Rockets versus space elevators. Space elevators versus rockets. From listening to altspace conversations on the subject, one could be forgiven for thinking that these are the only two ways to achieve orbit and beyond. But the reality is much more interesting. Follow me below the fold for more.
After SpaceX just completed their first successful Falcon 9 flight, the old conversations started back up -- "Now we need to move beyond rockets to space elevators, the logical next step" However, space elevators are hardly the only non-rocket game in town. In this diary, we shall explore some of the possibilities.
First, let us explore the "rocket-like" vehicles and those that still require significant rocket stages for more than just a kick or OMS stage. By comparison, a conventional rocket is:
Tech readiness: 9/9
Development/Research cost: high
Launch cost per unit payload: very high
Aircraft-assisted launch
There are a variety of methods with which an aircraft can launch a spacecraft. This is actually a fairly mature tech; the Pegasus rocket is launched in this way. Use of an aircraft gets the rocket past the atmosphere, reducing its drag and allowing its engine(s) to be optimized for space. It provides it a small amount of orbital velocity. And perhaps most importantly, it lets you launch your craft from virtually anywhere, which reduces legal liability and red tape and facilitates equatorial launches. A rocket can be:
- Carried under the wing of a conventional craft. This limits rocket size to whatever will A) not unbalance or overly strain the craft, B) not drag along the ground, and C) fall within the aircraft's carrying capacity.
- Internal stowage in a conventional craft. Experiments have been done in this regard. A parachute pulls the rocket out of an open cargo bay, it falls, drops the chute, then fires. The rocket size is limited to A) what will fit inside the craft, and B) fall within the aircraft's carrying capacity.
- Carried under the body of a custom-designed twin-fuselage aircraft. This is the approach of the suborbital "SpaceShipOne". The rocket is limited to A) what will not drag along the ground, and B) what falls within the aircraft's carrying capacity.
- Mid-air refueling. As proposed by "Black Horse" and "Black Colt". A pure rocket or hybrid rocket/airbreathing craft takes off with minimal fuel, docks midair with a fueling tanker to transfer fuel and/or oxidizer, then fires rockets to orbit. The advantages of this approach versus just taking off fully loaded is that A) an airbreathing craft (airplane) carries your fuel to altitude for you, and B) you can use much lighter landing gear and structural supports if you don't have to bear the full weight of the vehicle on the ground.
- Tow launch. Like mid-air refuelling, but the rocket is towed to altitude by the fueling tanker, then refuelled. The advantage of this approach is that it eliminates the need for a midair docking.
There's one big problem to aircraft-assisted launch: namely, the carrying capacity of aircraft. The pegasus rocket uses four stages and can only get a tiny payload to space. SpaceShipOne is merely suborbital, and nowhere close to orbit. You need a very favorable combination of stages, a low payload, a high ISP, and an extremely large aircraft to make aircraft-assisted launch work.
Tech readiness: 9/9
Development/Research cost: high
Launch cost per unit payload: high
Scramjets
Conventional ramjet operation does not work at extreme speeds. As the airflow inside a ramjet reaches supersonic velocies, internal shockwaves make sustaining a combustion front impossible. A scramjet burns its fuel at supersonic velocities, using one of a range of techiques (for example, using a small amount of a hypergolic fuel such as silane to maintain combustion). While scramjets bear very poor lift to drag ratios, the incredible ISP advantage they offer from airbreathing operation makes them intriguing. A popular variety of scramjet is the "waverider", in which the "engine" has only one side; the other side is virtual, formed by the shockwave of the spacecraft itself.
Scramjets cannot operate at very low speeds and cannot achieve orbit on their own. Various scramjet hybrids or stages have been proposed.
Tech readiness: 2/9
Development/Research cost: very high
Launch cost per unit payload: medium-high
Skylon (SABRE)
The SABRE engine concept is a successor to the (less likely plausible) LACE concept. In LACE, incoming air is chilled by the liquid hydrogen fuel in order to liquefy the oxygen component. This oxygen is then burned with the liquid hydrogen fuel in a rocket engine.
The problem with LACE is that it requires a tremendous amount of liquid hydrogen be brought onboard in order to cool the air, and more to overcome the extra vehicle drag. SABRE, by contrast, is a "rocket engine/precooled air turboramjet". In each engine, small turbine/compressor accelerates the vehicle to ramjet speeds, where a ram takes over compression. At higher speeds, the liquid hydrogen fuel is used to cool, but not liquefy, the incoming air. This should allow the engine to near the maximum theoretical velocity for a ramjet -- Mach 5.5 -- as well as operating at high altitudes.
SABRE is to be used on an innovative launch vehicle called Skylon. The large mass of hydrogen and low mass of oxidizer gives the vehicle a high surface area to mass ratio, which simplifies reentry heat shielding requirements. The very high ISP of atmospheric operation allows the vehicle to be an SSTO; combined with horizontal takeoff and landing, its turnaround time and maintenance should be low. For the development process, probably the best example to look to is the SR-71 Blackbird; Skylon is 2x larger, 1.5x the speed under turbojet power, 3x the takeoff weight, and 1.3x the cruising weight.
