Earth to orbit concepts Archives - Opening the High Frontier

Starship Congress 2017, Combination Launch Systems

How a Combination Launch System Works

A combination launch system works by reducing the total speed a launch vehicle needs to achieve in order to reach low Earth orbit. For a launch vehicle to reach low Earth orbit on its own without the assistance of a combination launch system, it needs to accelerate to a very high apparent velocity. In addition to achieving the actual speed of low Earth orbit (7,800 m/s), it also needs additional speed to overcome the force of gravity that is working to pull it down as it boosts for orbit and even more speed for overcoming the aerodynamic drag that wants to slow it down. There are also other issues that require additional speed to overcome but these are the three largest. It is the total of all these speeds that are known as the apparent velocity, or the total speed required for orbit. When all of these speeds are combined together, the total amount of speed required to reach low Earth orbit when launching due east from Cape Kennedy is approximately 9,100 m/s. This is the total speed required for orbit.

For a launch vehicle to reach orbit and do it affordably it also needs to be single stage, completely reusable, and carry a large enough payload to make it all worthwhile. Unfortunately, it is not possible to build such a launch vehicle with current technology. The reason for this is the amount of propellant that it takes to get to low Earth orbit using chemical rocket motors. Using the highest performance chemical rocket motors available, a Single Stage To Orbit launch vehicle would need to be 87% propellant when it leaves the launch pad. That leaves only 13% for the launch vehicle and payload. That is not enough to make a Single Stage To Orbit launch vehicle completely reusable and still carry enough payload to make the cost per pound to orbit mass market affordable.

The closest we have come to building such a launch vehicle was the partially reusable Two Stage To Orbit Space Shuttle. It had a propellant fraction of 87.2%, an empty weight fraction of 12%, and a maximum payload fraction of 0.8% when flying to the International Space Station. It was also significantly more expensive to fly than any expendable launch vehicle.

This is where the combination launch system with non-rotating skyhook comes in. The first step of a combination launch system consists of either

an air-assisted launch

or a ground assisted launch.

A subsonic air-assisted launch using a carrier aircraft like Stratolaunch will reduce the total speed required for orbit by approximately 1,100 m/s. A 600 MPH ground assisted launch from a mountain top will reduce the required speed for orbit by approximately 900 m/s. An air-assisted launch will require the addition of wings to the launch vehicle. This will increase the empty weight of the launch vehicle as well as the drag. A ground assisted launch does not require this. The end result is they are both about the same when it comes to reducing the total speed required for orbit. Reducing the total speed required for orbit by 900 m/s will reduce the necessary propellant fraction from 87% to 84%.

If it is assumed that a fully reusable single stage launch vehicle requires an empty weight fraction of 15% to be built (3% more than the empty weight fraction of the Space Shuttle, or a 25% increase in empty weight), then either of these assisted launch concepts will make this kind of launch vehicle possible but the payload fraction will still be only 1%.

If it is assumed that a fully reusable single stage launch vehicle will reduce the cost to orbit by 90% compared to existing expendable launch vehicles, then this system will reduce the cost to orbit to approximately 1/10th of what it is today.

If this reduction to the propellant fraction is used to increase the payload fraction from 1% to 4% and the launch vehicle is left unchanged, then this system will reduce the cost to orbit to approximately 1/4th of what it is today.

Either approach will work, but neither of them by themselves will make spaceflight affordable to everyone.

Now add the non-rotating skyhook to the launch system. One design for a mature non-rotating skyhook has an overall length of 2,200 km and a lower endpoint velocity of 80% of orbital velocity for its altitude. This will reduce the total speed required for the launch vehicle by 1,560 m/s. Combine this with the 900 m/s velocity reduction that comes with a ground assisted launch and the total speed required for flying to the lower end of the skyhook becomes 6,640 m/s. If it is assumed that the same high-performance LOX/LH2 rocket motors are used, the required propellant fraction will drop from the original 87% to 77%. If it is also assumed that the fully reusable Single Stage To Skyhook launch vehicle can be built with an empty weight fraction of 15%, the payload fraction becomes 8%. This is 10 times the payload fraction of the Space Shuttle when it flew to the International Space Station. Now, keep in mind that this launch vehicle is a fully reusable Single Stage To Skyhook vehicle that is expected to cost 1/10th the amount to fly as any expendable launch vehicle and combine this number with the 10 fold increase in payload. That works out to 1/10th divided by 10 = 1/100th the cost in dollars per pound to orbit of any expendable launch vehicle flying to orbit without the assistance of a combination launch system. In other words, if the original expendable launch vehicle cost $10,000 per pound to orbit, the fully reusable Single Stage To Skyhook vehicle will cost somewhere in the neighborhood of $100 per pound.

