Space launch lite: the Swala concept
by John Hollaway
|It would be much simpler and efficient to have a vehicle with wings that would allow their lift to carry it up to the top of the atmosphere, and a motor that used the oxygen in the air for combustion.|
This article takes a closer look at the practicability of the scheme, which I am calling the Swala concept—“swala” is Swahili for the Grant’s gazelle, a very fast and lightly built member of what hunters call “plains game.” There is a reason for using an East African name, which I will come back to later
However, I have to admit that I am not one of that perhaps mythical fraternity: a rocket scientist. Nonetheless, I do have a good general scientific background arising from my career in the minerals and metals processing industries. In the last 35 years I have undertaken more than 400 consulting assignments in 34 different countries, and I first began to think about the challenges of space 30 years ago, after the Challenger disaster. I felt that there must surely be a better way to get up there other than using the battering ram approach it exemplified.
The Space Shuttle was undeniably successful: after all, only two out of its 135 missions failed. But its enormous cost, its complexity, and the feeling that this whole project was right at the safe limits of our technology haunted me. It would be much simpler and efficient to have a vehicle with wings that would allow their lift to carry it up to the top of the atmosphere, and a motor that used the oxygen in the air for combustion.
To take a vehicle to the edge of space without hauling up its own oxygen needed a motor capable of generating a velocity that allowed it to make best use of the thinning atmosphere. The ramjet is the obvious choice, but it is always argued that this is a very inefficient motor outside its designed speed. In fact this hardly matters; if it can attain Mach 5, as the literature states, it will be up there in less than two minutes.
As to the other objection to their use—that ramjets need a high initial velocity to function—it appears from practical experience that, above about 300 kilometers an hour, they begin to generate significant thrust. In fact, not much thrust is needed; this is not a matter of pushing the vehicle straight up. Once there is significant forward momentum the wings will impart lift.
The most relevant model for the Swala proposal is an early type of cruise missile, the Bomarc, which was a ground-to-air interceptor developed in the period when conventional aircraft carrying atomic bombs were the threat, not ICBMs. This used a rocket motor to blast off vertically and switched to a couple of ramjets once it was supersonic. These ramjets had to work hard, as they took the vehicle to Mach 3 and an altitude of 30 kilometers. To do this they had a thrust of over 50 kilonewtons apiece.
|Its most noticeable feature will be the launch track, perhaps 20 kilometers of dead straight, flat railroad, running west to east and as near the Equator as possible.|
I would expect that 100 kilonewtons would do for the approximately 20-ton spaceplane (including a 500-kilogram payload) that the Swala concept envisages, given that it will have the benefit of lift from its broad, if thin, wings. Indeed, I would hope it replicates the achievement of Martin Marietta’s prototype ASALM missile, which inadvertently achieved Mach 5.5 in 1980 when its throttle stuck.
The spaceplane would launch from a railroad trolley (I can’t think of a more dignified description of this unit) that has been accelerated to about 400 kilometers an hour by a linear motor of the sort that is used in the AirTrain at New York’s John F. Kennedy International Airport and elsewhere. It would be held to the trolley by electromagnets until strain gauges indicate that that the vehicle is indeed straining at the leash, at which it would be let free.
At whatever altitude and speed it achieves, once the thrust from the ramjets fades, they are dropped off, employing the systems and technology used for discarding the solid fuel boosters on more conventional launches. Parachuting down into the sea, they will be recovered and reused as they have no components that might be harmed by a dunking.
Now a solid fuel motor, which occupies most of the body of the Swala vehicle, fires up and takes it to low earth orbit at about 29,000 kilometers an hour. This is a simple system but its inflexibility may mean that thrusters will be needed to adjust speed and altitude.
Re-entry after offloading the payload would borrow techniques and technology from the shuttle. What may be possible is that the very low ballistic coefficient of the vehicle at this stage—just a shell with wings and a stabilizer—could make it possible for it to use titanium alloy alone for its surfaces, without a heat-resistant coating. With its wings, the vehicle could lose momentum more slowly than any of its predecessors.
Swala would land back on the trolleythat it was launched from, or another trolley, at perhaps 200 kilometers an hour. This is the part of the Swala concept that worries most people, yet it would use a variant of the aircraft instrument landing systems (ILS) that have been in service since the 1960s. The key difference is that the trolley adjusts its speed precisely to the arriving vehicle, hence the variable-frequency linear motor driving it. The homing beam sent out by the trolley guides the vehicle down, while the transmissions from the vehicle cause the trolley to match its velocity to the vehicle. This will require some advanced electro-mechanical and laser interaction, but much of it can be developed from existing designs and equipment. As soon as it touches down the electromagnets lock it in place.
The process would be vulnerable to sudden gusts of wind, but by scheduling landings to occur at dawn—when, as balloonists know, conditions are normally ideal—this should not be a threat.
An interesting aspect of this is that the launch and retrieval systems could be developed using a large fixed-wing drone mounted on skids sitting on the electromagnets. Using this proxy vehicle to perfect the scheme would mean that no chances have to be taken with the expensive prototype when it makes its maiden flight.
|Converts to the Swala concept begin to look on the conventional rockets used for getting things into space as resembling dinosaurs: massive, clumsy and inefficient.|
So where could the Swala project be sited? Its most noticeable feature will be the launch track, perhaps 20 kilometers of dead straight, flat railroad, running west to east and as near the Equator as possible to get the maximum benefit of the earth’s rotational velocity. (Ten kilometers should be plenty for the launch, but some leeway might be advisable for the landing.) As it happens, there is an area of high semi-desert to the east of Mount Kenya that should fit the bill, hence the Swahili name. However, it is within 100 kilometers of the border with Somalia, and Kenya has suffered a number of horrendous suicide bomber attacks from al-Shabaab, the local al-Qaeda franchise. The Swala project would have been too enticing a target for certain Somali youths anxious to get to all those virgins, and the idea had to be abandoned.
A safer site, albeit well to the north, would be in Abu Dhabi. Here the prototype could be developed in the southwestern desert adjacent to the “empty quarter” of Saudi Arabia. Abu Dhabi is attractive for another reason: the tourism potential of the project. This may sound a bit odd, but its airport is one of three airline hubs in the area, and there will be many readers who have waited long hours, often in the middle of the night, for connections either there or at Dubai or Qatar.
Stuck with perhaps three grim hours to kill, the prospect of a paying for a short return helicopter flight to the Swala site could be very attractive, particularly if the visit could coincide with a landing or takeoff. A permanent launch track could be established running across the neck east of Dubai between the Persian Gulf and the Gulf of Oman, with the discarded ramjets being parachuted into the latter. Tourists could be provided with a visitor center, observation platforms to see what is going on in the vehicle service bays, souvenir shops, restaurants, and even hotels for those who decide to break their trip for a day or so to be sure of seeing a landing or takeoff.
Converts to the Swala concept begin to look on the conventional rockets used for getting things into space as resembling dinosaurs: massive, clumsy and inefficient. Certainly satellites—think CubeSats—and space hardware in general is getting smaller and lighter. Yet the need for heavy-lift vehicles will persist; the Swala concept can be scaled up, but probably not to take a ten-ton payload. What it can do is assume the task of launching the numerous small satellites now planned for low Earth orbit, and also for retrieving the obsolete, damaged, or dangerous pieces of equipment that are increasingly circling the Earth. With a turn-around time of perhaps only a day or so, the Swala concept really could be “Space Launch Lite.”