The fastest vehicle upon leaving Earth’s sphere of influence (transplanetary injection) is the New Horizons probe, which was launched by an Atlas V in the A551 configuration with a Star 48B upper stage and was moving at about 16.5 km/s relative to Earth, and about 46.2 km/s with respect to Earth (exceeding solar escape speed). The Helios probes are actually moving faster at perihelion, but required less energy to achieve their orbit, their speed coming from gaining potential energy by flying closer to the Sun.
I think he’s asking more like, "If we threw all the resources we realistically could at this problem, just how fast could we make a probe go? How quickly could we get to another star using technology that it would be feasible to build within ten years or so?
I’m guessing we could get something going pretty fast with a combination of a large conventionally fueled rocket launched from orbit, coupled with a fission-powered ion drive.
How quickly could we get something to a nearby star if we had to launch it within ten years? Maybe a thousand years?
The use of a whopping big laser to nudge along an interstellar probe is a key plot point in Niven and Pournelle’s terrific first contact story The Mote in God’s Eye.
"Proxima Centauri has been suggested as a possible first destination for interstellar travel. Although the Voyager program spacecraft are anticipated to become the first spacecraft to enter interstellar space, they move relatively slowly, at about 17 km/s, requiring well over 10,000 years to travel each light-year. In comparison, Proxima is presently approaching at a rate of 21.7 km/s. However, it will only come as close as 3.11 light-years, and then move farther away after 26,700 years. Thus, a slow-moving probe would have only several tens of thousands of years to catch Proxima Centauri near its closest approach, and could end up watching it recede into the distance.
If current, non-nuclear propulsion were used, a voyage of a spacecraft to a planet orbiting Proxima Centauri would probably require thousands of years. Nuclear pulse propulsion encompasses several technologies which might enable such interstellar travel with a trip timescale of a century, beginning within the next century, inspiring several studies such as Project Orion, Project Daedalus, and Project Longshot."
Ion drives are horribly inefficient, and with some specific exceptions don’t scale up to any level useful for a large probe or spacecraft. They also tend to erode due to ion interaction with anodes or other structures, and even engine concepts like the VASMIR, which addresses (to some extent) most of these issues, generate enormous amounts of heat that have to be rejected somehow. They’re fine for stationkeeping or small continuous impulse propulsion where your energy source is sunlight and you want to maximize the effectiveness of a small amount of propellant (high effective exhaust velocity) but not very promising as a main vehicle propulsion system for anything over a few dozen kilograms.
The two most promising systems for increasing propulsion capability beyond the current chemical rocket systems are the ORION-type nuclear pulse propulsion and the fission-fragment engine. However, neither one has been demonstrated beyond a basic non-nuclear proof-of-concept (in the case of fission-fragment, by simulation only) and both would have to be massive, requiring dozens or hundreds of launches of super-heavy booster lift systems like the Atlas V 551 or larger and integration on orbit. I don’t think it would be feasible to talk about doing this in anything like a ten year timeframe even if you had a workable design in hand.
Nuclear thermal (NERVA, KIWI, Timberwind) appears to be prohibitive insofar as making a reactor compact enough to fit into a reasonable sized vessel, energetic enough to produce the larger thermal difference in order to achieve high efficiencies, and stable enough to be safely operated. Because of the way fission works you just can’t scale a reactor down linearly, and by the time you get it big enough to have enough output with adequate mass to operate in a stable, predictable regime, it is just too large to lift from Earth, and far too complex to assemble in orbit.
I was trying to stick to the 10 year time limit, and I don’t think those other technologies could be done in that time. VASMIR engines are already being tested. I’m not saying these things would propel a spaceship constantly for years - just that if we were going to try to make a rocket go as fast as possible in the next 10 years, they are the best candidate.
But maybe nuclear thermal is a better answer. NERVA engines have already been tested. They’re still not getting you to another star system for hundreds or thousands of years.
Indeed, but I’m not worried about getting to another star, just getting the ship out there on the Bonneville Space Flats and firing that mother up.
Could we really get a “fission powered ion drive” into space within 10 years?
Here’s my imaginary design for a realistically-powered space hot rod: A missle-shaped craft within a bundle of solid-fuel boosters that would fire and drop away in sequence. For the creme de la resistance, detonate a nuclear warhead against a concave butt-plate (is that what “nuclear pulse propulsion” is ?). All this would happen after the craft has been boosted into space and slingshot (slingshotted? slungshot?) around the Sun or Jupiter. So we’re already coasting at a pretty good clip, then we hit the accelerator.
So you have this sweet probe that takes 500 or 1000 years to get to the target and then it sends back all this cool info. Does anyone remember it and have the dates marked down on the calendar? Are the nations that sent it still coherent entities? Does anyone even know how to decrypt these ancient mothballed programs and work with this laughably out of date technology? Anyone fluent in 21st century terminology?
One would construct a very solid, permanent obvious structure with redundant equipment, plans, and instructions in several languages. Seal it up and leave some time capsule in the corner stone. Build some in various places.
While true, the National Labs have their own fabs to make somewhat antiquated yet long term reliable ICs. And not all ICs made are destined for consumer junk. That made for servers and phone systems has to be extremely reliable over its life, and is in fact very reliable. That is one of the reasons the military has moved to COTS devices.
None of this means that anything we have today will stand up to such a trip. After all, it is considerably longer than the entire history of ICs, even considering stress to accelerate failures. In any case, any such system will be made fault tolerant, since not even the best components are likely to be good enough. There is lots of theory behind the design of such a system, though it has never been implemented at the level needed because of expense, and because down here things now get obsolete long before they wear out.