If an empty weight fraction of 15% is not enough to build a fully reusable Single Stage To Skyhook launch vehicle, increase the empty weight fraction to 18%. That is 50% more than the empty weight fraction of the Space Shuttle. This will reduce the payload fraction to 5% and increase the cost of flying to the skyhook to $160 per pound. To get back to the $100 per pound launch cost it will be necessary to increase the length of the skyhook until the lower endpoint velocity is moving at 73% or orbital velocity for its altitude. This will reduce the propellant fraction of the launch vehicle to 74% and increase the payload fraction back to 8%.

This $100 per pound launch cost is what most people think of as affordable to everyone spaceflight. Think about that. If the seat weight per passenger is 200 pounds, that means the price for a ticket to the lower end of the skyhook will be in the neighborhood of $20,000. How many people would buy a ticket to fly to the lower end of the skyhook for that price? Would you?

What the view from the lower end of a skyhook will look like.

Impact on Lift-off Weight

Another issue that doesn’t get discussed very often is how the combination launch system and non-rotating skyhook affect the lift-off weight of the launch vehicle. Existing launch vehicles are large and that adds to their cost. The Space Shuttle had a lift-off weight of over 4.4 million pounds. The Falcon 9 has a lift-off weight of over 1.2 million pounds. The largest Delta launch vehicle has a lift-off weight of 1.6 million pounds. By comparison, the Boeing 737 MAX airliner has a take-off weight of 195,000 pounds. As the old saying goes, size matters, and it has a direct impact on cost.

What determines the lift-off weight of a launch vehicle is its payload fraction and the size of the payload that needs to be delivered. If the design payload size for a conventional launch vehicle along the lines of the Atlas or the Delta is 12,000 pounds, and the payload fraction for the launch vehicle is 1%, then the lift off weight will be 1.2 million pounds. If the design payload size for a fully reusable Single Stage To Skyhook launch vehicle using the previously mentioned ground accelerator and skyhook is also 12,000 pounds and it has a payload fraction of 8%, its lift off weight will be 150,000 pounds. That is 1/8th the lift-off weight of the conventional expendable launch vehicle and on top of that, it is fully reusable. It is also worth noting that its lift-off weight is less than the take-off weight of the Boeing 737 MAX.

Now notice the empty weight. If the empty weight fraction is 15% its empty weight will be 22,500 lbs. If the empty weight fraction is 18% its empty weight will be 27,000 lbs. By comparison, the empty weight of the Space Shuttle Orbiter was 172,000 pounds. Either way, imagine how much easier it will be to move the Single Stage To Skyhook vehicle around, to service it, and to prep it for flight.

When it comes to cost, smaller is definitely better.

What an air-launched fully reusable Single Stage To Skyhook launch vehicle might look like.

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Making Spaceflight Affordable

Making spaceflight affordable to everyone is the single most important issue that needs to be addressed if we are ever going to build a spacefaring civilization. Without it, there will be no cities on the Moon or Mars, no space colonies or orbiting hotels, no spaceports where you can go to purchase a ticket for a ride to orbit, and no orbiting factories or research stations other than those that are funded by the government. Like it or not, making spaceflight affordable to everyone is an absolute necessity that needs to happen if we are ever going to open the high frontier for large scale settlement and development. Unfortunately, making spaceflight affordable to everyone is a task that has been ignored for far too long.

Currently, the best idea for making spaceflight affordable to everyone is a combination launch system with a non-rotating skyhook. But that is not the only way to reduce costs. There are smaller ideas that will not by themselves make spaceflight affordable to everyone but will still significantly reduce costs. These are ideas that fall under the category of “working smarter, not harder.”

An example of one of these ideas is the way we currently resupply the International Space Station. The three spacecraft that are currently used to do this are;

the Russian Progress spacecraft which has a launch mass of 7,150 kg and a cargo capacity of 2,230 kg,

the enhanced Cygnus spacecraft which has a dry mass of 1,800 kg and a payload capacity of 3,500 kg when launched using the Atlas V launch vehicle,

and the Dragon spacecraft which has a dry mass of 4,200 kg and a payload capacity of 6,000 kg.

So in regards to making spaceflight affordable to everyone, the question becomes, how cost effective are these vehicles in terms of dollars per pound to orbit?

As part of the Commercial Resupply Services program, NASA paid SpaceX $1.6B for 12 resupply flights to the ISS using the Dragon spacecraft. Assuming that each of those flights carried the maximum possible payload of 6,000 kg (13,200 lbs), that works out to a cost per flight of $133.3M, and $10,100 per lb of useful payload delivered.

