Tuesday, November 01, 2011

Cruise Power

Recall three phase profile of thought experiment's typical interstellar profile from Sol to AC.
  1. Accelerate at g-force for up to a year. This achieves velocity, v, significant portion of c.
  2. Cruise at v for several years. Impractical to carry enough fuel to accelerate for entire interstellar flight.
  3. Decelerate at g-force for same duration as acceleration. This reduces ship's velocity down to operational speed at destination.

(Time, speed, and distance values in following paragraphs are examples only.)
Acceleration Phase. Spacecraft accelerates at g-force for 246 days to achieve a very high velocity of 50% c (distance traveled would be .189 LY = 11,890 AU). Then, spacecraft stops g-force acceleration to maintain constant velocity. The g-force which accelerates the spacecraft will also impart near Earth gravity throughout the vessel. Power needs for entire vessel can be bled off of propulsion system just like auxiliary power needs for planes, trains, ships and automobiles are currently bled off of their main power sources which primarily feed their movement source.
Above g-force and power needs also apply for the Deceleration Phase (also 246 days duration).
Cruise Phase. Having achieved a significant portion of light speed (.5c), travel time between stars is reduced from centuries to just years. (Example: distance to closest star, AC system, is about 4 LY; thus, Earth observer would measure a cruise time of 8 years.) Near Earth gravity is maintained by spinning the habitat section of the vessel just like asteroidal habitats will have done for centuries back at the Solar System. Just like habitats in the Solar System, interstellar cruise habitat's power needs will need the most effective fusion fueled power; this is the subject of this chapter.

For the long cruise phase of interstellar flight, propulsion is off and the ship travels at constant velocity. If no propulsion, how does the vessel satisfy the continuing power needs of the sizable onboard population. Vessel will need to transform from an accelerating g-force vessel to a spinning habitat and simulate gravity via centripetal force. (Note: Habitat is a hollowed out asteroids to be living quarters for relatively large populations). Sunlight will definitely not be available. Fusion power will have to be available, and it will have to be the most effective fusion power; that's where Helium-3 (3He) comes in. While sunlight might provide essential power for vessels close enough to sun (after all sunlight now powers the planet, Earth), sunlight density disperses quickly with distance, and vessels well beyond Earth orbit will need another long term reliable and safe power source, 3He is the best known candidate.
Selected events
  • 1950's to 1970's: Conceptual designs for both D-T and D-3He. These included
    1. Simple mirror reactors
    2. Toroidal reactors with magnetic divertors
  • Thought a new concept in the early 1960's when the D-3He design was introduced, high temperature superconductivity would greatly improve the design.
  • Recent advances have enhanced the feasibility of D-3He reactors for space applications. These include discovery of Lunar-3He resources.
  • Early authors on D-3He space-propulsion reactors: Englert (1962) and Hilton, et al. (1964). Their papers have many interesting ideas and several important engineering approaches.
  • Later authors, such as Roth, et al. (1972), introduce 'bucking' coils to extract a magnetic flux tube from a toroidal magnetic fusion reactor and gain thrust via the exhaust.
  • Arthur C. Clarke wrote in 1961: "The short-lived Uranium Age will see the dawn of space flight; the succeeding era of fusion power will witness its fulfillment.''

Fusion Factors for Space Applications


  1. High percentage of charged-particles efficiently converts fusion power to thrust or electricity.
  2. D-3He reduces radioactivity hazards.
    • No radioactive materials at launch.
    • Only low-level radioactivity after operation.
    • Reduced neutron flux reduces radiation shielding.
  3. Higher specific power values (1--10 kW-thrust per kg).
    • Much higher than for nuclear-electric or solar-electric propulsion.
  4. Fusion gives high, flexible specific impulses (exhaust velocities), enabling efficient long-range transportation.
  5. D-3He output energy greatly exceeds input energy.
    • Extremely high energy density.

We often hear that antimatter-matter annihilation is the best method for space-propulsion. However: the process of creating antimatter takes a lot of energy; as a matter of fact, it takes 104 times more energy to create than it produces when annihilated with matter. This is definitely not the case with fusion which promises an efficiency of more then 70%. Thus, antimatter will probably be of limited use for routine interplanetary travel.

Fusion Alternative: Antimatter?

