

PROs: ADVANTAGES OVER SPACE ELEVATOR (aka “beanstalk”)

1. VSH key advantage over SE is orbital flexibility. Like SE, VSH could locate at GEO to synchronize with a Terran nadir. Unlike SE, VSH is not physically required to be at GEO. VSH is much more flexible and has a range of orbit options.

2. Easier to Build and Deploy. Much smaller tether with less rigorous tensile strength requirements. Skyhooks may be viable for decades or more before a beanstalk.

3. Debris Unlike SE, VSH can more easily avoid other orbiting objects.

MORE PROs: DIVERSE POWER SOURCES

4. Electric Power. Tethers will likely contain electrodynamic materials (possibly superconducting) to generate electric power as they pass through Terran magnetic fields.

5. Solar Power. Tethers can also function as anchors for very large solar arrays.

CONs: CONSIDER FOLLOWING CHALLENGES

· Tether requirements overwhelm capabilities of nanotubes as well as other “super” materials (discussed in subsequent content).

· Radiation shielding must be considered. Tether length often puts Core and CW deep in Terran radiation belts, which necessitates radiation shielding to shroud those platforms and the elevators that service them.

SKYHOOK’s Chief Challenger: Rotovator

Rotating Skyhook (aka “rotovator”) imposes least logistics pain in building a space-based launch-assist tether. They require less mass (thus, less deployment effort) in orbit than vertical skyhooks not to mention considerably less than Space Elevators.

MAYBE ROTOVATOR HAS MORE VALUE THAN VERTICAL SKYHOOK!!!!

1) ROTATING SKYHOOK: ROTAVATOR

Many thanks to R. Hoyt for above illustration.

Rotating SkyHook (RSH) (a.k.a. “Rotavator”) might use a 600 km long, rotating cable suspended from an orbiting station. RSH leverages momentum exchange to accelerate payloads into orbit and beyond. This greatly reduces cost to orbit compared to traditional rockets. It works as follows:

· (RSH) orbits the Earth with a Counter-Weight (CW) at the far end. Rotating Skyhook system rotates around its center of mass much like a giant bola.

· A spacecraft or payload docks with the end of the cable as it passes near Earth’s surface, using its considerable angular momentum to continuously revolve about the station, the Sky Hooked payload rapidly gains altitude. (Skyhook’s orbital ascent could be a few hours, much quicker than SE or VSH. SE climbs its cable at a relatively slow 200 km per hour; thus, total climb time is 7 days from Earth surface to GEO.)

· Momentum exchange takes place when rotating cable flings the payload into a higher orbit or even into deep space, this exchange will slightly slow the skyhook's rotation and lose a bit of altitude.

· Fortunately, RSH can regain both momentum and altitude. It can use traditional rocket propulsion, electromagnetic induction as cable conductor slices through Earth’s magnetic fields or it can “catch” another deorbiting object to gain momentum.

SUMMARY: RSH is cheaper to build and to operate than the SE. Furthermore, a prototype has already been built (not so for SE.)

SFIA: SKYHOOK

Boeing’s HOSTAL Project

ROTAVATOR PRO’s and CON’s

PROs: ADVANTAGES OVER TRADITIONAL ROCKETS

1) Reduced Launch Costs: Skyhook's "slingshot" momentum decreases overall fuel requirements which greatly lowers overall gross weight of payload spacecraft.

2) Faster Transit Times: Skyhooks might enable faster interplanetary travel by quickly accelerating much lighter spacecraft to increased velocities.

3) Faster Turnaround Times: Rocket launches typically take months to accomplish. A synchronized rotating Skyhook can rendezvous same Earth location at same time daily.

MORE PROs: ADVANTAGES OVER THEORETICAL SPACE ELEVATOR (SE)

· Much Less Mass: Skyhook tether is much shorter than SE tether; thus, Skyhook deploys much quicker, cheaper and easier than SE without the onerous tensile strength requirement of a 36,000 km tether from Earth to GEO.

· History: A space tether was deployed in 1996 as part of a space shuttle mission. February 22, 1996, Space Shuttle (STS-75) deployed a rotating tether system. Known as the TSS-1R mission, it studied basic space tether behavior and space plasma physics. SE has yet to deploy anything.