NASA also paid Orbital Sciences $1.9B for 8 resupply flights using the Cygnus spacecraft as part of the same program. Assuming that each of those flights carried the maximum possible payload of 3,500 kg (7,700 lbs), that works out to a cost per flight of $237.5M, and $30,800 per lb of useful payload delivered.

Now divide the total cost of those two programs ($3.5B) by the total amount of useful payload that could have been delivered by those 20 flights if they had carried the maximum possible payload (220,000 lbs), and the cost average for those two programs works out to $15,900 per lb.

No matter how you look at it, that is a lot of money to pay for hauling the freight. Putting it in more mundane terms, it means that the 2 oz granola bar you ate the other day while standing in line at the grocery store would cost you $1,987 if you were on the International Space Station.

An Alternate Method (Working Smarter)

When used as an expendable launch vehicle, the Falcon 9 Full Thrust rocket can boost 22,800 kg of payload to a low Earth orbit that has an inclination of 28.5 degrees. When flying to the orbit of the International Space Station it can lift approximately 20,000 kg. Now assume that this payload is loaded into a pressurized cylindrical canister like the one used on the Cygnus spacecraft and that this container has a guesstimated empty mass of approximately 800 kg. That means the Falcon 9 rocket without the Dragon spacecraft could deliver a useful payload of 19,200 kg to the same orbit as the ISS. Now assume that a reusable on-orbit serviceable and refuelable version of the service module that propels the Cygnus spacecraft is kept at the ISS for retrieving these pressurized payload containers and that it uses approximately 400 kg of propellant in the process of doing that. That leaves a useful payload delivered to the ISS of 18,800 kg.

Think about this for a moment. This is not a big change. There is no new hardware to develop, and there is no new technology to develop. It is just a different way of doing what we are already doing with existing hardware. It is a simple change in how we operate.

So what is the big deal, why bother to change anything?

The cost of a flight on the Falcon 9 without the Dragon spacecraft is $62M. Now divide that cost by the 18,800 kg (41,360 lbs) of useful payload delivered to the International Space Station by this method. The result is $1,500 per lb to orbit.

The cost of that granola bar just dropped from $1,987 to $187.

This is an example of working smarter. While it still does not lower the cost of spaceflight enough to make spaceflight affordable to everyone, it definitely is a step in the right direction and it is something we can do right now. It is also not the only idea that we can use to reduce the cost of spaceflight.

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Stratolaunch and the X-15

Stratolaunch is the big, beautiful carrier aircraft that is being built by Vulcan Inc. for launching rockets into space. It has an empty weight of approximately 500,000 pounds, holds up to 250,000 pounds of aviation fuel, and is designed to launch a rocket that weighs up to 550,000 pounds. It is one monster sized incredible achievement on the part of Paul Allen and Vulcan Inc. Hopefully, it will become the first step of a combination launch system that will make spaceflight affordable to everyone. A fully reusable combination launch system that could eventually consist of Stratolaunch, an X-15 style first stage, a vertical landing suborbital upper stage, a suborbital spacecraft with a built-in upper stage rocket motor for carrying passengers and cargo, and a space station equipped with a basic 200 to 400-kilometer long non-rotating skyhook for the suborbital spacecraft to dock with.

While that might sound like a lot of parts and complexity in order to get to orbit, keep in mind it is a 100% fully reusable system that is less than half the size of existing rockets, that can be affordably built on a step by step basis using existing technology, and that will make spaceflight affordable to just about everyone. Also keep in mind, that as the skyhook is made longer, it will become possible to eliminate the vertical landing suborbital upper stage from the system. In addition, as the skyhook continues to be made longer, it will also become possible to add a Rocket Based Combined Cycle (RBCC) propulsion system to the X-15 style first stage so that it can fly directly to the bottom of the non-rotating skyhook without the need of any upper stages.

An example of how this step by step development might work is as follows.

In order for Stratolaunch to get started as a low-cost Earth to orbit satellite launch system, it needs two things. It needs a winged reusable first stage launch vehicle similar to the X-15, and it will need a small expendable two-stage solid propellant rocket. This is exactly what was proposed back in 1962 for making the X-15 into a low-cost satellite launch system.

The X-15

In June of 1952, the National Advisory Committee for Aeronautics (the predecessor to NASA) decided to expand its research aircraft program to include aircraft designs capable of speeds between Mach 4 and 10, and altitudes of 12 to 50 miles. This led to a number of paper studies that resulted in a joint program between NACA/NASA, the U.S. Air Force, and the U.S. Navy, to build a hypersonic research aircraft. In late 1954 it was decided that this aircraft was to be capable of Mach 6.6+ and be able to reach 250,000 feet or more of altitude. As with previous rocket-powered research aircraft, it was to be air launched. It was the beginning of the incredibly successful X-15 program that resulted in 199 flights, speeds in excess of Mach 6.6 (4,500 MPH), and altitudes in excess of 350,000 feet (66 miles). It was this program that also led to the idea of attaching a small expendable rocket to the underside of the X-15 for launching small satellites into orbit. If this had been done it would have become the world’s first combination launch system. Pictures of what this would have looked like can be seen here.