Tech readiness: 3/9
Development/Research cost: very high
Launch cost per unit payload: medium-high
From here, we'll get into the less rocketlike launch mechanisms. First, ballistic launch mechanisms. Note that all ballistic launch mechanisms need a kick stage to circularize their orbits (if not outright reach orbital velocity), and most are unsuitable for launching living payloads.
Super-cannon
Straight out of Jules Verne, this technology is surprisingly well-developed, and much of the story of it traces back to one man: Gerald Bull. Bull was a researcher with a knack for two things: brilliant artillery innovations and ticking off his superiors. Many of his innovations are still used in top-of-the-line artillery systems, such as giving shells a small rocket engine that fills in their low-pressure wake with its exhaust to reduce drag. Bull managed to get a project going called HARP, in which they connected multiple naval guns end-to-end, smoothed out the bores (in favor of self-expanding fins on the projectile), perfected sabot launch, and demonstrated that not only could they provide a good chunk of a projectile's delta-V, but that they could do it without damaging sensitive electronics systems.
Unfortuantely for Bull, despite getting projectiles up to half of orbital velocity, his funding was soon cut. He continued his artillery research as a private corporation, but served six months in jail for exporting weapons to South Africa -- something that he had previously been permitted to do. Bitter, he left the US and decided to sell his services to the highest bidder (anyone but the Russians). He found a buyer in Saddam Hussein, and commissioned the Babylon Supergun project; Hussein's fee for funding the project was assistance with his missile program, which Bull provided. This work in particular likely led to Bull's assassination in 1990, by individuals largely suspected to be tied to the Mossad.
A related program for a supergun traces back to World War II. The Germans created a special cannon: the V3. It used multiple successive charges and incredibly precise timing to continue boosting the projectile -- a difficult technique called "blast acceleration" later tested by HARP. It proved an ineffective weapon, as it was very easy to target.
Tech readiness: 4/9
Development/Research cost: medium
Launch cost per unit payload: medium
Ram accelerator
A ram accelerator takes an entirely different approach to the problem of the force on a projectile dropping as it goes down the barrel. Instead of repeatedly boosting back pressure, they make the shell of
the projectile shaped like the core of a ramjet and fire it down a tube of fuel-air mixture. Thus, the core accelerates tremendously as it travels, with the thrust always behind it. Mixtures with progressively higher speeds of sound are used, separated by thin membranes, down the length of the barrel; this helps the ram keep the flame behind it burning properly, as ramjets perform poorly at hypersonic speeds.
Tech readiness: 4/9
Development/Research cost: medium
Launch cost per unit payload: medium
Light gas gun
A light gas gun is a conventional gun which deals with the problem of maximum acceleration of a projectile due to its speed of sound in a different manner: rather than having the exhaust of an explosive accelerate a projectile, the explosive compresses a large ram, which presses a light gas like hydrogen against the projectile. The much higher speed of sound allows for higher barrel exit velocities.
Tech readiness: 4/9
Development/Research cost: medium
Launch cost per unit payload: medium
Slingatron
The Slingatron is a concept that proposes to use a spiral-shaped tube with the payload beginning in the center, which is shaken around in a cylindrical motion to spin the projectile along the walls of the tube until it emerges out of the spiral with high velocity.
Tech readiness: 1/9
Development/Research cost: high
Launch cost per unit payload: medium-low
Railgun
A railgun uses two charge-carrying rails with opposite polarity over a long track. These create strong magnetic fields. The projectile forms a conductive path between them, developing its own magnetic field tangential to that of the tracks. This propels the projectile down the track.
The primary downside to railguns is heavy wear on the rails at high velocity.
Tech readiness: 4/9
Development/Research cost: medium
Launch cost per unit payload: medium-low to medium
Coilgun
A coilgun addresses the wear problem with railguns: there are no rails to wear. Coilguns are comprised of a series of electromagnetic coils which are activated in sequence such as to attract a projectile on approach and repel it upon departure.
The primary disadvantage to coilguns for spacecraft launch is high-speed switching. The coil needs to be brought from no current or an inverted flow to a full flow in the desired direction in a tiny fraction of a second, with split-second timing.
Tech readiness: 3/9
Development/Research cost: medium
Launch cost per unit payload: medium-low
Now, off to the old altspace favorite: structures in tension!
Space elevator
Ah, the space elevator -- the old sci-fi fallback! What could be more romantic than a giant technological beanstalk to the stars? Part of it (climbers) is simple enough that college students can compete on designs, furthering interest. Travel is low-G and, unlike ballistic launch, does not suffer from extreme atmospheric drag.
