3m-home.htm
Anyone wishing to copy this material may do so freely, though the ideas I've expressed remain mine and I would appreciate the credit. My e-mail address, oglenn@earthlink.net, may or may not continue to be functional.
I would like to thank all the folks out there who've contributed through your comments and questions as well as all the sources I've read. Had there been more of me this is one of the areas I would like to have pursued more.
Glenn
Tri-Mode comes from a general concept for designing low cost spacelift systems. That concept, or philosophy, is to use alternative accelerators (vice rockets), each in their most effective flight regime.
Using this concept I deduced that the most efficient spacelift system would use three modes of acceleration; mechanical, airbreathing, and rocket.
While working in spacelift planning and research I realized that we've essentially reached the technological and economcal limits of rocket propulsion. Rocket engines are on the order of 98% efficient in terms of energy conversion. We're pushing the limits of available materials and any additional improvements are in the margins and very expensive.
There are improvements that can be made in rockets like using more exotic materials to reduce a few pounds here and there. Because rockets place only a couple percent of their liftoff weight into orbit there is almost a pound-for-pound increase in payload, at least for a single stage vehicle. This relates to some large dollars savings but don't really pay for the research, design, and testing of such materials. Plus, like engine efficiency, materials have very little room for improvement (barring the invention of unobtanium).
Technology development has historically followed an "S" curve. That is, a new technology is introduced, improvements pick up speed, then flatten out as they near the limits of improvement. An example is propellor driven aircraft. Following the Wright brother's first flight there were years of working out the bugs and designing truly useful aircraft. Improvements in performance then came quickly and gave us a wide range of civil, commercial, and military aircraft. But they were limited by the propeller. Then came jet engines which were very buggy at first. Within a few decades we went from propellers to Mach 3+ spy planes and jumbo jets.
Spacelift has reached the end of what can be done with rockets alone and it's time to move on.
Realizing that rockets were dead ended as far as offering low-cost spacelift I went back to the basic question of what we were trying to do. Get into orbit, yes, but what does that mean? Low Earth Orbit (LEO) is about 100 miles up and moving at 17,000 miles per second. So the goal is to add altitude and velocity, or energy, to mass. At low Earth orbit (LEO) about 10% of the energy is potential (altitude) and 90% is kinetic (velocity). We can already do that with rockets but the cost is a couple of orders of magnitude higher than desired.
Velocity being the biggest part of the goal I started looking for alternative accelerators. A range of alternative accelerators was considered, as well as altitude additives such as balloons. For now I'll skip the lengthy description of each one I looked and summarize the results.
Mechanical accelerators come in a variety of forms but a number of them provide highly efficient energy conversion (acceleration) at ground level. This efficiency decreases as velocity increases and costs increase exponentially due to the long runs involved. At an acceleration of 3 Gs, that of the Space Shuttle, it takes about four miles to achieve Mach 2.
Airbreathing accelerators also come in a variety of forms. Turboprops and turbofans are highly efficient but are limited to subsonic speeds, which can be covered by mechanical accelerators which are more efficient. Turbojets are less efficient that turbofans at low speeds but are capable of about Mach 2. Afterburners have been used to exceed Mach 3 and Mach 5 turbo-ramjets have been proposed for commercial transports.
Ramjets have no thrust at zero velocity but exceed that of rockets above Mach 0.5 and exceed turbojets above about Mach 2.5. Extensive literature searches and interviews with ramjet experts shows that ramjets can reach at least Mach 7, and possibly as high as Mach 11, over half the velocity needed for orbit. Ramjets have been flying for over 60 years, the open literature describes Mach 5.5 flight achieved, are simple, and have a number of advantageous traits. In 1951 NACA (NASA's predecessor) produced a report, L50L27, that describes testing ramjets that accelerated a vehicle from Mach 1.33 at 2,300 ft to Mach 3.12 at 67,200 ft. Had the ramjets been larger and the fuel more than minimal the vehicle would have put many rockets to shame.
I looked at scramjets but after 40 years of testing, about $4B invested around the world, they are still at a NASA Technology Readiness Level of about 4 (out of 9). I've talked with analysts and they agree that, even if they work, they will be very limited in their ability to propel vehicles. If they do work, however, they could be used to improve on the already impressive ability of conventional ramjets.
Above the atmosphere I found nothing that could reasonably replace rockets.
I looked at different ways to reduce the number of accelerator modes (or stages) but was unable to do so.
For the mechanical accelerator I found a tradeoff between two methods, depending on final velocity. Up to about Mach 0.5, the speed at which ramjets become more efficient that rockets, something like a catapult was more efficient and cost effective. From there to about Mach 2, possibly 3, pneumatics may be the best. There are many examples in industry of building large diameter water pipes (up to 20 ft) up to hundreds of miles long. Steam and other gasses can provide more than enough force to achieve supersonic exit speeds. The downside of such speeds is the sonic boom.
Because of the wide velocity range of the ramjet, it's simplicity, and potential for improvements, I chose it for the second accelerator, or stage. There will need to be a tradeoff between the mechanical and airbreathing modes to determine the economically optimum transition speed. One minor variation would be to use two ramjet stages, one for subsonic and low supersonic speeds, and the other to achieve hypersonic speeds (above Mach 5).
The rockets will have to be sized based on the limits of the ramjets but by this time the vehicle will be outside the atmosphere and into the environment where rockets are at their most efficient.
Note: This is a working draft so far
This Page Last Updated 22 Sep 98