The X-15 was designed for a maximum dynamic pressure of 2,500 pounds per square foot, a positive load factor of 7.33 g’s, and a maximum temperature of 1,200 degrees F. The skin of the X-15 was made of a nickel alloy called Inconel X, and the internal structure was mostly titanium. Sixty-five percent of the structure was welded.

The propulsion system used on the X-15 was the XLR-99 rocket motor which had a vacuum thrust of 57,000 pounds, a vacuum specific impulse of 279 seconds, a propellant flow rate of 213.8 pounds per second, and weighed 910 pounds. It was a variable thrust motor that could be throttled between 50% and 100% thrust and was restartable in flight. The propellants for the motor were anhydrous (water-free) ammonia and liquid oxygen.

The X-15 carried 8,400 pounds of fuel and 10,400 pounds of LOX in its internal tanks and had a useful burn time of 85 seconds. The empty weight of the X-15 was 15,000 pounds (of which 1,300 pounds was instrumentation) and the launch weight was 33,800 pounds. The top speed of the X-15 using internal propellant was 4,150 MPH (Mach 6). This speed was also the maximum it could achieve without supplemental thermal protection for the airframe.

An interesting proposed follow-on program to the X-15 that did not get built was the addition of a highly swept delta wing that was expected to increase its top speed from Mach 6 to Mach 8.

Additional pictures of a model of this concept can be seen here.

The Expendable Rocket

The small expendable rocket that was proposed for launching satellites from the underside of the X-15 was called the Blue Scout. It was a two-stage solid propellant rocket that was made from the 2nd and 3rd stages of the Scout rocket. The 1st stage of the Blue Scout had a launch weight of 4,424 kg and an empty weight of 695 kg. It had a vacuum specific impulse of 262 seconds. The 2nd stage had a launch weight of 1,400 kg and an empty weight of 300 kg. It had a vacuum specific impulse of 311 seconds. The pylon for attaching the Blue Scout to the underside of the X-15 had an estimated weight of 500 pounds. The amount of payload that could be delivered to low Earth orbit using this system was estimated to be 150 pounds. The total weight of the Blue Scout with payload and pylon came out to approximately 13,500 pounds. The top speed of the X-15 when carrying this additional weight was estimated to be 2,280 MPH.

So what is the purpose of all this information?

To rough out the design of a reusable first stage launch vehicle for Stratolaunch.

Use the highly swept delta wing version of the X-15 for the basic configuration. Build it with Inconel-X skins and a titanium interior. Build them two at a time, as prototypes, using soft tooling, 3D printing, and welding as much as possible. Plan on refining the design based on lessons learned every time a new pair is made. Make them as unmanned, computer-controlled remotely piloted vehicles. Use a modern LOX/Methane rocket motor. Make this vehicle large enough in proportion to the expendable rocket that it can reach a speed of 4,150 MPH before launching the rocket. Consider using carbon-carbon for the leading edges and nose of the vehicle in order to increase the launch velocity. This will increase the size of the reusable parts of the launch system while reducing the sizes of the expendable parts which will reduce the cost of getting to orbit. In order to keep development costs to a minimum, build the expendable two-stage rocket using existing solid-propellant rocket motors and hardware as much as possible.

Once the reusable delta wing X-15 style first stage vehicle and the two-stage expendable rocket are operational, start working on a vertical landing, LOX/Methane powered reusable upper stage rocket to replace the first stage of the expendable rocket.

Once that is done, start working on the reusable spacecraft with built-in rocket motor for carrying passengers and cargo to the bottom of the non-rotating skyhook. This built-in rocket motor to supply the remaining velocity for matching speed with the lower end of the skyhook and for landing when it returns to Earth if it is a vertical lander. The amount of propellant that it will need to carry will depend on the length of the non-rotating skyhook and how it lands. This spacecraft could be a vertical landing spacecraft like the Dragon V2 being developed by SpaceX or a horizontal lander like Dream Chaser.

Why the Delta Wing X-15?