Unfortunately, space elevators have a number of huge problems. The first one is tensile strengh. Technically, you can make a space elevator with any tensile strength. But to make one practical, you need a tensile strength of at least 100GPa, and preferably 120GPa or more, with the density of graphite. Single-walled carbon nanotubes have been theorized to be that high with that density, but the strongest ones ever measured are barely over 60GPa. And these are individual tubes; the strength of bulk fabrics is far lower than that of its component fibers. The tubes form ropes held together weakly by pi bonding and van der waals force. These are themselves strung together to form fabrics. Achieving 10GPa at the density of graphite on a fabric that long would be quite the feat. 100GPa may ultimately prove to be be physically impossible.
Even if you can achieve this, the safety margins in space elevators are appallingly low, with countless threats to their integrity. The transit time in the radiation belt is high. And recent simulations show that space elevators may likely be dynamically unstable. While space elevators are electrically powered, which is quite efficient, they have to transmit that power from the surface to the climbers. Small-receiver power beaming is grossly inefficient over the distances involved -- single-digit percent. Sadly, I cannot endorse this beautiful concept. The one thing I can say in their favor is that, contrary to popular perception, a falling space elevator poses no threat to anyone. By their very nature, they must have extremely low mass per unit length. A space elevator tether falling on you won't hurt any more than a nylon stocking falling on you.
Tech readiness: 1/9, possibly 0/9
Development/Research cost: very high
Launch cost per unit payload: medium-low
Skyhooks and Rotavators
A skyhook is basically an undersized space elevator. By virtue of not having to reach all the way to earth, their tensile strength requirements are reduced. Unfortunately, so is their utility. One must fly up to the base of the skyhook, achieve its relative velocity (often a good chunk of orbital velocity), then climb it. And, since the skyhook isn't tethered to the ground, it must re-boost itself back to its original position.
A rotavator is a skyhook which rotates around its axis. This allows it to have a lower tip speed at the base. It still needs reboost, but some designs allow for returning payloads to provide the reboost. Rotavators require precise timing in their operations.
Both skyhooks and rotavators may suffer from the same dynamic instabilities affecting zero-gravity zero-atmosphere high-tension tethers that affect a space elevator.
Tech readiness: 2/9 at best
Development/Research cost: high
Launch cost per unit payload: medium
We will now discuss kinetically-suspended launch structures. They provides the potential for use as both a space launch track and a large energy storage system. Because of this, unlike a space elevator, there are much lower power losses during the acceleration of a vehicle, allowing for the cheapest possible access to orbit.
Space fountain
The best way to understand the space fountain, and all kinetically-suspended structures, is to picture it this way. Imagine you've got a falling plate overhead and you've got a basketball. You throw the basketball upwards at the plate before it hits you. The ball bounces off the plate with greater velocity and the plate flies back upwards. It starts to fall again. But you throw the basketball again. If your aim is flawless, you can keep the plate roughly suspended in the air.
Now, imagine that your basketball was automatically routed back upwards toward the plate -- instead of you throwing it, it simply took a loop. Imagine that your system was lossless. Your plate will hover without any additional energy input.
Now let's make it practical. Instead of basketballs, you use a continuous stream of particles in a near-vacuum. At the top is a magnet that redirects them back downward, reversing their velocity and accelerating them back to Earth. At the bottom is another magnet, as well as a cyclotron that re-accelerates the particles to compensate for any system losses (which there always will be). You now have a tower suspended in air with no unusual tensile strength requirements -- just a relatively low constant input energy. This is a space fountain.
Like most kinetically-suspended structures, it provides the potential for use as both a space launch track and a large energy storage system. Because of this, unlike a space elevator, there are virtually no power losses during the acceleration to orbit.
Tech readiness: 2/9
Development/Research cost: high
Launch cost per unit payload: low to medium, depending on how orbital velocity is reached in space.
Launch loop
The "Launch Loop" is a particularly interesting kinetically-suspended structure. A long loop of iron at the equator spins rapidly inside an evacuated tube. Its centrifigual force supports it to reach up to near space, while weighted cables shape it into a track for acceleration to orbital velocity. The evacuated tube ends in space, where it is no longer needed. The tube is kept separated from the loop by magnetic levitation. Craft are launched atop the tube/track by magnetic acceleration against the spinning ribbon (approximately 50% efficient).
Unlike the space fountain, the Launch Loop naturally provides a long track for steady acceleration to orbit. Like all kinetically-suspended structures, it stores a great amount of energy; however, its length gives it an additional use (power transition).
I personally find the Launch Loop proposal the most promising for dramatic launch cost reductions. Loftstrom's calculations suggest that a $30B loop could launch 6 million tons to Low Earth Orbit per year at a stunningly low cost of only $3/kg. No exotic materials required.
Tech readiness: 2/9
Development/Research cost: high
Launch cost per unit payload: low
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Are these the only non-rocket ways to get to space? Hardly! But they're a nice starter. Who knows how we'll utimately move beyond our planet, but there certainly are plenty of possibilities.
I'll leave you now with Carl Sagan and Stephen Hawking singing A Glorious Dawn":