JOHT 83: Getting High
Sep. 17th, 2020 09:06 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
stickmakerThe Joy of High Tech
by
Rodford Edmiston
Being the occasionally interesting ramblings of a major-league technophile.
Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. If you find a mistake, please let me know about it.
Getting High
As this is written, we just had a Soyuz launch vehicle failure with a fortunately successful abort. This underlines that getting from the surface of the Earth into space is still a difficult, expensive and dangerous enterprise. Even though we're not doing it the way SF pioneers such as Jules Verne (huge canon) or Herbert George Wells (antigravity) chronicled. (Though, honestly, if antigravity worked the way Wells described that would almost be pleasant. Unfortunately, it doesn't.) Right now, we're stuck with using chemical energy, via rocket. Which means they still sometimes literally blow up in our faces. If only there were another way...
Konstantin Eduardovich Tsiolkovsky (Russian: Константи́н Эдуа́рдович Циолко́вский, thank you Wikipedia) was a small-town high school mathematics teacher. However, thanks to the efforts of one his own teachers, he developed early in his life a fascination with space and the exploration of it. He not only evaluated - through mathematics and thought experiments - various methods of reaching space, but also some of the engineering details necessary for living and working there. Though he early on concluded that rockets were the only practical technology available for reaching orbit, he evaluated other methods. One of these was the orbital tower, inspired in part by the Eiffel Tower, completed when he was 38 years old.
Tsiolkovsky knew that at a certain altitude above the Earth the orbital period was the same as the planet's rotational rate. (Arthur C. Clarke later used this idea to invent the communications satellite.) If you could get directly to that altitude you could avoid much of the energy expenditure needed to first gain low orbital velocity. You'd only need to supply the change in potential energy. So if you had a tower that high, with an elevator...
Tsiolkovsky knew that no materials were strong enough to build such a tower, but he defined the mathematics required for its operations. However, many materials are far stronger in tension than in compression. What if you treated the distance between geosynchronous orbit and the ground as something to be spanned by a tension structure, rather than a compression one? You just need a counterweight a bit past geosynchronous orbit, to keep the structure taut.
There are several materials - some more practical than others - which have sufficient tensile strength to span that gap. As these have been studied and production methods developed the concept of the orbital elevator (or beanstalk) has become more practical. Several organizations are today evaluating the engineering and economics of eventually building a working beanstalk; though, as usual, science fiction went there first. In Fountains of Paradise Clarke proposes a form of diamond whisker as the material. Since that novel was written other substances have been discovered which are even stronger. These days, the target material is Buckytubes (Buckminsterfullerene); these are carbon nanotubes, single-walled or double-walled, hollow strands of carbon atoms in a hexagonal matrix, which have fantastic tensile strength. This material can already be made in the laboratory, and the lengths being produced are gradually approaching values which would make a beanstalk possible. Even technically feasible.
Most plans call for starting by a putting a spool of cable made from Buckytubes in low Earth orbit and unreeling it, with tidal forces causing a weight at the upper end to rise as a weight on the lower end descends. Once that first, small cable is established a lightweight climber will go up, unrolling more cable on the way. This will be added to the original, probably with dabs of advanced epoxy. Repeat with gradually heavier climbers carrying heavier cables until you have built a strong enough cable to handle cargo. The climbers remain at the upper end - further out than geosynchronous altitude - to add mass to the counterweight. Not surprisingly, this process of building a beanstalk is referred to as bootstrapping. :-) Most current plans have individual beanstalks far smaller than those in Clarke's story, but more of them. The bottom anchor would likely be a modified oil drilling platform in the distant ocean, far from shipping. This would be self-propelled, so it could tow the lower part of the cable out of the way of major storms or orbital debris. (Yes. The part of the cable crossing low Earth orbit counts as "lower." :-)
A beanstalk would be an impressive structure, and an impressive sight. Imagine standing at the Earth-based anchor and staring upward, watching this - according to current designs - tapered, flat ribbon ascending into the sky to the vanishing point and beyond, with no visible means of support.