Making spaceflight affordable to everyone is all about cost. The X-15 is flight proven concept that worked that was affordable to build and operate. That minimizes both risk and development cost when building a new vehicle. Both of which are necessary if launch costs are to be kept to a minimum. As for delta wings, they are lower in drag, don’t get as hot at high speeds, are lighter in weight, structurally redundant, simple to build, and have been used on just about every high-speed aircraft ever built. The Avro Arrow, the F-102, the F-106, the SR-71, the B-58, the XB-70, the X-24B lifting body, the Concorde, and the Space Shuttle, to name just a few. There were even a number of proposed space shuttle designs from the 1950’s that had delta wings.

Wernher von Braun’s “XR-1”, 1955.

Darrel Romick’s “Meteor”, 1956.

Boeing X-20, “Dyna-Soar”, 1957.

Another more modern delta winged version of the X-15 with expendable upper stage rocket that could be air-launched by Stratolaunch is the XS-1.

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Skyhooks and Space Elevators

A skyhook is a proposed space transportation concept that will help make spaceflight affordable to everyone. When used as part of a combination launch system it will make the building of a spacefaring civilization possible on a commercial basis. There are two kinds of skyhooks, a rotating skyhook, and a non-rotating skyhook.

A non-rotating skyhook is a much shorter version of the Earth surface to geostationary orbit Space Elevator that does not reach down to the surface of the Earth. It is much lighter in mass, can be affordably built with existing materials and technology, and in its mature form, is cost competitive with what is thought to be realistically achievable using a Space Elevator, assuming materials strong enough to build a Space Elevator ever become available. It works by starting from a relatively low altitude orbit and hanging a cable down to just above the Earth’s atmosphere. Since the lower end of the cable is moving at less than orbital velocity for its altitude, a launch vehicle flying to the bottom of the non-rotating skyhook can carry a larger payload than it could otherwise carry to orbit. When the non-rotating skyhook is long enough, Single Stage To Skyhook flight with a reusable launch vehicle becomes possible at a price that is affordable to just about anyone.

Another way to understand the non-rotating skyhook is to think of it as a momentum exchange device that consists of a space station in a higher altitude, higher energy, elliptical orbit, with a cable that hangs down to just above the atmosphere. When a suborbital spacecraft coming up from the Earth docks at the lower end of the cable, it pulls the space station down into a slightly lower more circular orbit. In effect, the space station gives up some of its energy to the arriving spacecraft so that the arriving suborbital spacecraft can stay in orbit instead of falling back to Earth. When the spacecraft lets go of the lower end of the cable to return to Earth, it gives that energy back which allows the space station to return to a higher altitude, higher energy, more elliptical orbit. The end result is that the energy that is exchanged between the non-rotating skyhook and the arriving spacecraft and then returned to the skyhook when the spacecraft departs, gets used over and over again every time a spacecraft makes a trip to the skyhook. This exchange and reuse of energy reduces the amount of propellant the launch vehicle needs to carry which allows it to carry more payload. Less propellant also makes for a smaller, lighter, and more affordable launch vehicle. More payload means that the cost of the launch can be spread out over a larger amount of cargo. Both of these changes reduce the cost per pound of getting to orbit. When the skyhook cable is long enough, airliner like operations to space become possible at airliner like prices.

History

The idea for a non-rotating skyhook evolved from the idea of an orbital tower which was first proposed by Konstantin Tsiolkovsky back in 1895. The orbital tower consists of a really tall tower that goes from the surface of the Earth all the way to geostationary orbit. Its purpose was to provide an economical way of getting to orbit so that the human race could start building a spacefaring civilization. The reason for wanting to build a spacefaring civilization was to avoid the projected collapse of our civilization at some time in the near future due to overpopulation. A collapse that is considered by many to be inevitable if we remain a single planet species. So why not use rockets? Konstantin Tsiolkovsky knew about rockets. After all, he is the person who first worked out the mathematics for using rockets to travel through space to other planets. As a result of that work, he knew just how uneconomical chemical powered rockets are, which is why he wanted to find a better way of getting into orbit. He got the idea for the orbital tower as a result of a trip to Paris, France where he saw the Eiffel Tower. While he knew that such an orbital tower could not be built, he felt certain that the existence of a theoretical solution to the rocket problem would eventually lead to a real world solution that could be built. He was right.

The idea of the orbital tower led to the creation of the space elevator concept, another idea that cannot be built. That led to the idea of a rotating skyhook, a type of rotating space elevator that rotates in the plane of its orbit like a two spoke wheel rolling across the top of the atmosphere as it orbits the planet. While this idea can be built with existing materials, it also has three very significant operational problems that have yet to be solved.

The first of these is the very short amount of time that is available for an arriving spacecraft to hook up with the end of the cable. A rendezvous window that is literally only three to five seconds long. This is what engineers and scientists like to call a “non-trivial problem.”

