From Ctrl+Alt+Del Comic…
Robert Bussard first wrote up the Interstellar Ramjet in 1960 – 52 years ago now. Scooping up one’s fuel whilst en route makes possible high levels of time-compression thanks to relativity.
The typical flightpath would be roughly as follows…
Taking 10 years to Alpha Centauri and about 100 years to reach 40 light-years. “Fast” but not as fast as it might get, with continuous boosting. One problem with extreme relativistic travel is the dust and gas between the stars becomes a lethal energy shower. Protection becomes the big question above about 0.6-0.8 c.
NASA Technical Report… Eagleworks Laboratories: Advanced Propulsion Physics Research
Harold White, Paul March, Nehemiah Williams and William O’Neill form the Advanced Propulsion “Eagleworks” which is exploring edge-fo-science concepts, like the Quantum Plasma Thrusters (Q-Thrusters) and Warp-Drives. Very much a neglected field of “just barely what we know” applications of advanced physics to NASA’s mission. The Q-Thruster performance really caught my eye – they’re talking a 1 year manned-mission to Neptune. To put that in perspective, Neptune is 4.5 billion kilometres away or about 60 times further away than Mars. Travelling there in 1 year needs ~0.02 m/s2 acceleration all the way and a top speed of ~570 km/s. The incredible thing, to me, is that such research isn’t trumpeted from the roof-tops, but half the time I suspect we’ve become wary of disappointment. “Breakthrough” physics doesn’t always turn out the way we expect it to.
Three of my Icarus Interstellar colleagues, Kelvin Long, Richard Obousy and Tabitha Smith, are attending the Nuclear & Emerging Technologies for Space conference. Lots of innovative engineering for nuclear power and propulsion – new energy conversion systems that turn radioisotopic heat (usually from Pu-238) into electrical power, and new designs for space-capable fission-reactors.
Plus some alternative technological work is being reported on. At least two Icarus Interstellar presentations, one with my name on it as co-author (I did a lot of background research for it, but Rich and Kelvin did the writing and Power-Pointing.) The second covers ignition of fusion via antimatter. Tabitha Smith is our resident nucleonics expert and is very keen to see a revival of nuclear thermal propulsion for space applications. Another concept – which I didn’t see an abstract for – is the fission-fragment rocket, which also has potential as an advanced fission reactor.
Then there’s the real fringe, but with solid experimental work to back it. Low-Energy Nuclear Reactions (misnamed “cold fusion”) were represented by two papers, one which discussed the physics and another which described a new “cold fusion” radiothermal heat-source. LENRs work by funky solid-state physics which confines deuterons in much closer proximity than they would ever experience in free states, like gas or plasma. This allows fusion tunnelling probabilities to go way, way up and so-called pycnonuclear reactions to occur. Small particles of palladium or platinum can soak up large amounts of deuterium gas and, once suitably “loaded”, in the right conditions, emit large amounts of heat for indefinite periods of time. Xiaoling Yang & George Miley’s battery produces about ~350 W/kg – a bit less than Pu-238’s 550 W/kg, but without the very nasty radiation. Not as power dense as a fission reactor, but enough to power space-probes. Given better thermoelectrics (also presented on at the conference) that heat might produce ~50 W/kg of electricity to power a probe’s electronics or even a low-thrust engine.
What kind of engine though? The presentation by Harold (“Sonny”) White and Paul March presents a propellantless drive, which they dub the Quantum Vacuum Plasma Thuster or Q-Thruster. It works… in test-rigs. The idea is simple – it uses the virtual particles of the vacuum to create an “ion wind” for push. Thus no onboard reaction mass required. Whether it will work in space is something they’re seeking to test in an orbital payload in the near future. Presently they quote the Q-Thruster making 0.1 N per kW of power, massing about 10 kg per kW. That’s pretty good compared to the current generation of ion-driven probes like “Dawn”, currently orbiting Vesta, which turns 2.6 kW(e) into 0.092 N thrust at maximum jet-power. Better still, there’s no propellant involved. No tanks, no piping, no flow-controls. Given power, a Q-thruster will push and push…
Imagine a nano-satellite, about 10 kg mass. Its power source is 3 kg of Miley battery – thus 100 W(e) to power the Q-thruster, and 50 W(e) for the probe’s instruments. That gives it a push of 0.01 N. A mass of 10 kg, that means ~4 kg of power/propulsion and 6 kg of instruments. The acceleration is ~0.001 m/s2, which sounds low, but isn’t really. “Dawn”, for comparison, masses ~1,290 kg at injection into deep-space and yet gets a top thrust of 0.092 N. “Dawn” also holds 450 kg of Xenon propellant in its tank, which it would exhaust in ~4.65 years at top thrust. Its maximum acceleration is just ~7E-5 m/s2, or 14 times less than our nanosat.
