Habcrafts and Cyclers

RobertDyck · 2014-10-20 13:27:10

Terraformer wrote:

I wonder how much the original shuttle plan would cost to develop today... mind, 11 tonnes to orbit is a lot for a reusable spacecraft. I presume they planned on launching vertically?

NASA requested proposals for a shuttle in 1968. Here are the responses from 1969. Both launched vertically, land horizontally.

Shuttle LS A - Lockheed with NASA's Marshall Space Flight Center, orbiter based on X-24B shape

Shuttle MDC - McDonnell Douglas, orbiter based on HL-10 shape

GW Johnson · 2014-10-20 14:59:34

I was talking turbine inlet temperature with respect to the Eurofighter 1800 K issue. It might be possible to get to 1800 K with a ceramic turbine, but only in small diameters, or only for very short engine lifetimes. There are no metal alloys capable of sustained, long-life operation at turbine inlet temperatures above 2200 F, even with active cooling by air, or by sacrificial liquids.

When you compare 2200 F turbine inlet temperature (exit from the combustion chambers) with the incoming unburnt air temperatures (inlet temperature), you see where the Mach 3.6 (to at most about Mach 3.8) limitation on turbine comes from. As you get to those speeds, there is no "room" left to heat the air in the combustors by burning fuel. No heat addition, no frontal thrust, period. Any altitude.

Yep, I was the one quoting speed limits for the SR-71. And with very good reason. I know the J-79's that pushed the SR-71 (and the A-12) were advertised as "air turborockets", but they really were not. What they had was unique at the time (late 1950's): an air bypass direct from about stage 3 or 4 of the compressor to the afterburner duct, and even that was limited to only 0-25% of the inlet captured massflow. It was simply impossible to isolate the turbine core from the super-high inlet air temperatures if flight speeds became excessive.

A "real" air turborocket can do that isolation function. There are none. There never have been any. Although, possibly, there could be.

BTW, the blackbird could fly steady-state higher than 80 kft, but only at speeds around Mach 3 to 3.5. Nearer 100 kft, actually.

I roughed-out a design for a parallel-burn turbine / ramjet aircraft way back in 1985 that was intended to fly over hostile territory at Mach 5 / 100-150 kft. The FBI confiscated all my design notes, even though I did this from 100% open sources. Turns out, somebody else inside the government was looking at exactly the same thing. "Surprise, surprise", -- Gomer Pyle.

To do the same mission today, I would use parallel-burn rocket and ramjet. Lighter, simpler, cheaper. Like the old Rocket-Racer aircraft, but with a big ramjet in the fuselage, and an airframe actually intended for hypersonic flight. (And it ain't what you think.) The problem isn't propulsion, it's heat protection, when you exceed Mach 3.5 to 4. Plain old ramjet can take you to Mach 6. We've already done it, nearly 40 years ago.

I see little point to flying within the atmosphere at speeds above about Mach 5 or 6. The heat transfer (and the drag) are just too high. Which is why I think scramjet is useless as a launch technology, only useful as a one-shot tactical missile technology for Mach 8 to 12 stuff. Better to go exoatmospheric by the time you want to exceed those Mach 5 or 6 speeds. Simple as that.

As for launch costs and re-usability, take a look at the expendable-booster unit costs I have posted over at "exrocketman". ULA's Atlas-5 and Delta-series all have around $2500/lb in the 15-20 ton to LEO category. Falcon-9 at 13 tons is $2400/lb. The Russian and French launchers fit the same curve of unit cost to LEO as these. That curve projects $1000/lb to LEO at 53 tons, and $1000/lb is Spacex's current projection for Falcon-Heavy! The same curve projects under $500/lb in the 100-ton class, while NASA's best estimates project $2500/lb for SLS. (Of course, NASA has long been infamous for way-underestimated costs.)

You hang the propellant margin on that for reusability, and you reduce payload. You raise your unit cost to LEO by exactly the same factor, all else being equal. But, it ain't. Pure and simple.

If you can reduce your logistics tail, then you can reduce total launch cost for a given first stage thrust size, which also impacts unit payload cost to LEO. Reduce that more than you drive it it up for re-usability propellant margins, and you can get a net reduction in unit cost to LEO with reusable systems. It's all about simplifying and reducing support manpower costs. THAT is where Spacex has pioneered so successfully, unique in the industry.

I think for smaller-mass payloads (like crews) to LEO, there is a niche for spaceplanes about the size of Dreamchaser at under $2500/lb, given the right booster rocket. I think for larger-mass payloads (like 15-50 ton modules to be assembled on-orbit), it'll be very hard to beat the simple expendable booster rocket, at about $2500/lb down to $500/lb, depending upon size. I think that will likely be true for most of the 21st century, unless we achieve some massive physics breakthrough like "warp drive". (I am not holding my breath for that breakthrough, though.)

GW

Antius · 2014-10-21 05:36:38

Some interesting replies. The general implication is that present day high costs are the result of poor scale economies, which require amortising large fixed costs in design and manufacture, over a small number of units and launches. This could be solved either by a reusable launcher (lower manufacturing costs) or a simplified expendable. In both cases the key appears to be to increase overall launch volume.

