Virgin Galactic's suborbital passenger vehicle SpaceShip Two. Image copyright Virgin Galactic.

Antipodal Bomber
Tech Level: 9
Suborbital Passenger Rocketplane
Tech Level: 11
Suborbital Military Transport
Tech Level: 12
Suborbital Passenger Scramjet
Tech level: 14

A suborbital flight is any launch that reaches at least the edge of space (about 100 km/60 miles up), but does not have enough velocity to enter a full orbit. Once reaching a target altitude, a craft may deadfall or glide back to earth, or it may use powered flight to maneuver to a particular destination.

Suborbital flights are mostly simple ballistic trajectories. The vehicle will launch upward and angle itself so that its momentum would carry it up and over the edge of the atmosphere. The bigger the initial launch velocity, the higher the arc and the farther vehicle can theoretically go. However, the vehicle will want to avoid going fast enough to enter orbit, as the whole idea of the technique is to use the craft’s own momentum to carry it back down to its destination with minimal additional thrust.

Suborbital flight has usually been a precursor to full orbital travel. Most nations, militaries, and private organizations that have ongoing space programs have usually achieved suborbital trajectories before attaining full orbital flight with their spacecraft. (Though there are some exceptions, most notably the Soviet manned space program.)

The first suborbital launch occurred in 1944, when a German V-2 rocket achieved an altitude of 189 kilometers. The first manned suborbital flight came in 1961, by American astronaut Alan Shepard. Two years later, the X-15 rocket plane, piloted by Joseph A. Walker became the first reusable aircraft to achieve the suborbital milestone. And most recently, in 2004, Mike Melvill piloted SpaceShipOne in the first privately-funded suborbital flight. ICBMs and other missiles often use suborbital trajectories to reach their targets in a short amount of time, and is a well-proven technology.

Suborbital flight has usually been a precursor to full orbital travel. Most nations, militaries, and private organizations that have ongoing space programs have usually achieved suborbital trajectories before attaining full orbital flight with their spacecraft. (Though there are some exceptions, most notably the Soviet manned space program.)

Currently, Virgin Galactic and others are planning on tourist-oriented suborbital flights to the edge of space and back. In the near future, a number of interests are looking into the possibility of using suborbital travel for point-to-point jaunts, allowing passengers to makes trips to anywhere on Earth in a handful of hours or less.

There are some limits and complications, however. The upper ceiling for suborbital arcs for passenger flights is about 500 miles up. Beyond that point lie the Van Allen radiation belts, and passengers at that altitude could be exposed levels of radiation that may be hazardous to their long-term health. Because of weight considerations needed for aircraft, a suborbital passenger vehicle probably cannot be too heavily shielded.

In order to extend its range, specifically designed suborbital craft may use a maneuver called "skip-glide." The vehicle would control its angle of descent, and use its flat-bottomed shape to ‘skip’ off the outer edges of the atmosphere, much like a stone skipping off the surface of a pond. It could do this repeatedly, using skip-glide hops of hundreds or even thousands of kilometers long to reach anywhere on the globe. Of course, the passengers may be in for a rough ride, as each ‘skip’ would bring quite a bit of potential turbulence and put considerable thermal stress on the aircraft (it is, after all, a type of aborted re-entry.)

Tech Level: 9

An antipodal bomber uses skip-glide and another maneuver called boost-glide to reach almost anywhere in the world to deliver its payload.

Antipodal bombers were first envisioned in Germany in the early 1930s, and the Nazi regime in its latter days had tentative plans to research the technology, but nothing except some preliminary designs were ever made.

The idea was revived in the late 1950s with the X-20 project, nicknamed "Dyna-Soar" (short for Dynamic Soarer.) Unfortunately, budgetary constraints in the mid-1960s prematurely ended the project before it could even begin test flights. A decade later, however, much of the design work and testing for the X-20 concept was later revived for the Space Shuttle.

Both iterations called for an advanced, delta-winged, rocket-powered bomber to be boosted to the edge of space on top of a more conventional vertical launch vehicle. The size and power of the launch rocket would determine the initial velocity and altitude of the bomber-glider.

Because of the powerful boosters available to it, including the Titan and Saturn rockets, the X-20 version could also have theoretically achieved orbit. In this case it would use a "boost-glide" technique to reach its target. It would initiate a de-orbit burn, still using its shape and the proper angle to ‘skip’ off the outer atmosphere. At its lowest point, it would deploy its payload, in most cases nuclear bombs. After the skip, it would use its rocket motors to push itself back into orbit. This was thought to be a rather brutal maneuver for the pilot, as he would be pulling a very uncomfortable number of G’s both de-orbiting and accelerating up to altitude again.

To aid in a boost-glide mission, the X-20 was often depicted as having an attached rocket booster on its aft end, to give it the extra fuel and thrust needed to make repeated burns both into and out of the edge of the atmosphere. This booster would be jettisoned prior to final descent.

Many consider the antipodal bomber, and especially the X-20 program, one of the great ‘what-if’ scenarios of modern manned space flight. If the project had gone ahead, it would have given the US a reusable winged spacecraft fifteen years before the Space Shuttle, and would have been a potential game changer for space flight for decades afterward.

However, it was intended mostly for military reconnaissance and long-range nuclear strike missions, which have since proven much cheaper to accomplish with automated satellites and ICBMs, respectively. So even if the X-20 project had gone forward, it would have had a hard time finding the proper niche in which to justify its continued great expense.

Still, the technology for the antipodal bomber is long since proven, and it would be interesting to see if any of the emerging fledgling space powers will ever resurrect the technology for other applications.

