Sunday, January 5, 2014

The year in review and Propulsion Physics

Wishing everyone all the best for 2014, clear skies and smooth waters. 

Finished all my studies in marine engineering (I did get my next Chief Engineer's licence) and I'm back into Physics. Thought it would be interesting to look at 2013 in hindsight and sum up where we are up to. To be clear here I am looking at alternatives to chemical rockets for propulsion purposes in getting hardware from ground to Low Earth Orbit. As previously noted, this is a make or break issue for long term manned deep space exploration including interstellar travel.

There are plenty of studies looking at the Physics in getting hardware light-years from Earth however few that look at how to get hardware 100Km or so into orbit without using chemical rockets. Most of the interstellar related propulsion studies I've seen don't have applications for at least 100 years, although interesting they demand a fusion or other high energy shipboard power plant or an extensive beamed power source. The solar sail solution is elegant however past Jupiter's orbit is somewhat less effective because of the diminished solar radiation intensity and the very large sail area looks very impractical. They also don't address how the starship will be built in the first place and presumably assume an extensive space based shipyard / infrastructure. To sum up all the current starship designs proposed for interstellar travel have major problems, are impractical for interstellar travel and the cost would be astronomical, although the physics is sound. So there's still a lot more work to be done in this area before man is ready to launch an interstellar starship to other star systems.

An alternative propulsion system to chemical rockets on the other hand is achievable within a more reasonable time frame (say 30 years). The space elevator is feasible although considerable progress in the fabrication of carbon nanotube fibres has to be achieved before this gets anywhere near to reality (apart from the large dollar investment).

The space elevator concept. Image: Space Elevator Wiki
Gravity Control Propulsion (GCP) would be another logical (more elegant) alternative however the underlying Physics of General Relativity (GR) still hasn't been worked out. Until this is understood the viability of GCP cannot be answered (although the basic requirements from GR have been established). The validity of the current alternative models to GR are not yet clear. Most of the models that I have looked at don't even attempt to come up with predictions that can be verified by experiment today. It is also not clear why some areas of research in Physics concentrate on finding a unified model of physics in quantum gravity as Nature has not made this a clear requirement.

A basic requirement for GCP: a flat spacetime metric within a curved gravity well. Image: CI
Are there any other viable alternatives? Unfortunetly the other options such as electromagnetic catapult launches, tethered pendulums from orbit, external nuclear pulse propulsion etc are also problematic to say the least, again requiring either considerable infrastructure, radiation fallout, extremely high costs or payload launching capability is so small that we might as well continue to use chemical rockets which are at least affordable for small payloads until an elegant solution is found. We'll see what 2014 has to bring for Propulsion Physics.

CI.

Chemical rockets, the only means today to get into orbit. Image: NASA.

3 comments:

  1. Christopher PhoenixJanuary 8, 2014 at 10:15 AM

    Hi- I only recently found your blog, but I always enjoy a good blog on interstellar flight and physics-related topics. Wishing you a Happy New Year!

    The high cost and limited payload capacities of today's launch rockets are a major bottleneck. A lot of the studies on interstellar propulsion concepts side-steps the launch and construction questions, as the authors are focusing on basic feasibility studies of a particular interstellar propulsion scheme... and they are probably assuming that by the time we would begin considering interstellar travel, we would already have the necessary orbital infrastructure to build large structures in space.

    Some authors, however, do discuss how to build a starship. Most starship designs are too excessively large and/or fragile to be practically launched intact from Earth. We could send them up piece by piece, like ISS, but this would be very costly- and rather like eating a bowl of rice with tweezers, one grain at a time (at least with today's rockets!). Instead, they say it is far more likely the starship will be constructed in space from materials obtained from the Moon and asteroids, like the plans for O'neil colonies would have done- or the shells of starships could be partially assembled and launched from low-gravity planetoids and moons for finishing and fitting-out in orbit.

