Remember that saving fuel also saves more fuel. For example, the Saturn V burned about 160 tons of fuel alone (exclusive of oxidizer), before it cleared the tower (~400', ~120m). That is over 5% of the total launch mass. Starting the rocket at 30,000 feet (9,000m) would save a large amount of fuel that requires more fuel to lift.

When SpaceX was possibly going to be providing the rocket portion, it would have had only 5 Merlin engines. Higher altitude and less weight of fuel means less thrust needed, which means less engines.

This would be at least the 4th airplane-rocket combo to launch a satellite (3rd rocket family). And another advantage of the plane, it can be flown right on the equator for maximum free power.

Ah! I had wondered about a Merlin configuration for such a process.

Okay here is some info from existing [URL="https://en.wikipedia.org/wiki/Pegasus_(rocket)"]Pegasus[/URL] air launched rockets:
[QUOTE][B]Launch profile[/B]

In a Pegasus launch, the carrier aircraft takes off from a runway with support and checkout facilities. Such locations have included Kennedy Space Center / Cape Canaveral Air Force Station, Florida; Vandenberg Air Force Base and Dryden Flight Research Center, California; Wallops Flight Facility, Virginia; Kwajalein Range in the Pacific Ocean, and the Canary Islands in the Atlantic. Orbital offers launches from Alcantara, Brazil, but no known customers have performed any. The capabilities of Alcantara are superfluous to other sites, without being any more convenient.

Upon reaching a predetermined staging time, location, and velocity vector the aircraft releases the Pegasus. After five seconds of free-fall, the first stage ignites and the vehicle pitches up. The 45-degree delta wing (of carbon composite construction and double-wedge airfoil) aids pitch-up and provides some lift. The tail fins provide steering for first-stage flight, as the Orion 50S motor does not have a thrust-vectoring nozzle.

Approximately 1 minute and 17 seconds later, the Orion 50S motor burns out. The vehicle is at over 200,000 feet (61 km) in altitude and hypersonic speed. The first stage falls away, taking the wing and tail surfaces, and the second stage ignites. The Orion 50 burns for approximately 1 minute and 18 seconds. Attitude control is by thrust vectoring the Orion 50 motor around two axes, pitch and yaw; roll control is provided by nitrogen thrusters on the third stage.

Midway through second-stage flight, the launcher has reached a near-vacuum altitude. The fairing splits and falls away, uncovering the payload and third stage. Upon burnout of the second-stage motor, the stack coasts until reaching a suitable point in its trajectory, depending on mission. Then the Orion 50 is discarded, and the third stage's Orion 38 motor ignites. It too has a thrust-vectoring nozzle, assisted by the nitrogen thrusters for roll. After approximately 64 seconds, the third stage burns out.

A fourth stage is sometimes added for a higher altitude, finer altitude accuracy, or more complex maneuvers. The HAPS (Hydrazine Auxiliary Propulsion System) is powered by three restartable, monopropellant hydrazine thrusters. As with dual launches, the HAPS cuts into the fixed volume available for payload. In at least one instance, the spacecraft was built around the HAPS.

Guidance is via a 32-bit computer and an IMU. A GPS receiver gives additional information. Due to the air launch and wing lift, the first-stage flight algorithm is custom-designed. The second- and third-stage trajectories are ballistic, and their guidance is derived from a Space Shuttle algorithm.

[B]Carrier aircraft[/B]

[B]The carrier aircraft (initially a NASA B-52, now an L-1011 owned by Orbital) serves as a booster to increase payloads at reduced cost. 40,000 feet (12,000 m) is only about 4% of a low earth orbital altitude, and the subsonic aircraft reaches only about 3% of orbital velocity, yet by delivering the launch vehicle to this speed and altitude, the reusable aircraft replaces a costly first-stage booster.

The single biggest cause of traditional launch delays is weather. Carriage to 40,000 feet takes the Pegasus above the troposphere, into the stratosphere. Conventional weather is limited to the troposphere, and crosswinds are much gentler at 40,000 feet. Thus the Pegasus is largely immune to weather-induced delays and their associated costs, once at altitude. (Bad weather is still a factor during takeoff, ascent, and the transit to the staging point).