· Ease of Deployment: Unlike a GEO based space elevator; the much shorter skyhook would not touch the surface. Safer for Earthlings and much less infrastructure required.

CONs: CONSIDER FOLLOWING CHALLENGES

· Complex Maneuvering: Hook at rotating cable’s end, docking with Skyhook requires rigorous precision.

· Atmospheric Drag: Even at high altitudes, atmospheric drag can decay the skyhook's rotation and orbital path.

· Lift Capacity. Unlike SE, Skyhook requires a suborbital vehicle for payload to ascend from surface to hook.

SKYHOOK’s Chief Challenge: Tether Material.

Future tethers must withstand intense forces to rotate and accelerate payloads.

Traditional materials: bronze, steel, even kevlar, are far too weak.

Which materials might work for a very long tether???

2) TETHER MATERIAL ...

... for either Space Elevator or Skyhook

will likely come from a combination of following fabrics.

CARBON
NANO-TUBE (CNT)

GRAPHENE
SUPER LAMINATE (GSL)

BORON NITRIDE
NANO-TUBE (CNT)

CNTs use carbon atoms to form cylindrical molecules with unique and exceptional properties. They are essentially rolled-up sheets of graphene. Thus far, CNTs have not yet been manufactured in large quantities. CNT diameters typically measure in nanometers, and lengths extend only to mere micrometers; much more work to do.

KEY PROPERTIES:

High Strength: Much stronger than steel, CNTs have enormous tensile strength.

Conductivity: CNT’s excellent electrical conductivity can exceed that of copper and aluminum. If CNT cable passes thru Earth’s mag fields, expect electric current.

Heat Transfer: Able to withstand high temperatures, CNTs can transfer heat efficiently; thus, useful quality for thermal management applications.

GSL is a strong, lightweight material made by stacking atom thin layers of graphene. It is a leading candidate for space elevator tethers due to its exceptional strength and light mass. The material's high tensile strength (130 GPa) in the x/y direction makes it ideal for withstanding the immense forces involved in a space elevator. More details:

Composition: GSL is multiple layers of graphene, a single-atom-thick sheet of carbon atoms arranged in a hexagonal pattern with strong bonds.

Strength: Graphene is 200 times stronger than steel. GSL’s stacked graphene layers provides remarkable tensile strength.

Space Elevator (SE) Application: GSL's strength-to-weight ratio makes it a promising candidate for the SE tether; it can withstand enormous tension.

Ongoing Research: The International Space Elevator Consortium (ISEC) is actively researching GSL behavior under various thermal conditions. ISEC hopes that GSL commercial production might begin within 10 years, with SE as the primary user.

BNNTs are nanostructures with alternate boron and nitrogen atoms in a hexagonal lattice, much like CNTs.

KEY PROPERTIES: High value potential for many high tech applications.

Structure: BNNTs are rolled-up sheets of hexagonal boron nitride (h-BN), like CNTs are rolled-up graphene sheets.

Electrical Insulation: BNNTs are great insulators; this contrasts with CNTs, which are metallic or semiconducting. BNNTs have a wide bandgap (≈ 6 eV).

Thermal Stability: BNNTs exhibit excellent thermal stability, withstanding high temperatures (up to 900°C in air).

Mechanical Strength: BNNTs have high mechanical strength and stiffness.

Chemical Stability: They are relatively inert and resistant to oxidation; thus, they might well thrive in harsh environments.

SYNTHESIS METHODS: Manufacture BNNT via various methods, including:

Arc Discharge: Leverage an electrical arc between boron and nitrogen sources.

Laser Ablation: Laser vaporizes boron which then reacts with nitrogen gas.

Chemical Vapor Deposition (CVD): Gaseous precursors form BNNTs on a substrate.

WHICH WORKS BEST???? PERHAPS ALL THREE...

materials will contribute to the manufacture and deployment of extremely long tethers for both Space Elevator (SE) and Skyhook (SH); maybe interwoven throughout entire tether. In any event, SH requires a much shorter tether with less mass; thus, SH tether tensile strength requirements will be much less rigorous than SE; thus, “Hook” might deploy in a few years while “Elevator” deployment will likely take decades if ever.