How long would a Miley LENR battery last? If 0.1 of the mass is deuterium (palladium really does soak it up) and the energy is from D+D -> He4 fusion (as suggested), then 100,000,000,000 seconds of heat is available at 350 W/kg. About ~3,170 years. How far would it go in that time at 0.001 m/s2 acceleration? By that stage it has reached 1/3 lightspeed (100 million m/s) and travelled over 520 light-years. In theory it could go into orbit around Alpha Centauri in 404 years, or fly past in 286 years at 0.03 c. Closer to home it would whizz past Pluto in a bit over a year.
Enough speculation for one night. Ciao!
A plan from 1990 for an antimatter powered starship of gargantuan size. The starship is a “terraformed” asteroid, wrapped in a multi-layer artificial “sky”/Whipple Shield. With just ~100 people to start with, the thing must mass multi-million tonnes, all just to be nice and homey for the crew. Are such colossii really needed to carry humans to the stars? Other designs have appeared from time to time and appeal more to the claustrophile than the claustrophobe. Dana Andrews  estimates a toroidal starship 200 metres round, but only a few metres wide in its rim, might house ~20 people for a mass of ~200 tonnes. His vehicle uses a magnetic field and an electron stripper – a thin sheet of durable material perpendicular to the direction of travel – to protect againt the ISM and cosmic-rays. Space dust and neutral particles pass through the electron stripper, which ionizes them and thus allows them to be deflected by the magnetic field. The toroidal habitat is large enough for a mini-menagerie and dwarf trees, and provides a nice running track through the middle, though I do wonder how 20 people would manage in such a confined space for ~20 years of starflight. Yet if the first star-farers aren’t “Flatlanders” (to borrow Larry Niven’s phrase for Earthlings) but “Belters” who are used to space’s yawning void, then such a space might seem very homey indeed.
 Andrews, Dana G., “Things to do While Coasting Through Interstellar Space”, 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Fort Lauderdale, Florida. AIAA 2004-3706.
Spencer & Jaffe, in 1962/3, produced this JPL study which examined the feasibility of interstellar probes, finding there was no physical reason why fusion or fission propelled multi-stage vehicles could not approach the speed of light. With five stages and a mass-ratio of ~240,000, a D-He3 fusion propelled vehicle could reach ~0.8 c. However the initial mass would of necessity be very large and the engine power levels required would be unlike anything in previous experience. However the physics was clear, interstellar travel was feasible. But how fast would a probe accelerate to burn-out speed? Heating limitations meant the acceleration that could be achieved would be low – a nominal 5 light-year mission would require five stages and 55 years. Fusion plasma containment vessels and high-temperature radiators could only handle so much waste heat from the reactions – about 20% would be lost as chiefly x-rays via bremmstrahlung.
Dwain Spencer explored the characteristics of fusion-propelled probes in a more focussed piece:
Fusion Propulsion System Requirements for An Interstellar Probe
His work reinforced the original finding that fusion propelled probes would be hampered by heat-rejection issues. In one sense nothing has changed since, as magnetic confinement fusion reactors inherently absorb a large fraction of waste heat from the reactions they contain. Newer materials might reduce the intercepted x-rays, instead returning them to the fusion plasma, but waste heat needs to be efficiently handled for fusion rockets to achieve reasonable voyage times.