D-3He Fusion Capabilities and Space Development Fusion is the only option that potentially achieves the most important regime for Solar-System travel: exhaust velocities of 105 to 106 m/s. (NOTE: 1,000,000 m/sec = .003c)Implications for space development Although the emphasis of this document has been space propulsion, relatively small mass penalties would allow the magnetic fusion systems described here to produce electricity or to be used for materials processing by the hot fusion plasma. These applications, along with fusion's propulsion capabilities, would enable
  • Human settlements on the rocky planets or in space;
  • Scientific outposts near the gas-giant planets and elsewhere
  • Access to the vast resources of the asteroids.

  

Great Slide Shows

for more content from presenter (includes other type material) see
University of Wisconsin's
(Click above for slide show.)



(Click above for slide show.)
.. for more content from presenter (includes other type material) see General Atomics fusion energy educational Web site
(Click above for slide show.)
Question: Can we do a mass analysis of 150 million reactions per second? Recall that a mole has 6x1023particles (atoms or molecules or ions). Consider that a theoretical "pure" reaction between 2D + 3He results in 1p + 4He. Of the two products, 4He is charge neutral and will continue to move randomly in the fusion chamber; however, the proton has a positive charge and can be channeled by a magnetic field to become "thrust" for a space vehicle. According to our definition of mole, molecular weight of a substance, expressed in grams, one gram of protons (essentially, Hydrogen without the electron) has 6x1023 positive ions with atomic weight, 1.0. If all the 150 million reactions produced a proton and were properly channeled by the magnetic field, total weight would be 1.5 x 108 protons / 6x1023 protons/gram = 2.5x10-16 grams, an infinitesimal amount of mass. Proof of principle, but obviously a long way to go. Kulcinski has made some essential contributions to the research of fusion propulsion; perhaps other scientists have also made some progress in this area. U.S. astronaut-scientist-entrepreneur, Franklin Chang-Diaz explains the plasma drive: “...rocket engine of the future. As plasma is released through an exhaust nozzle, it creates the rocket effect and pushes the engine (in opposite direction). ... a plasma engine is more efficient and faster. In fact, a plasma-driven rocket could push a cargo from Earth to Mars in ninety days, about twice as fast as solid or liquid fueled rockets.” (Many thanks to Wired Science contributor Adam Rogers for an excellent interview..edited for following.) Astronaut Franklin Chang-Diaz has spent more time in orbit than most people spend in airplanes. Since 1986, he's logged over 1,600 hours on board the Space Shuttle. Now retired from NASA, he's President and CEO of the Ad Astra Rocket Company. At facilities in Houston and in his native Costa Rica, Franklin Chang-Diaz is developing some revolutionary rocket engine technology. Ad Astra is building a spacecraft propulsion system, Variable Specific Impulse Magneto-plasma Rocket (VASIMR).VASIMR is a very different type of propulsion system. For comparison, consider current chemical rockets which combine reactant chemicals (such as hydrogen and oxygen) for a quick burn to produce a lot of heat. VASIMR exhaust gets much hotter then chemical reactants without burning. It heats a gas until it becomes plasma, the stuff of stars; the hotter the exhaust, the faster the rocket. VASIMR works great for interplanetary voyages, but it's not suitable for Earth launches. Plasma is by far the most plentiful substance in the universe, but it can't exist in the atmosphere; it needs a vacuum. Thus, we could use it for transport between planets. Use a traditional chemical rocket to achieve orbit; then, enter the interplanetary vehicle and travel to Mars. Of course, we would some sort of a transporter to get us to orbit and then from orbit, we would move on with plasma drive, just like the Starship Enterprise…. Plasma drive is much more efficient than a traditional chemical rocket. To supply the moon for many, many years, plasma gets stuff to the moon cheaply. Plasma drive is much faster than a chemical rocket if you give it range. You can realize that speed capability on a mission to Mars because the engine stays on. When you get to the mid-point of your journey, vessel needs to flip around and start decelerating. Otherwise, you miss the planet and then you miss your exit, you can't just turn around. You could only realize that speed capability in a, on a mission to Mars or beyond. And that is, um, because the, the engine is on all the time. You can think of it as on continuously halfway and then when you get to the midway, the mid-point of your journey, you flip around and you start decelerating because otherwise of course, you miss the planet and then you miss your exit, you can't just turn around. The M in VASIMR stands for "magneto-plasma" because magnetic fields are essential. Plasmas are about a million degrees and cannot be contained by any material. Fortunately, plasmas respond very well to magnetic and electric forces; thus, we create an invisible flux tube, an invisible sleeve with the shape of a nozzle. It's a force field and so instead of a cone, like on the bottom of a standard rocket, that's just a magnetic field. A magnetic nozzle. Chang-Diaz's original research was in fusion, so that confining plasma with a magnetic field comes results from that. Origin of this whole technology comes from research in magnetic fusion. Confine the plasma and hold it away from any material walls to heat it and compress it to thermonuclear temperatures to ignite the plasma. Phase of development is the VASIMR. We’re about to test a flight-like prototype, in the JPC laboratory in Houston. With the results of that test, we will then design two flight engines. Space travelers to Mars may get a quicker trip with plasma-powered engines. At Johnson Space Center’s (JSC) Advanced Space Propulsion Laboratory, Dr. Franklin Chang-Diaz is working on a plasma rocket to reduce travel time to Mars from the current six months to about three months. Variable Specific Magnetoplasma Rocket (VASIMR) uses radio waves to heat hydrogen to plasma state which achieves extremely high temperatures (millions of degrees). This greatly increases rocket performance which improves with hotter exhaust; thrust greatly exceeds that of chemical reactants which only reach thousands of degrees in a conventional rocket engine. Plasma, the fourth state of matter, is electrically charged gas made up of ions. Plasma typically occurs in environments of high pressure and temperature, such as stars. Soon, Chang-Diaz hopes to test the engine in a vacuum chamber. While these lab tests will be powered from the local grid, operational tests will require their own power. Near Earth tests will use solar arrays. Deep-space tests will likely require nuclear power to provide the wattage needed to heat the plasma. A persistent obstacle to fusion energy has been the difficulty of containing hot, electrically charged plasma. Ironically, the traditional plasma confinement problem led Chang-Diaz to consider leaking plasma for spacecraft propulsion. He realized that allowing plasma to leak out in one direction would create thrust on a spacecraft. Thrust from the plasma engine could boost a spacecraft for a longer time and with better efficiency than conventional engines. Plasma engines would have longer and stronger thrust than conventional rocket engines. (Specific impulse.)
VASIMR is a high power, electro-thermal plasma rocket with a very high specific impulse (Isp) and a variable exhaust. In VASIMR’s magnetic configuration (“asymmetric mirror”), plasma is injected, heated and directed out of the vehicle to propel it forward.
  1. First stage injects and ionizes propellant gas via a helicon discharge.
  2. Second stage further heats the plasma via cyclotron resonance.
  3. Third stage acts as a magnetic nozzle by directing the plasma momentum opposite to direction of travel.
The magnetic field insulates nearby structures from the very high plasma temperature (>1,000,000°K.) This high performance magnetic field needs high temperature superconductor windings. High temperature superconductors can operate at higher temperatures then normal superconductor. However, even high temperature superconductors need cooling; this is accomplished by a combination of deep space radiation supplemented by cryogenic propellants (hydrogen, deuterium, helium or mixtures).