Still, even if the technical issues of the beanstalk are solved we'll probably need rockets of some sort to get that first reel of cable into space. (In Clarke's story, the material had to be manufactured in space. Buckytubes can be made down here.) Even once beanstalks are built we'll need launch vehicles for some usages. Then there's the situation of continued major need for rockets if there are some serious technical - or political - problems with the beanstalk.
In 2005 the European Space Agency published an anthology of stories about the beanstalk concept, which had one of my short stories among them. The book is available from Lulu, at: http://www.lulu.com/shop/bradley-edwards/running-the-line/paperback/product-487807.html
(Amazon has the book, but only the first edition and for a collector's price.)
Conventional launch vehicles continue to improve. However, the improvement is evolutionary rather than revolutionary. We now know enough about rockets and propellants that any sort of sudden increase in performance is very unlikely. Not even barring the use of more exotic propellants than liquid oxygen and liquid hydrogen. Which, trust me, are both pretty exotic, already. However, when chemistry isn't enough, physics can be.
A laser thermal rocket throws the normal limitations for rockets out the window. In fact, they can use a chemically inert reaction mass, such as water. Most proposals, though, use chemically reacting propellants, for several reasons. These launch systems would use ground-based lasers to add energy to the reaction mass in any of several ways. The simplest is to just aim the laser straight up the exhaust into the combustion chamber. This has some limitations, of course, and points out one reason most of these schemes involve chemically active propellants. If you lose the laser - even if only because the rocket is tipping over at altitude to build up speed for orbit - you still have thrust. A conical aerospike would be a particularly good engine design for this, since the laser (or, more likely, lasers) could shine up the middle and through a window in the combustion chamber without having to go through the exhaust. A linear aerospike would also work, depending on the details of laser and engine design.
Some of these concepts use a continuous power laser. However, it is easier to get high peak power using pulses. Additionally, pulsed laser beams propagate better through the air, with lower losses. Also, with very sharp, intense pulses you can create Laser Supported Detonation waves. These are not only a more efficient method of using lasers to add energy to the reaction mass, they can even cause chemical changes which increase thrust, depending on what reaction mass you are using.
Even solid materials could be used for reaction mass in this way, through laser-induced rapid surface ablation. A sharp laser pulse dumps enough energy into the surface of the material that it vaporizes extremely rapidly, with little of the energy being transferred to the material beneath.
People have been working on airbreathing first stages - and even complete vehicles with multi-cycle engines which breathe air during the first part of launch and turn pure rocket when higher - for several decades. Such configurations are often proposed for single-stage to orbit concepts. The airbreathing liftoff idea is attractive; you use oxygen from the air while it's dense enough, doing away with carrying the oxidizer needed early on. However, airbreathing engines are heavier than pure rockets of the same thrust (in part due to air only being part oxygen) and combined cycle engines are even heavier. You can remove some of that weight by going with slightly reduced engine efficiency. Which is also a bad thing for a launch vehicle.
Using lower thrust during the airbreathing stage and instead adding aerodynamic lift also helps. Horizontal takeoff requires less thrust for a vehicle which generates aerodynamic lift than does vertical launch. The vehicle starts generating lift once it begins moving through the air, even if the thrust is lower than the vehicle weight at this stage. (Something used in the movie version of _When Worlds Collide_.) However, the gain in efficiency from low-thrust airbreathing engines must be balanced against the increased period in which aerodynamic drag is significant. A horizontal takeoff launch vehicle headed directly for orbit must climb pretty steeply.
If you have an existing aircraft capable of high-altitude flight you can modify one of those to use as a first stage. In this case, the time needed to get to launch altitude is less important. This approach is promising enough that several companies and even entire national industries are backing development of it.
In fact, there have for decades been many actual operations which put objects into space using an aircraft as the first stage. Even the Bell X-1 used a modified B-29 to get reasonably high before most of its flights. While none of the flights by the X-1 or its derivatives reached space (at that time defined as an altitude of fifty miles; today one hundred kilometers is used) those paved the way for aerospace craft which did, such as the X-15 rocket plane. In 1984 a modified F-15 fighter launched an anti-satellite missile in a test. In 1990 Orbital Sciences became the first private space launch company when it launched a payload into orbit using a three-stage Pegasus rocket. This was dropped from a B-52; one of those used for the X-15 launches, in fact. A similar system, using a Lockheed L-1011, is still in operation.