Tech Level: 11

Rocketplane technology goes back to the dawn of the space age. Chuck Yeager used one in 1947 to break the sound barrier. Several rocketplanes, particularly the X-15, carried out suborbital flights in the mid-1960s. Most recently, in 2004, rocketplane SpaceShip One became the first privately-funded effort to send a man into space.

Virgin Galactic and other companies including Space Adventures, Starchaser, Blue Origin, Armadillo Aerospace, XCOR Aerospace, and Rocketplane Limited are actively pursuing the development of suborbital tourist vehicles. Virgin Galactic, with the development of its SpaceShip Two system, seems to be closest to producing the first commercially available flights.

SpaceShip Two uses an updated version of the X-20 scheme, with the rocketplane launching from an advanced high-altitude carrier jet. So far there are only plans for it to take passengers on short excursions out of the atmosphere. Basically the carrier craft would spiral up, the rocketplane will launch on a steep trajectory, and passengers would experience a few minutes of weightlessness before the return flight would commence. It would land at the exact same facility from which it was launched. Other commercial space venture developing tourist flights are planning vehicles with similar capabilities.

Future versions of these early suborbital efforts hope to introduce point-to-point travel and ferrying passengers up into full orbit to dock with small "space hotel" stations.

Tech Level: 12

ICBMs are a long-proven suborbital technology that can deliver a payload anywhere in the world in under two hours. But does the technique have to be limited to just carrying nuclear bombs? A new joint DARPA-Air Force project called SUSTAIN (Small Unit Space Transport And InsertioN) is researching the use of suborbital flight to deliver US marines and vital support equipment to any point on the globe they may be needed, many hours ahead of what conventional transports are capable of. The actual vehicle, which is still in the design phase, is code-named Hot Eagle.

Though the project has yet to settle on an initial design, some experts speculate that it may evolve similarly to Virgin Galactic’s SpaceShip Two concept. A high-altitude carrier aircraft would lift the suborbital transport into the upper atmosphere, and after detaching, Hot Eagle would use its own rocket motor to boost itself into a suborbital trajectory. It would then glide down to its destination. Including the carrier aircraft flight, the system could deliver a squad of up to 13 marines and equipment to any suitable landing site in the world in under four hours.

However, the question invariably comes up, what can a squad of 13 marines, or even several spaceplanes worth of them, be able to accomplish in any wartime situation dire enough to require immediate intervention? The Hot Eagle spaceplane, no matter its final design, will likely still depend a decent landing strip at the very least in order to touch down safely, meaning its not very likely it could inject the troops directly into a hot zone. Both the heat from reentry and its sonic booms as it approaches the ground will make stealthy insertion non-viable. Plus, because of the weight considerations needed for suborbital craft, it couldn’t be that well armored, and very likely would not have enough fuel to lift itself off again.

So, basically, once it touched down, Hot Eagle and the squad it carried would be there for the duration, vulnerable to attack, and most likely on its own. Sending troops as well as a very expensive spaceplane into a potentially dangerous situation without support does not sound like a sound tactical strategy, no matter the situation.

Rather, instead of being used for troop insertion, Hot Eagle may finds it true use rather as a means of rapid response to disasters and non-military crises. If an outbreak of a deadly disease is reported, for example, Hot Eagle could deliver experts, vital medicines, and equipment needed to deal with the outbreak in a much more timely manner than other means of transport. A number of other disasters and accidents that could benefit from rapid transport of experts and specialized equipment to the affected area could include nuclear accidents, earthquakes, tsunamis, mine collapses, and chemical or biological terrorist attacks.

Tech Level: 14

Dedicated suborbital craft such as SpaceShip Two and its kin are likely to be precursors of true reusable orbital launch vehicles, as both orbital flights and ferrying passengers to space hotel stations become mainstays of the space tourist trade decades from now.

Its likely that these new reusable craft would be pressed into service both for orbital insertions as well as suborbital travel. In fact, from Tech Level 13 on or so, suborbital and orbital interface capabilities will probably merge completely in many vehicles. This is mostly due to the fact that the difference in performance and cost between orbital launch vehicles and suborbital-only craft is narrow to begin with, and it will probably become more practical and economic to produce spacecraft types that can fulfill both roles as the technology advances.

However, this may not always be the case. Dedicated suborbital travel may be taken up by light scramjets, similar to the way smaller jets fly the routes that big airliners eschew. A scramjet is a Supersonic Combustion Ramjet, or SC ramjet. In a conventional jet or ramjet engine, baffles or turbines slow the flow of air through the engine to allow the combustion of fuel. In a scramjet, the airflow isn’t hindered at all, and the engine allows for combustion as air and fuel flow through the engines at supersonic speeds.

Theoretically, super high-performance scramjet engines could achieve well over Mach 15, but more practically scramjets will probably max out around Mach 8 or 10. However, when combined with a rocket engine in the same housing (this is called a combined cycle engine—the engine is capable of acting as a regular jet, a scramjet, and a rocket), the scramjet is capable of bursting up to enough speed to take it into a suborbital trajectory.

The big advantage of scramjets over previous suborbital designs is they do not need a carrier aircraft or separate booster. They can be launched from and land directly at any conventional commercial runway. Because of that, the technology could open up affordable suborbital travel to the mainstream. However, because it uses much of its onboard fuel for take-offs and landings, a scramjet passenger craft would not have a great deal of reserve for maneuvering in orbit, even if capable of reaching such speeds. Thus, all but the largest scramjets will likely remain suborbital craft.


Article added 20 December 2008