NOTE: Granting that Dr. Franklin Chang-Diaz is a brilliant astronaut-scientist, and I am just a simple layman; however, I’d like to further leverage his brilliant ideas about the VASIMR. First, I wholehearted agree that a three month trip to Mars is much better than a 6 month trip. Thus, I readily concur that a plasma drive performs much better than a chemical drive. However, I think we can do much better. If we can figure out how to accomplish constant g-force acceleration throughout the flight, we can travel to Mars in DAYS!!! The way that first comes to mind is an onboard particle accelerator which speeds up a specified quantity of plasma particles to significant portions of light speed. Resultant momentum could impart g-force to a very large space vehicle. I think the VASIMR device could serve a truly useful role as the particle source for the particle accelerator. A controlled flow of plasma into the particle accelerator would be far more useful then the same controlled flow directly into space. Top speed from VASIMR exhaust particles would be about 1/3 of 1%c (one million m/sec), while this greatly exceeds particle speed of chemical reactants (thousands of m/sec), it is much much less than what a particle accelerator can do (eventually high 90's percentiles of light speed). Variable flow from VASIMR is extremely important to maintain g-force throughout the flight. Recall that force requires energy expenditure; thus, constant force requires energy in form of fuel which leads to fuel consumption. Fuel consumption reduces mass of spaceship which in turn requires a fuel flow adjustment to maintain accurate g-force. Daily difference discusses this differential effect in more detail. Δ = %TOGWDay = ffDay/MShip vExhffExhMShip%TOGWDay (d * c )(n * ffsec )(mega-ffsec )(Δ)
d

n

(106 * ffsec)
(%MShip/Dy)


































































































