    This is a bit more viable, but does mean waiting until interplanetary travel is commonplace and extensive infrastructure exists throughout the solar system. And we still have to get enough mass into orbit in the first place to jump-start space mining. But once there, raw materials are readily available, and abundant energy from sunlight, microgravity, and hard vacuum are all assets for mining, refining, and construction.

    Then again, if we get tired if lifting all the little bits of spaceships and equipment into orbit piecemeal, there are entertaining old ideas like Dandridge Cole's Aldebaran- a giant nuclear-pulse launch rocket designed to meet the massive launch requirements of the far futuristic year of 1990(!). This used the Helios internal nuclear pulse rocket, cousin to the more famous Project Orion, only the explosions happened inside a (very) large combustion chamber.

    Every spaceflight begins on Earth, so a complete program for starflight must consider how we will literally get the project off the ground- hopefully more studies will be done to bridge the gap between paper starships that assume Star Trek's orbital shipyard and our current launch capabilities.

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  2. Hi Christopher,

    Yes ideally the starship would be built complete on the ground and somehow either launched or be able to get itself to orbit from the ground using its own propulsion system. Using rockets for this is just not an option for a decent size ship. The Orion project had some merits however having radioactive exhaust material in Earth's atmosphere wouldn't go too well I think with people.

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  3. "The space elevator is feasible" I contend that even if you could build it, you couldn't run it.

    1) van Allen belts. Material radiation degradation (add solar hard UV); being fried passing through in either direction.
    2) The filament must be non-conductive, or you have a 22,300 mile-long wire as the magnetosphere billows re solar flares and solar coronal mass ejections. That's a transformer. ZAP! Orbiting through the Earth's static magnetic field doesn't sound so good either. The huge energy surges convert from filament velocity, wanging it out of orbit.
    3) What powers the elevator? (2) is a bucking bronco.
    4) Only the endpoints are in stable orbit. The rest is moving at the wrong speed vs. altitude. The minimum energy curve for dropping the filament from geosynchronous orbit is not a straight line, it is a spiral or thereabouts.
    4) Say the super-filament average masses around 7 kg/foot, like suspension bridge cabling. 22,300 miles masses about 830,000 tonnes. Worldwide production of polyamides (e.g., kevlar) is some 70,000 tonnes/year - and it's an easy synthesis from cheap starting materials.

    OK, we have a ground to geosynchronous orbit dielectric beanstalk. The elevator itself travels at 100 mph (The world's fastest elevator is 38 mpg, Taipei 101). 10 day trip. What's in its fuel tank, paying for mgh? The sun shuts off 50% of the orbit in Earth's shadow re parasitic mass solar panels and batteries. The elevator cannot be small for net payload economics and propulsion. A many tonnes massive elevator cannot grab onto the beanstalk for the transferred stresses of not being in equilibrium orbit at any altitude, re Roche limit.

    OK, one goes up as one comes down, transferring energy. Piddles aside, what is the economic beanstalk geometry that allows them to safely pass each other at 200 mph relative while impressing opposite orbital stresses (tangentially one too fast, one too slow) on the beanstalk all the way up and down? Plucked guitar string. Not so good, even if you had a million tonnes of business plan polished shitanium, or its Federally-subsidized big brother bullshitanium - and a way to get it all up there in the first place.

    Space propulsion is momentum, mv (getting where you are going) versus energy, (mv^2)/2 (paying for momentum). Self-contained mass and energy sum to rocketry. External feed of either or both is not optimistic for large short term momentum change. New physics would be helpful. Tapping into the multiverse, finding a hot source and a cold sink, offers the known hazard of a short circuit. Matter-antimatter offerings are only half as large as Official Truth. Hadron-antihadron annihilation boils off half its energy as unharnessable neutrinos. Electrons-positrions are about 0,.03% of matter's mass.

    But wait! The low Earth orbital energy of a lump of anthracite coal is its enthalpy of combustion! Dear Reader need only fill in the blanks. More studies are needed.

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