Air launching reduces range costs. No blastproof pad, blockhouse, or associated equipment are needed. This permits takeoff from a wide variety of sites, generally limited by the support and preparation requirements of the payload. The travel range of the aircraft allows launches at the equator, which increases performance and is a requirement for some mission orbits. Launching over oceans also reduces insurance costs, which are often large for a vehicle filled with volatile fuel and oxidizer.

Launch at altitude allows a larger, more efficient, yet cheaper first-stage nozzle. Its expansion ratio can be designed for low ambient air pressures, without risking flow separation and flight instability during low-altitude flight. The extra diameter of the high-altitude nozzle would be difficult to gimbal. But with reduced crosswinds, the fins can provide sufficient first-stage steering. This allows a fixed nozzle, which saves cost and weight versus a hot joint.[/B]

A single-impulse launch results in an elliptical orbit, with a high apogee and low perigee. The use of three stages, plus the coast period between second- and third-stage firings, help to circularize the orbit, ensuring the perigee clears the Earth's atmosphere. If the Pegasus launch had begun at low altitude, the coast period or thrust profile of the stages would have to be modified to prevent skimming of the atmosphere after one pass.[/QUOTE]

Our attempts to launch a rocket from a plane in [URL="https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiqkdD5-aHUAhUQ0IMKHdFuDYkQFggoMAA&url=https%3A%2F%2Fkerbalspaceprogram.com%2F&usg=AFQjCNEkP92dTnDB7qrlaay0UWNorsrR2g&sig2=o6T5EWzIs_UxH-2roiQp_A"]Kerbal Space Program[/URL] have not ended well for the Kerbals involved.

:razz:

[QUOTE=kladner;460391]I wonder how the development costs would compare with those of a linear accelerator. Of course, those would be best sited at higher altitudes, near the equator. This puts a number of constraints on the idea.
[Edit: Oh. OK. The plane exists. Do the intended payloads?]
Still, for smaller launches this might be helpful. The catapult might have a lot to recommend it, if it could fling freight capsules into orbit.[/QUOTE]An orbital velocity linac would have serious thermal problems for its projectile. Hitting a thousandth of an atmosphere at 20km/s is enough to make meteors burn and/or explode. Remember what happened over Siberia (at least) three times in the last 110 years. Hitting half an atmosphere is seriously interesting.

The cheap way around the problem is to build a very rugged rocket and sent that on a high sub-orbital trajectory. The expensive ways are to arrange for a vacuum ahead of the projectile, either contained in tube or by pushing the air out of the way just in time, either through powerful lasers or with sacrificial mass accelerated in the same linac.

[QUOTE=Uncwilly;460395]This would be at least the 4th airplane-rocket combo to launch a satellite (3rd rocket family). And another advantage of the plane, it can be flown right on the equator for maximum free power.[/QUOTE]

And another advantage is you can get above weather, which scrubbed SpaceX's Thursday launch, and has a 40% probably of scrubbing today's.

SpaceX feed is now live: [url]http://www.spacex.com/webcast[/url]

[QUOTE=chalsall;460448]SpaceX feed is now live: [url]http://www.spacex.com/webcast[/url][/QUOTE]

When I reincarnate, I hope to do great things.

[QUOTE=chalsall;460452]When I reincarnate, I hope to do great things.[/QUOTE]

They pulled this off. As expected.

In the Dragon cargo is a cool kind of solar panel that will be tested but is not currently going to be an addition to the ISS.