.866

2

52.95

0.163%


































































































































Given
1
√(1-d2)

c*√(n2-1)*ffsec
g
0.283%
√(n2-1)

































































































































Given exhaust particle speed is 86.6% light speed, then exhaust particle size doubles; thus, n =2. As particle speed further increases, size also increases, triples for n=3; quadruples for n=4, etc.; however, this example considers only the case of doubling.
All factors considered, g-force ship mass is about 53 million times size of orginal fuel mass. For example, if original fuel per second (fuel flow, ffsec) is 1.0 kgm, then the size ship which will be propelled 10 m/sec faster is 52.95 million kg = 52,950 metric Tonnes. Another example, if the ship size is 100,000 mT, then ffsec= 100,000 mT/52.95M = 1.88 kg = 1,888.36 grams.
Fuel consumption reduces mass of spaceship which in turn requires a fuel flow adjustment to maintain accurate g-force.
GW0 = 100,000 mT
GW1= (1-Δ) 100,000 mT
GW1= (0.99837) 100,000 mT
GW1= 99,837 mT
Thus, at end of first day of g-force flight,
ff1 = 99,837,000 kg/52,950,000 =1,885.5 gms
Thus, there is a slight difference in fuel flow from start of flight to end of day 1 (due to decreased energy needed to g-force propel a slightly lighter ship). This necessitates an adjustable "throttle" to continuous adjust plasma flow into accelerator. This could be done via a "math model" which considers above factors. A simpler implementatin might be a "servo" model which continuously compares weight of known mass onboard ship to what it would be on Earth. Quick example, does a 100# weight still register 100 lbs on a scale onboard ship, just as it did on Earth. This is very similar to a well known thermostat which continuously compares actual temperatures to desired temperature.
Question: He-3 will definitely be needed to keep us warm during the flight, but will it help propel the ship???
Typical distance from Earth to Mars, about .5 AU = d = 75,000,000 km.
dAcc = d/2 = 37,500,000 km = dDec
Assume typical trip time about 3 months = t = 90 days.
tAcc = t/2 = 45 days = 3.888 x 106sec= tDec
vAvg = dAcc / tAcc = 37.5 x 106km /3.888 x 106sec= 9.645 km/sec = tDec
To determine rate of acceleration, assume initial velocity is zero (v0 = 0 km/sec).
Recall d= a * t2/2
Thus, a = 2 * d / t2 = 2 * 37.5x106 km / (3.888 x 106sec) 2 = 4.96 x 10-6km/sec2
vFin = a * t = 4.96 x 10-6 * 45 days * 86,400 sec/day = 19.2845 km/sec
vAvg = vFin/2 = 19.2845 km/sec /2 = 9.642 km/sec

Recall Momentum Exchange Equation.
mExh * vExh =MShip * VShip
Divide each side by one second.
mExh * vExh

1 sec
=MShip * VShip

1 sec
Redistribute
mExh

1 sec
* vExh =MShip * VShip

1 sec
Mass per second = fuel flow
ship velocity per second = acceleration.
ffSec * vExh =MShip * a

Assume max vExh = 106m/sec
Assume ship size = 5 metric Tonnes = 5x106gm
Round a ≈ 5x10-3m/sec2
ffSec =MShip * (a*sec)

vExh
=5x106gm * 5x10-3m/sec

106m/sec
=.025 gm=ffSec
Given a fusion propulsion system; then, we can assume exhaust particle velocity of 1 million m/sec. Thus, a particle exhaust rate of .025 gram/sec will propel the 5 metric Tonne (mT) vessel a mere 5 millimeters/sec faster for each second of powered flight. Recall Avogadro's number 6.023x1023 particles per mole; thus, .025 grams of protons is 1.505x1022 protons which must pass through ship's exhaust nozzles.
Without the fusion propulsion system, the vessel have to endure a constant velocity 6 month trip. With the fusion propulsion, the trip is a much shorter 3 months. If I had to choose between the above two choices, I'd take the shorter trip.
However, this thot exp. introduces a third choice, a particle accelerator propulsion system. Present day accelerators routinely take ions to near light speeds; thus, thot exp. assumes exhaust particle velocity of .866c which introduces a relativistic growth factor of 2 (particle is going so fast, it doubles in size). Considering both this enormous velocity and relativistic growth, thot exp. assumes that particle exhaust flow of .1 gms/sec will propel a 5 mT vehicle 10 m/sec faster for every second of powered flight. This exhaust performance introduces g-force which simulates near Earth gravity to the powered portions of flight. Furthermore, a trip to Mars is reduced from months to just days, a much more reasonable trip time.
TRADE-OFF: Technology Requirement. This fusion propulsion vehicle requires a lot more technology then the constant velocity vehicle. If we consider the technology advance as an order of magnitude higher; then, the particle accelerator propulsion vehicles is several orders of magnitude greater in technology then the fusion vehicle. As a matter of fact, thot exp. suggests using a fusion reactor as an initial stage in the accelerator propulsion system.
VASIMR system strikes me as good candidate for this initial stage. VASIMR's variable thrust would prove very useful as explained earlier.