While nuclear engines under the NERVA program (KIWI, Phoebus, and the Pewees) were tested, none of them were flyable engine designs. They were all proof-of-concept and development test articles. Ditto for the Timberwind engines developed by the Space Nuclear Thermal Propulsion program as part of SDIO. I do happen to know a couple of people who worked on NERVA and associated programs, and as the program was shut down in 1972, they’re all in retirement, which means that a lot of the heritage knowledge about how to make such a system work (which often doesn’t seem to make it into reports and documentation in a coherent fashion) is effectively lost. Timberwind is more recent, but if it is anything like other cancelled SDIO programs, the principals packed up the information and stuck it in their attics. In short, nuclear thermal propulsion is effectively about at the same state it was in 1960, aside from having better simulation and computational tools.
Sure. NASA planned to do that in the last decade. “Project Prometheus” was supposed to give us advanced nuclear power supplies for space missions as well as nuclear powered drives - nuclear thermal drives and nuclear ion drives. Nuclear thermal drives use a nuclear plant to heat up the propellant to very high temperatures and then blow it out the back of the rocket. Nuclear ion drives use nuclear power to generate electricity, then use the electricity to create the ion propulsion.
If we were willing to spend the money and go into a crash program, we could be flying spaceships with these drives in them. There is even a prototype high power ion drive (the VASIMR drive) which is scheduled to be added to the space station in a couple of years for testing. A drive like that could be powered by a fission reactor.
Ion drives are a well-proven technology. They’ve been used on multiple space missions (Deep Space 1, Smart 1, Hayabusa, Dawn, etc). Even the Soviets used low-power ion thrusters in their space program. We’ve also been powering spacecraft with nuclear thermal generators (Cassini, for example).
The flaw is that chemical rockets very rapidly reach the point where the weight of the fuel becomes self-limiting. Chemical rockets have a very low exhaust velocity. That means the amount of thrust you get for a given amount of mass is fairly low.
For example, the energy available from a solid rocket booster is about 3 MJ/kg. The energy available from the VASIMR engine, which has a much higher exhaust velocity, is about 43,000 MJ/kg.
The problem with ion engines is that they don’t throw much mass out the back for any given period of time, so their overall thrust is relatively low. You can’t launch a rocket off Earth with them. But because their propellant mass requirement is so small, you can run them for very long periods of time once in space, and let your speed build up. That’s why ion rockets can ultimately propel spacecraft to much higher speeds than can chemical rockets. Although Stranger on a Train makes a good point that you have to shed waste heat, which will limit the power of the rocket or require that it be shut down periodically to let waste heat radiate away.
Your nuclear bomb idea is feasible. There have been a number of designs of spacecraft that propel themselves by dropping small nukes out the back and detonating them against pusher plates. Theoretically, they could go pretty fast. We just don’t know how to build them.
Yeah, I didn’t mean to imply that they were tested in the sense that they’re sitting on a shelf all ready to go - just that the the technology is not beyond our grasp.
Do you not think we could fly engines like that within ten years given a suitably large amount of funding and other resources?
That it won’t actually be going all that fast. Solid propellant motors have somewhat lower specific impulse (the amount of impulse you get per unit of propellant mass), so they’re great at generating large amounts of thrust at low altitude where I[sub]sp[/sub] isn’t all that critical, but not great in vacuum (though their lack of slosh and other fluid dynamics issues makes them preferential for simplicity in kick motors). For each solid motor you strap on, you have to add more propellant to carry them along; eventually, you hit a point where there it is no longer advantageous to add more boosters.
As far as using a “concave butt-plate” to absorb the impulse from a nuclear device means that the vehicle absorbs the initial undamped shock. The ORION actually has a two-stage spring-piston system to absorb and distribute the impulse.
Also, there are treaty restrictions for designing and developing nuclear bombs in space. So these so-called “Orion” designs never really made it past the theoretical stage. You could see why the Soviets might not have felt comfortible with us building a space ship that launched nuclear bombs out the back.
Crazy idea. Maybe instead of rockets the answer is rail gun like launchers with small probes.
Imagine something like a space fountain, but on a grander scale. Probes would be accelerated to some nontrivial fraction of C, by being guided down a magnetic loop then when the probe is moving so fast it’s gonna break away the magnet is turned off and the probe is flung off. Probes that need to be bigger than the system’s capacity could be launched in pieces, with each piece moving a little slower than the one behind it. They’d rendezvous and assemble along the trip as the fastest caught up with the slowest.
Some problems are:
How the heck would such powerful magnets be built? I don’t even think there’s enough data to even start.
Slowing it down on the otherside. If you want it to stay in the other star system it’s going to need a breaking propulsion system that can be broken down into small pieces.
It’d be a weapon. A mass driver.
A strong plus would be reusability. Once the launcher is built it can be used for more missions.
Plus since it wouldn’t have to carry it’s own fuel it could theoretically hit some fast speeds. Missions could be completed in human life times, at least to a few nearby systems.
It wouldn’t need nearly as much coolant. All heat generated from propulsion would be left back in the Solar System. Further passive systems could be used for breaking, such as a solar sail that opened up and used the target star’s solar wind to slow down.
First of all, you’re probably describing a coil gun, not a rail gun: A rail gun requires that the projectile remain in contact with the rails to complete a circuit, and that introduces friction, plus resistive losses in the long rails. Second, as Stranger on a Train mentions in post 31, such a device would need to be insanely large, have insanely high acceleration, or more likely both.