The second problem is maintaining the synchronization between the rotation rate of the skyhook with its orbital period. Since the rotating skyhook is in an elliptical orbit, the rotation rate of the cable needs to be in sync with the amount of time it takes to orbit the Earth so that the lower end of the cable will be at the bottom of its swing when the rotating skyhook is at the low point of its elliptical orbit. When a spacecraft docks with the lower end of the rotating skyhook at the low point of its orbit, it pulls the skyhook down into a lower orbit with a shorter orbital period. Since the rotation rate of the cable does not change when the rendezvous occurs, the rotation rate of the rotating skyhook is now out of sync with the new orbit. The rotating cable will need to be brought back into sync with the orbit before another spacecraft can use the system. This is another non-trivial problem.

The third problem with the rotating skyhook has to do with how the release orbit of the spacecraft occurs one-half a rotation after a spacecraft docks with the cable at the bottom of its swing. This linkage of the release orbit to the time of arrival causes a problem in that only a very small percentage of the release orbits will be pointed in the right direction for a spacecraft that is going to the Moon and beyond. The only solution to this is to limit the departure speed of the spacecraft to a speed that will take it to a higher altitude orbit where the spacecraft will use its onboard propellant to circularize its orbit and wait until it is in the correct position to boost for its final destination. This noticeably limits the usefulness and cost advantage of the rotating skyhook for manned spaceflights to the Moon and beyond.

It was the search for a workable solution to all these problems that led to the creation of the non-rotating skyhook. A skyhook that can be affordably built and operated with existing materials and technology and that doesn’t have the problems of the rotating skyhook.

For more detailed information about what a non-rotating skyhook is and how it works, go here.

A 200-kilometer long basic Non-rotating Skyhook configured to receive a suborbital spacecraft coming up from the Earth.

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Combination Launch Systems, part 2

Component One The first component in a combination launch system is a choice between using an air assisted launch as shown in this video

or a ground assisted launch as shown in this picture.

argus2

As with any competing concepts, there are advantages and disadvantages to both of these systems. Some of the advantages and disadvantages are technical, some are operational, some are financial, some of them vary depending on the other components of the combination launch system, and some are political. There are also a number of design variations to both of these systems, the selection of which is usually determined by the specific needs and goals of the developer. For example, an air launch can be either subsonic or supersonic, and a ground assisted launch can be on rails, with maglev, or trackless. Ground assisted launch can also be enclosed in a tunnel as shown in the picture, out in the open on a mountainside, or on a vertical tower.

Component Two The second key component in a combination launch system is making the launch vehicle reusable. There are a number of ways this can be done with the best choice being determined by the total amount of velocity reduction that is made possible by the air assist/ground assist launch, the non-rotating Skyhook (component three), and the type of propulsion system being used on the launch vehicle (component four). If it is an all-rocket powered launch vehicle with an entry level length Skyhook, the best choice for the initial launch vehicle will be a reusable first stage/expendable upper stage configuration. If air-launched, the launch vehicle will use a winged horizontal landing first stage, if ground accelerator launched on a steep enough slope so that wings are not needed, a vertical landing first stage like the ones being developed by SpaceX and Blue Origin will be best.

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As the length of the Skyhook is increased and the velocity reduction for the launch vehicle increases, it will become possible to combine the expendable upper stage of the launch vehicle with the spacecraft. This will make the launch vehicle into a fully reusable two stage to Skyhook launch system which will further reduce the cost of getting to orbit. As the Skyhook continues to be made longer and the velocity reduction continues to increase, it will eventually become possible to make the launch vehicle into a single stage to Skyhook vehicle. This will reduce the cost of getting into space even more.

Component Three The third key component in a combination launch system is a non-rotating Skyhook (see section 3.2.1 on page 7). A non-rotating Skyhook is a vertically oriented cable that is attached to a space station. Since the speed of orbit goes down with increasing altitude, the lower end of the cable is moving at less than orbital velocity for its altitude, and the upper end of the cable is moving faster than orbital velocity for its altitude. The end result is that a launch vehicle arriving at the lower end of the cable does not have to go as fast as it would need to without the Skyhook. This reduced velocity requirement allows the launch vehicle to carry a larger payload which reduces the cost of getting to orbit.

non-rotating_skyhook_with_spaceplane

A non-rotating Skyhook works on the same principles as an Earth surface to geostationary orbit Space Elevator. The main differences between them are that the Skyhook is much shorter, it does not reach down to the surface of the Earth, and it can be affordably built with currently existing materials and technology.

Component Four The fourth key component in a combination launch system is a combination air-breathing and rocket motor propulsion system. This works by reducing the amount of oxidizer the launch vehicle needs to carry which makes for a smaller and more affordable launch vehicle. The reduced propellant requirement also increases the payload fraction and thereby reduces the cost to orbit.

