danielravennest

Talking about destinations assumes a mission orientation, and I think this is the wrong approach. Instead, I think NASA should think in terms of high leverage technologies and capabilities. Forgive my language, but designing new missions with the same crappy technology will not get you far in a limited budget environment. These two areas are the ones I think are most important:

Propulsion

Chemical rockets reached their technical limits 50 years ago when the first Centaur LOX/LH2 engine was developed. Later cryogenic engines have not improved significantly because there is no more energy in the fuel to use. At best you can get marginal improvements. The only way to get radical improvements is to stop using chemical rockets as the main method to travel in space. I will mention a few candidates, but the main point is move away from a technology that has reached a dead end.

One candidate is to use air-breathing engines up to around Mach 5. There are various engine cycles that could be used, but the important thing is by using oxygen from outside the vehicle, you can get about three times the fuel energy/kilogram as with LOX/LH2. Above Mach 5 the increased drag and heating makes it better to use rockets, so air-breathing propulsion is mainly useful for a first stage.

Another candidate is electric propulsion. Ion and plasma thrusters already have demonstrated 6-10 times the exhaust velocity of cryogenic engines, and solar panels to power them have doubled in efficiency in the last decade. They should receive a lot more attention than they are.

The Supply Chain

Bringing everything from Earth is ultimately too expensive and unsustainable. We should put a lot more emphasis on extracting resources wherever we are, establishing a production base, and making products like structural elements, fuel, food, and other things you need for long term operations. Modern computers and networking makes it possible to do a lot of the work by remote control. So we can put the robots to work preparing for when the humans arrive, and to assist us once we get there.

Some of this technology is getting worked on, but far too little effort is going into it, and I have not seen a real integrated systems approach to the supply problem. For example, if you extract fuel for a plasma thruster from asteroid materials, and use part of that fuel to bring back the next load of raw rock, it becomes self-sustaining. But you don't discover that synergy if you look at materials processing and propulsion in isolation. You need to look at them as an integrated process where each supports the other.

-- Sincerely,

Dani Eder

Formerly with Boeing's Space Systems Division, now writing a book on space systems engineering and doing conceptual design of future projects

[EDIT] For those who asked Mr. Garan to reply, he did, here, but it's kinda lost in the thousands of other comments:

http://www.reddit.com/r/IAmA/comments/14f7gt/i_am_nasa_astronaut_ron_garan_i_want_to_do_an_ama/c7ddnrk

The original idea for a space elevator was proposed in 1895, by Tsiolkovsky, and unfortunately it is still the one shown in all the color illustrations. We really need to move away from 19th century designs and into the 21st, to ones that are actually feasible:

• Do not try to span the whole of the Earth's gravity well (ground to GEO) because the highly non-linear mass ratio of such a large structure will kill you.

• Do not try to build a single cable 35,000 km long because it has too much exposure to meteor and debris impacts, and will therefore not survive.

The solution is to use a short, rotating, multi-cable structure with a tip velocity of a few km/s. In low orbit, at the bottom of rotation, it subtracts a few km/s from the velocity a launch vehicle needs to reach it. At the top of the rotation it adds a few km/s and can put you into a higher transfer orbit.

One that size is very feasible with existing materials like carbon fiber. With multiple redundant cables, and load sharing every few km, any debris impacts are not catastrophic, they become a maintenance issue.

Of course, even one that size would be a big project, and you need the traffic to justify building it. For now, I would suggest building a variable-g facility in orbit, which we need anyway to learn about the long term effects of low gravity on, for example, Mars. It would also teach us about using artificial gravity on long missions, and get some practice building rotating structures. Over time you can work up to building things like space elevators, but you can't expect to just jump to something that big without some smaller projects to get experience.

SpaceX made a huge step in cost by using modern production equipment, and doing most of the work in one factory. Past projects like the Space Station (which I worked on for many years) was terribly inefficient by comparison, since it had suppliers all over the country.

You can definitely save money by not throwing away the whole rocket every time, but the ability to bring things back costs you in payload, which you cannot increase because there is no more energy in the fuel, and landing gear and heat shields cost you in weight. It is easier to make a reusable vehicle if the first stage is air-breathing, which gives you more margin to add the systems to bring you back.

One of the ideas I worked on at Boeing was "Jet assisted rockets". Imagine a "booster ring" with a number of fighter jet engines mounted vertically around the ring. A standard rocket sits on top of the ring. The booster ring takes you up to Mach 1.6 and 15 km (50,000 ft) altitude, at which point the rocket lights up. The ring then comes down to a vertical landing. That is off the shelf technology, and the head start for the rocket lets you then add the equipment to recover the upper stages without losing too much payload.

Only if we are ready to build a permanent research station. A "flags and footprints" mission that does not leave us with a permanent base is not worth doing.

I propose a chain of fuel stations and greenhouses, in High Earth Orbit (like L2), in a transfer orbit to Mars, on Phobos, and then the surface of Mars. The first two use asteroid materials for building materials. Each station acts as a forward base to produce the fuel and food and other supplies to establish the next one. Once you have a supply chain that reaches the Martian surface, you can keep a full base going at reasonable cost.

Using asteroid materials also solves the radiation problem, since you have ample shielding. It also gives the crews in transit to Mars something to do on the way - make fuel and food for the next trip. There are 10,000 asteroids near Earth, and many more in outer orbits, so whatever orbits you pick, there are going to be a number of them within easy reach.

Asteroid miner here. That paper is completely off in it's analysis.

First, it assumed missions from Low Earth Orbit. A better location is near the Moon, where you can use Lunar gravity assists in both directions, and don't have to climb down the Earth's gravity well every time.

Second, it assumed chemical propulsion to get to the asteroids. Electric thrusters (ion and plasma) are 4-7 times more efficient.

Third, if you are mining asteroids for fuel, you can do that in more than one place. Just like you can go from New York City to Los Angeles by refueling your car several times, you can travel all over the Solar System with a network of refueling stations.

Fourth, 100% of asteroid materials can be used for something. If nothing else, you can use it as radiation shielding for your crew modules, or a counterweight for artificial gravity.

Orbital Mining section of a textbook I have written, if you think I'm joking about asteroid miner:

https://en.wikibooks.org/wiki/Space_Transport_and_Engineering_Methods/Orbital_Mining