Such air launches continue to be used for high-speed vehicle and propulsion system test flights. This includes several successful flights by scramjet powered vehicles. If the scramjet can be made practical that could provide an airbreathing "second stage" for launch vehicles. Theoretically, a craft propelled in this way could even reach orbital velocity, by rotating upside down so that as the centripetal force increases with speed, the vehicle can use lift to stay in the atmosphere longer. Once going fast enough, the vehicle can rotate back upright and fly out of the atmosphere. Rockets are used for both attitude control and to obtain the desired final orbit.
There are many other technologies which might be useful for getting from the ground into space. Electromagnetic launchers of various types have been extensively and repeatedly examined. However, many proposals require already having access to space to implement.
The rotating tether can be thought of as a short beanstalk with a center of mass in low Earth orbit and which alternately dips its ends into the atmosphere. There a fast-flying airbreathing (and perhaps rocket boosted) payload is grabbed and lofted as that end of the cable swings upwards, heading back into space. Bodies in space can be captured and swung into the atmosphere, maintaining the rotational momentum of the object. Meanwhile, the rotating tether continues to circle the Earth.
The launch loop is an endless loop with one focus on the ground, where it is powered to overcome losses from atmospheric drag and transferring momentum to payloads, to fling them into space. It works like that trick where a rotating sprocket hurls a drive chain upwards, until it reaches its physical limit and turns downward. Except that the launch loop is much larger, and moving at a significant portion of orbital velocity.
Related to this is the orbital ring. This is a ring around the Earth at the height of low orbit but which moves at well above orbital velocity. (It's like a smaller version of Niven's Ringworld.) Fast enough that centripetal acceleration provides enough support for two (or more) opposed elevators from the surface to a magnetically coupled upper end. The distance of the elevator ride is much shorter than that to geosynchronous. Like the Ringworld, though, it would be unstable and require constant position holding through some means.
The space fountain is similar to both of the above concepts. It has one end on the ground, with a station in space supported above the atmosphere by magnetic fields acting on the upper portion of the moving loop.
The slingatron is a large disc - perhaps more than a kilometer across - which does not rotate with respect to the Earth. However, it is on an eccentric shaft which moves the entire disc in a small (compared to the size of the disc) circle. A spiral path leads from the center of the disc to the edge. Introduce a payload at the center and each "sling" moves it further outwards and increases its speed. At the exit it is at or close to orbital velocity.
Even the old idea of a space launch canon has been experimented with as a first stage for small payloads. Gerald Bull's Project HARP (High Altitude Research Program) of the Sixties set an altitude record for canon-launched projectiles. It was intended to be the first stage for a Martlet solid propellant rocket. Three installations were built for this research. In November of 1966 the third gun - operating in Yuma, Arizona - fired a 180 kg Martlet 2 projectile at a muzzle velocity 2,100 m/s. This reached an altitude of 180 km, a record which still stands as of 2018. (I have not been able to determine if the solid rocket motor of the Martlet 2 was actually fired in this test.)
There are many more concepts involving real physics than have been explored here. There are also many involving speculative physics which are far less likely to even be taken seriously. Let's just say that, while rockets are it for now and the near future, at least for the upper part of the launch, looking further ahead reveals multiple possibilities.
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Date: 2020-09-18 12:14 am (UTC)Which is about the same as the low-end range for fission powered rockets.
Fusion rockets would be interesting, but their exhaust is a major catastrophe. Between the temp and the velocity,, they'd really chew things up.
But if you had the exhaust inside a duct/shroud open at the front a "low power" jet would drag a l;ot of air in and expel irt at increased velocity and temp.
So you could use a fusion ram-rocket for takeoff and landing and crank it up as the air got thinner (and you were farther from anything to damage).