There are many ways to make a combination air-breathing and rocket motor propulsion system. They can be rocket/ramjet combinations, ducted rocket/ramjet combinations, ducted rocket/ramjet/scramjet combinations, and so on. All of them have different costs, different weights, and different performance advantages. Some of them will require extensive development effort, some will not. The one that reduces the cost to orbit the most will depend on the details of all the other components of the combination launch system, the amount of development work required, and the flight rate.

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It’s All About Speed!

When a rocket takes off from the surface of the Earth and flies into orbit it increases its velocity by approximately 9,100 meters per second. That breaks down to 7,800 meters per second for the speed of orbit and 1,300 meters per second for drag and gravity losses. That is a lot of speed and it takes a lot of propellant to go that fast.

An example of just how much propellant is required is the Space Shuttle. Sitting on the launch pad waiting to take-off, the Space Shuttle was 85% propellant, 14% launch vehicle, and 1% payload. If Earth to orbit spaceflight is ever going to be affordable to everyone, the launch vehicle will need to be both fully reusable, and able to carry a large enough payload that it makes it worth all the trouble. Up to now, that has not been possible. To make the Space Shuttle fully reusable it would have been necessary to make it both larger and heavier which would have required a larger propellant fraction and that would have made the payload go to zero. Obviously, not a very workable solution.

This is where reducing the speed to orbit comes in.

There are a number of ways to do this. One is to use a ground accelerator that is located on the side of a tall mountain to boost the launch vehicle up to 600 MPH before starting its engines. This reduces the speed to orbit in two ways, by the speed added to the launch vehicle by the ground accelerator, and by reducing the drag and gravity losses that would have been incurred by the launch vehicle if it had accelerated to this speed and altitude on its own.

Another way to reduce the speed to orbit is to use a Skyhook at the upper end of the flight profile. Skyhooks can be short or long. The best way to use a Skyhook is to start small while the flight rate is low and gradually grow it into a longer and stronger version as demand increases. For this example, a short Skyhook, like the one shown in this video was selected.

The total velocity reduction made possible by the 600 MPH ground accelerator and 200-kilometer long basic Skyhook used in this example is 1,060 meters per second. This reduction in velocity will triple the amount of useful payload that can be delivered to the Skyhook compared to the same expendable launch vehicle flying to a space station without a ground accelerator or Skyhook. Increasing the amount of useful payload by a factor of three will reduce the cost to orbit to 1/3 of what it was without the ground accelerator and Skyhook. If it is assumed that the first stage of this launch vehicle is made reusable like the first stage of the Falcon 9, it then becomes reasonable to assume an additional 50% reduction in launch costs. This will reduce the cost to orbit to 1/6 of the cost of flying the expendable version of this launch vehicle without the ground accelerator and Skyhook.

And this is only the beginning. The longer the Skyhook becomes the lower the price becomes. Once the Skyhook is long enough it then becomes possible to use a fully reusable single stage launch vehicle that will reduce the cost even more. Best of all, the 600 MPH ground accelerator, the basic Skyhook, the reusable first stage launch vehicle, they can all be affordably built right now with existing materials and technology.

For more information about this and other related cost reducing concepts, read the book “Opening the High Frontier”.

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Combination Launch Systems

A combination launch system is a launch system that consists of multiple launch technologies that work together to boost a payload into orbit for a small fraction of the cost of current launch vehicles. It works by reducing the amount of velocity that the rocket-powered components of the launch system need to achieve. This reduces the propellant fraction and increases the payload fraction of the launch vehicle to such a degree that airliner like operations to orbit and beyond using a fully reusable launch system becomes possible.

There are many different launch concepts that we can build today that can be combined together to do this: subsonic air-launch; supersonic air-launch; ground accelerators; combination air-breathing and rocket motor propulsion systems; reusable launch vehicles; and, Skyhooks. All of these concepts can be combined together to reduce launch costs.

There are also many ways to combine these concepts to make a combination launch system. They can be as simple as using a vertically oriented ground accelerator to boost an existing expendable launch vehicle up to 600 MPH prior to igniting the rocket motor. They can be as complex as combining air-launch with a reusable launch vehicle that uses a combination ramjet, scramjet, rocket motor propulsion system, that flies to the lower end of a non-rotating Skyhook. The hard part is finding the combination that has the lowest user cost to orbit for the lowest possible initial investment, that is sized for the existing launch market and has the potential to grow as the market grows.