Source document. Fusion research began in 1951 in the United States under military auspices. After its declassification in 1957, scientists began looking for a candidate fuel source that wouldn't produce neutrons. Although Louie Alvarez and Robert Cornog discovered 3He in 1939, only a few hundred pounds (kilograms) were known to exist on Earth, most the by-product of nuclear-weapon production.

Source document. 3He fusion is ideal for providing internal power to spacecraft and interstellar travel. It offers the high performance fuison power, 3He reactors would require less radioactive shielding, lightening the ship's internal infrastrucutre.
Lunar supplies The possibility that helium-3 may be widely found on the Moon has led to discussions ([2], [3]) as to whether it could be used as an energy source. Yet to be determined is the exact quantity of helium-3 which the solar wind traps and deposits on the lunar surface. It may be so scarce as to be beneath the point of economic recovery. The temperature required for helium-3 fusion is ten times higher than conventional D-T fusion, which itself has yet to be achieved at the break-even point (to clarify, fusion experiments have produced Q values >1, ie where energy output exceeded energy input; however break-even here probably refers to ignition of the plasma, otherwise known as a 'burning plasma') . Accordingly, helium-3 seems less likely than other reactants for use in fusion power generation, though it cannot be ruled out completely.
Fusion
Helium-3 undergoes the following aneutronic fusion reaction, among others, although this is the one most promising for power generation:D + 3He → 4He (3.7 MeV) + p (14.7 MeV)The appeal of helium-3 fusion stems from the nature of its reaction products. Most proposed fusion processes for power generation produce energetic neutrons which render reactor components radioactive with their bombardment, and power generation must occur through thermal means. In contrast, Helium-3 itself is non-radioactive. The lone high-energy proton produced can be contained using electric and magnetic fields, which results in direct electricity generation.However, since both reactants need to be mixed together to fuse, side reactions (D + D and 3He + 3He) will occur, the first of which is not aneutronic. Therefore in practice this reaction is unlikely to ever be completely 'clean'. Also, the temperatures required for D + 3He fusion are much higher than those of conventional D + T fusion, so it is unlikely that this type of fusion will be achieved before the problems with conventional fusion are worked out.A common myth is that due to the rarity of helium-3 on Earth, any reliable sources of the fuel have to come from other bodies in space. This is untrue. Helium-3 is a byproduct of tritium decay, and tritium can be produced through neutron bombardment of lithium, boron, or nitrogen targets.
BIS's earlier Daedalus study used D-3He fuel but had to simply assume that the physics would work.
The Daedalus interstellar concept was worked out to the finest detail by the British Interplanetary Society in the 1970's. Powered by nuclear fusion, Daedalus's gross weight would be 49,000 metric tonnes and would require 27,000 tonnes of Helium-3 for fuel (not available on Earth but readily available in Jupiter's atmosphere).

Daedalus was an unmanned spacecraft and the project planned for a one-way trip to Barnard's Star. About 6 light years away; thus, an average speed of 0.15c (15% light speed), travel time would take about 50 years.
Recall Avogadro's number 6.023x10^23 particles per mole; thus, .025 grams of protons is 1.505x10^22 protons which must pass through ship's exhaust nozzles.


(Click above for slide show.)
Plasma, the fourth state of matter, is made up of electrically charged ions. Plasma typically occurs in environments of high pressure and temperature, such as stars where the environment is way to hot and heavy for matter to exist as solid, liquid or even gas. Thus, plasma comprises the overwhelming majority of existing matter; humans see plasma when they observe stars; however, they also see plasma in neon signs and flourescent bulbs.

posted by JimODell at 12:41 AM

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