Freeman Dyson, based on his work on “Project Orion”, led him to sketch a high performance interstellar version of “Orion” with a wholly different relationship to reaction heat. Instead of absorbing and re-radiating the heat, blow it away with the propellant in a massive thrust, with a very short heating period. Interstellar “Orion” used pure deuterium fusion devices with very high burn-up fractions (“burn-up” is the fraction of fusion fuel that actually fuses) which allowed high-performance. Dyson argued, for reasons of energy efficiency, that the mission velocity be kept to a low multiple of the effective exhaust velocity – his fusion devices had an exhaust velocity of 15,000 km/s (0.05c) and his “Orion” had a mass-ratio of just 4, meaning a total delta-vee of ~20,000 km/s. This meant trips to Alpha Centauri lasting ~130 years. A slightly higher mass-ratio would drop that to just 100 years – “Project Icarus” time-frames.
Daedalus assumed He3 sourced from Jupiter, but the other three gas giants would be better due to Jupiter’s massive gravity-well. Uranus seems the best choice – as I jokingly put it, He3 from the Gas Mines of Uranus – for accessing this very attractive fusion fuel. However the energy requirements reported in this NASA paper are quite at odds with the figures derived by Bob Parkinson for Daedalus, a discrepancy I can’t yet explain. There’s more helium and helium-3 than what Parkinson assumed, so the task is easier, but the energy levels are an order of magnitude higher. This means extracting sufficient He3 is a mammoth task and a significant fraction of any interstellar effort.
For that reason I advocate developing deuterium propulsion. Pure deuterium fusion puts out about ~1/3 its energy as neutrons (versus 4/5 for D-T fusion) making it challenging to operate at high power-levels. However the prospect of ultra-dense deuterium, 1 million times denser than liquid deuterium, makes this potentially a very good choice indeed. Quite possibly ultra-dense deuterium forms naturally inside super-Jovian planets, which makes for the intriguing possibility of planetary collisions causing deuterium fusion explosions, with all sorts of exotic isotopic anomalies arising… much like what’s seen in our Solar system.
Norman Schulze’s massive study covers the major options and their state of development as of 1991. 20 years on and his good advice has been largely ignored, not for technical reasons, but because the vision of a long-term interplanetary future seems unpalatable for the powers-that-be. Why?
Massive amount of information on all the fusion options, as of 1991, including the Field Reversed Configuration, which is a favourite approach of Rostoker and co. Schulze computes the performance of different fusion rocket concepts, though the performance figures are surprisingly low. An Isp of 270,000 seconds for a probe to Alpha Centauri, which means multi-century mission times. Dyson’s “Orion” and “Daedalus” got 1,000,000-1,5000,000 seconds, though the designs aren’t as elaborated as Schulze’s.
Just 6.97 parsecs away, which is 22.7 light years – a century via a fusion rocket or laser-sail or an episode on “Star Trek”. Given large solar collectors at 0.1 AU from the Sun and the mass-beam projectors parked at both ends, then a manned starship might do the trip in ~26.4 years at 0.9c. At 1 gee it takes exactly 2 years Earth-time to accelerate to 0.9c, while travelling ~2.5 light-years to get up to speed, then brake, thus cruising ~20.2 light-years. Total trip-time for the travellers is 12.6 years. The trick for mass-beam starships will be getting up to speed in a reasonable aiming distance for the mass-beam projectors. Pushing hard at 25 gee would allow cruise speed to be reached in just 0.05 light-years, but will require the crew enduring 21 days of high-gees at each of the journey. In theory they could float in oxygenated fluid, but in practice that’s largely an under-developed technology which only features in SF – like “The Forever War” (Joe Haldeman) and “Fiasko” (Stanislaw Lem).
One-gee allows 0.5c to be reached in just 0.15 light-years, which might be more feasible if the high-gee option deters would be relativistic travellers. Of course if we had a 1 gee space-drive, just how long does the journey take? On board the ship it’s 6.27 years, while in the Galaxy’s reference frame it’s 24.6 years of travelling. However from the PoV of an observer on Earth a return signal indicating arrival at GJ667C takes ~47.3 years, while a signal indicating the ship’s departure only arrives 1.9 years before the 1-gee starship does, for the observer at GJ667C. Relativity messes with our perceptions in more ways than one.