Combination launch systems are not new. The Wright brothers used a catapult to boost their first airplane up to flight speed. The U. S. NAVY still uses catapults to launch fighter planes from the decks of aircraft carriers. The world’s first reusable rocket plane, the Bell X-1, was air-launched from a B-29. Probably the best-known combination lunch system was the X-15 rocket plane that was air-launched from a B-52. In 1967 they even test flew a mock-up of a scramjet on the X-15. There were also studies that examined the possibility of supersonic air-launching a rocket-ramjet-scramjet powered delta winged version of the X-15 from the back of the XB-70 at Mach 3 that had a small expendable upper stage rocket for carrying satellites into Earth orbit.

Unfortunately, all of these ideas fell by the wayside in our rush to beat the Russians to the Moon back in the 1960’s. It wasn’t until SpaceShipOne flew in 2004, the start of Virgin Galactic and Stratolaunch shortly thereafter, as well as SpaceX and Blue Origin developing reusable first stage launch vehicles, that we have seen a serious effort to develop combination launch systems again. The only negative about this is that none of these new combination lunch systems go far enough.

While all of these concepts, ground accelerators, air-launch, reusable first stage launch vehicles, air-breathing and rocket propulsion systems, and Skyhooks, will reduce the cost of getting to orbit, none of them, by themselves, will make spaceflight affordable to everyone. To do that we will need a launch system that combines almost all of these concepts together.

Once we have that, the solar system is ours.

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Why do Rockets Cost so Much?

In the late 1960s and early 1970s, America used the Saturn V rocket to go to the Moon. The first stage of that rocket had an empty weight of 130,000 kilograms and carried 2,160,000 kilograms of propellant. It was used only once and then thrown away.

The second stage of the Saturn V had an empty weight of 40,100 kilograms and carried 456,100 kilograms of propellant. Like the first stage, this stage was used only once and thrown away.

The third stage of the Saturn V had an empty weight of 13,300 kilograms and carried 106,600 kilograms of propellant. This stage was also used once and thrown away.

The total empty weight of those three stages was 183,400 kilograms.

By comparison, a Boeing 747 has an empty weight of 183,000 kilograms. The 747 can fly 15 hours per day, 11 months per year, and has a useful life of 20 years. It also carries three hundred plus passengers per flight.

Each Saturn V made only one flight and carried only three passengers.

This is why spaceflight that is based on using expendable rockets costs so much.

This is also why SpaceX and Blue Origin are working so hard to develop reusable rockets.

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Skyhook, a Journey to Orbit and Beyond!

One of the key concepts discussed in the book “Opening the High Frontier” is the idea of a combination launch system. This is a concept that combines multiple launch technologies together in order to reduce the velocity that the rocket-powered components of the launch system need to achieve to reach orbit. One of those launch assist systems is called a non-rotating Skyhook.

This video shows how a non-rotating Skyhook works.

It starts with an Orion spacecraft on a suborbital flight path that will take it within reach of a crane located at the lower end of the Skyhook. Upon capture, the crane docks the Orion with the Lower Endpoint Station. The Midpoint Station on the Skyhook, which was positioned at the upper end of the Skyhook cable for the rendezvous, then starts moving down the cable to the Lower Endpoint Station. Once the Midpoint Station and Lower Endpoint Station come together and dock, the ion propulsion system on the Midpoint Station is activated in order to start raising the orbital altitude of the Skyhook. While this is going on, the crew and passengers of the Orion spacecraft will transfer to the Midpoint Station and the Orion spacecraft will be transferred to one of the docking ports at the upper end of the Midpoint Station. Next, the Midpoint Station undocks from the Lower Endpoint Station and starts moving up the cable to the Upper Endpoint Station. Upon arrival, the passengers, crew, and Orion spacecraft are transferred to the Upper Endpoint Station, and the Midpoint Station starts back down the cable to the lower end. Once the Midpoint Station has arrived at the lower end of the cable and the Skyhook is at the proper orbital position, the Orion spacecraft is released from the Upper Endpoint Station to a higher orbit.

The power for all these orbit changes comes from the ion propulsion system on the Skyhook. Since the ion propulsion system is much more fuel efficient than a chemical rocket motor, the amount of propellant that needs to be carried into orbit is greatly reduced. The reduced velocity required for flying to the lower end of the Skyhook also increases the payload fraction of the launch vehicle and allows for the use of a smaller reusable launch vehicle, all of which reduces the cost of getting to orbit.

The Skyhook shown in the video is called a Basic Skyhook. In the beginning, when combined with an air-launched reusable first stage launch vehicle, it has the potential of reducing the cost to orbit by 85%. Over time, as the Skyhook is made longer, the launch vehicles get better, and the flight rate increases, it has the potential of reducing the cost to orbit to $20,000 per person.

I strongly recommend watching the video full screen, with the sound turned up!

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