Instead of rigid solar panels it is flexible and rolled up under some tension to spring open "like a party favor." [url]https://www.nasaspaceflight.com/2017/06/spacex-falcon-9-crs-11-dragon-iss-100th-39a/[/url]
[QUOTE]ROSA is a prototype for a new compact solar array, which will be deployed at the space station to test its deployment mechanism and demonstrate it in orbit, although this will not form a permanent part of the station’s power generation system.[/QUOTE]

[url]https://www.nasa.gov/feature/roll-out-solar-array-technology-benefits-for-nasa-commercial-sector[/url]
[QUOTE]The Roll Out Solar Array (ROSA) is one of the options eyed by NASA that could power an advanced solar electric propulsion spacecraft that makes possible such endeavors as the agency’s Asteroid Redirect Mission—plucking a multi-ton boulder from an asteroid’s surface, and then maneuvering that object into a stable orbit around the moon for human inspection and sampling.

Tapping into ROSA technology allows the conversion of sunlight into electrical power that drives the ion thrusters of a solar electric propulsion spacecraft. ROSA is expected to enable a number of space initiatives and is a cost-saving plus to transport cargo over long distances beyond the Earth.

Commercial satellite platforms

One of the companies that NASA STMD contracted with is Deployable Space Systems (DSS) of Santa Barbara, California to work on the ROSA. DSS has partnered with Space Systems Loral (SSL) of Palo Alto, California to apply the technology into SSL’s heritage commercial satellite platform. SSL builds some of the world’s highest power satellites.

Higher power increases satellite capability, particularly for applications like broadband and ultra high-definition television, explains Al Tadros, SSL vice president for Civil and Department of Defense Business.

“We get more power by using larger solar arrays. But efficiently packaging them for launch and then deploying those big arrays by a spacecraft has been the challenge.”Tadros says. “What the work on ROSA has done is develop a technique to deploy very large surface areas of flexible solar arrays, doing that efficiently with low risk,” Tadros adds. “It’s more power without increasing the mass dramatically.”

ROSA is an enabler for SSL business, Tadros adds. “Higher power can translate to more payload and more revenue for our customers.”

Scalable solar wings

In general, the solar array rolls up around a spindle to form a compact cylinder for launch. Those solar wings are then deployed via strain energy in rolled booms that form the outer sides of the structure. A lightweight mesh material supports strings of photovoltaic cells that churn out electrical power.

What’s more is that ROSA is scalable. It can be configured and combined with other ROSAs for very high power levels, Michael Ragsdale says, Research and Development project manager at SSL.

Deployment of the innovative ROSA is straightforward, a two-stage process that takes roughly ten minutes, Ragsdale says. “It’s a simple mechanism that controls the array deployment and that increases its reliability.”

SSL is working closely with space agency researchers at NASA’s Glenn Research Center in Cleveland, and its contractor DSS to qualify ROSA to become an integral part of SSL’s 1300 satellite platform product line.

Technology testing

The U.S. Air Force has funded a test flight of the ROSA mechanism, now scheduled for a SpaceX launch in Spring 2017 to the International Space Station, where it will be deployed in space.

ROSA is groundbreaking, a lightweight technology that rolls up and stows into a very compact volume, explains Brian Spence, president of DSS. “NASA’s investment in ROSA was important to elevate the maturity level of the technology and I am pleased to see a good return on investment of taxpayer dollars.”

DSS is a small business founded some eight years ago, Spence notes, and brought to bear its cadre of engineers in solar array, deployable boom, mechanism and composites design to bring ROSA to flight-ready status.

“It’s very unique and innovative, different than anything that’s been done before,” Spence points out. “However, it’s also extremely simple. That aspect of the technology really lends itself well to being accepted by end users, like SSL.”

Looking into the future, Spence envisions ROSA technology as key for NASA’s solar electric propulsion needs as well as robotic and human journeys to Mars and beyond. For example, excursions on Mars can benefit by deploying solar arrays, he adds, and then retract them for point-to-point travel across the rugged landscape of the Red Planet.

“I think that we’re at the cusp of something really big here,” Spence concludes.[/QUOTE]

[QUOTE=xilman;460418]An orbital velocity linac would have serious thermal problems for its projectile. Hitting a thousandth of an atmosphere at 20km/s is enough to make meteors burn and/or explode. [/QUOTE]

There is of course the fact that low earth regime orbital velocities are roughly 8 km/s, not 20.