SFIA: SPACE ELEVATOR MATERIALS

RICE UNIVERSITY’S GRAPHENE PROJECT

Other New Materials: Zylon Dyneema

SKYHOOK’s 2^nd Chief Challenge: Elevating From Ground to Hook.

Skyhook requires a suborbital vehicle for payload to ascend from surface to hook. Obvious choices include:

1) ROCKETS; however, we’re trying avoid expensive, risky rockets.

2) AIRCRAFT are very flexible, and they can hover for many hours.

3) DIRIGIBLES (aka “blimps”, lighter than air ships) are not as fast as aircraft, but they can hover for days, weeks, even longer with virtually no fuel.

History of Airborne Spacecraft Launches

1949, Rockoon (combine rocket + balloon) was a sounding rocket carried into upper atmosphere via a gas-filled balloon; at altitude, it separated from the balloon to ignite and travel much higher.

August 12, 1960, Project Shotput launched Echo 1, first passive communications satellite, Echo 1 was a large reflective balloon designed to bounce signals back to Earth. It gained an altitude of 300,000 kilometers and speed of Mach 10 (10x speed of sound).

On April 5, 1990, Pegasus Rocket launched from a NASA B-52 aircraft at 39,000 feet and 530 mph. At altitude over open ocean, Pegasus deployed from host aircraft into a free-fall for five seconds before igniting its first stage, rocket motor; thence, orbited satellites in 10 minutes.

Today, John Powell’s Airship To Orbit (JP ATO)

routinely uses airships to deliver payloads

as high as 300,000 kms at Mach 10.

See following details.

3) AIRSHIP TO ORBIT (ATO)

Airship to Orbit (ATO) is an JP Aerospace enterprise which proposes a series of airships to routinely transport payloads from Earth to orbit, with no need for risky rockets. ATO three-stages are described below.

1. Atmospheric Ascender Airship would transport payloads from the Earth's surface to the Dark Sky Station at 140,000 feet.

2. Dark Sky Station is a transfer point for cargo and personnel, a construction facility and a telecommunications hub.

3. Orbital Ascender Airship is a massive, lighter-than-air vehicle that would ascend from the Dark Sky Station to orbit.

ATO advantages might include:

· Leverages readily available “old” tech (i.e. blimps).

· Much cheaper than current launch systems.

· Much safer than risky rockets.

Challenges include:

· Orbital velocity with plasma engines (new tech).

· Achieve lift at extreme altitudes.

Wikipedia Article: Airship to Orbit

J P Aerospace Home Page

ATO PRO’s and CON’s

ATO PROs: ADVANTAGES OVER TRADITIONAL ROCKETS

· Manufacturing Costs: Blimps are fabric; modern rockets are huge steel monoliths.

o Spacex rockets cost less than legacy rockets, but still very expensive.

o JP ATO more capable than blimps; still much less $ than cheapest rocket.

· Operating Costs: Blimps (and other airships) use virtually no fuel.

· Launch Locations: ATOs launch/go anywhere; rockets need fixed infrastructure.

MORE ATO PROs: ADVANTAGES OVER TETHER SOLUTIONS

· Reduced Mass: ATO requires no tether. HOWEVER, SH requires 600 kms of high tensile strength fabric, very heavy; SE tether is much more massive cause it requires 36,000 km of fabric from ground to GEO and another 64,000 km more from GEO toward space.

· Launch Locations: ATO can launch from anywhere and hover anywhere.

o SE is fixed location with rigorous queue restrictions.

o SH has fixed orbit with rigorous perigee/apogee schedules.

· Faster Turnaround: Deploying ATO is easy (it’s been done several times.) Deploying SE is still theoretical; deploying SH is still prototypal.

ATO CON: Chief Challenge is achieving orbit after Ground Ascender reaches altitude.

Maybe Skyhook can help!!!!

Airship's Best Feature: ATO can efficiently float payloads from the ground to a high atmospheric platform.

Rotavator's Best Feature: RSH can quickly scoop payloads from an atmospheric platform and fling them into space.

Can we combine best of each?

4) HYBRID SOLUTION:

Combine Rotovator & ATO

ATO LIFTS PAYLOAD OFF THE GROUND