Reusable rocket vehicles would be a far more practical proposition if they could be:

1. Refuelled in orbit using non-terrestrial propellants;

2. Fitted with re-entry shields manufactured in orbit, using non-terrestrial refractories;

3. Fitted with expendable engine liners, constructed from refractory materials and partially cooled by sacrificial liquids. Sacrificial liquids not only cool the engine surface, but also thicken the boundary layer, reducing heat fluxes.

The first allows the vehicle to cancel part of its orbital velocity prior to re-entry, which significantly reduces the heat fluxes. It also improves payload to higher orbits, by allowing refuel in very low earth orbits.

The second effectively removes the payload penalty of the reusable system by obviating the need to carry that heavy heat shield into space. Every kg of material in a 400km orbit is carrying 30MJ of energy relative to the Earth’s surface. All that heat must be transferred to the atmosphere by radiation and convection during re-entry. To decelerate a large vehicle carrying that much specific energy over a period of just 10 minutes, very high temperatures and heat fluxes are inevitable. Erosion and cracking of even the best refractory heat shield is unavoidable and the mass of sacrificial liquids required to achieve active cooling would be prohibitive. Re-entry shields cannot therefore be reusable. If a simple, expendable heat shield can be made in orbit using lunar or asteroid rutile, then the biggest obstacle to reusability is removed.

The third measure would increase engine mass and would add a modest propellant burden, but would also permit a truly reusable engine, by incorporating an expendable and easily replaceable liner. As the heat fluxes at the nozzle throat run into several MW per square metre, it would appear to be very difficult to avoid erosion and cracking of the surface using regenerative cooling alone.

RobertDyck · 2014-10-21 06:13:58

Be very careful. Manufacturing in space is very expensive. It requires transporting heavy equipment into space, which is the primary expense. And manufacturing equipment developed for use on Earth depends on gravity; designing for space would require redesigning custom manufacturing equipment.

I have suggested asteroid mining use option 2. Mine a metal asteroid for precious metals: gold, silver, platinum, and all platinum group metals. This will produce copious quantities of left-over materials: primarily iron and nickel, but also other industrial metals. The left-over metal will be left at the mine site, the asteroid, to be picked up by some future operation. If you want to build something big in space, you may want to use it. But primary mine operation will just leave industrial metals at the asteroid. Some of that left-over material will be used to make an aeroshell to return precious metal bullion to Earth. Convert nickel, chrome, molybdenum, and a tiny bit of aluminum and carbon, to make an alloy called Inconel 617. NASA already identified that alloy for metal heat shields. Use a two part mould to form a heat shield, and another two part mould to make a back shell. No parachute, no control rockets, no other add-ons. Have a reused robot spacecraft carry that aeroshell back to Earth, aim at a desert somewhere and just drop it. Tell everyone to stay out of the way. It will just be a big piece of shaped metal falling out of the sky. Because it'll be filled with about 3 metric tonnes of refined gold, platinum, etc., you would want a helicopter with armed guards to meet it. Then send a flat bed truck with a truck crane to pick it up.

That works for asteroid mining. No humans onboard, and it's expected to crash. Not something for crew. Manufacturing would be involved, but just barely simple enough to do on-site in space.

Non-terrestrial fuel: Again be careful. You could mine a carbonaceous chondrite asteroid for ice and tar. That can make rocket fuel, and working fluids that a metal asteroid will need for smelting: carbon monoxide and hydrogen. But asteroids with ice tend to boil off that ice when they get too close to the Sun. The best guess by scientists is that an asteroid would have to be as far as Mars to retain ice. That makes the Moons of Mars ideal candidates. But asteroid 3552 Don Quixote has already been identified as dripping with water; it has a thin coma like a comet. But it's beyond Mars. Near Earth Asteroids are the obvious target because they're close, so short trip times and minimal fuel. But you have to go farther to find one with water you can mine for fuel.

One reason I say "be careful" when talking about non-terrestrial fuel, is the Moon guys will try to sell you stuff. But the Moon doesn't have water, or carbon, or any fuel. They will claim they found water, but it's concentration is a cup of water over an area the size of a football field. And that's the richest, most concentrated spot at the Lunar south pole. Any mining expert will tell you it isn't worth harvesting. And the only carbon or nitrogen is in the same deposit, and even less concentrated than water. John Wickman did figure out one fuel you can make on the Moon; he called it Lunar Soil Propellant. It's aluminum powder and liquid oxygen. First mine Lunar soil for aluminum ore, then smelt aluminum (which involves volatiles). Then grind the aluminum as fine as flour. Then your options are mono-propellant or bi-propellant. Mono is suspend the aluminum in liquid nitrogen. Most engineers will question whether this is safe for human travel. The alternative is to blow aluminum powder into the combustion chamber with nitrogen gas, and add liquid oxygen separately. But that requires nitrogen. There isn't any nitrogen on the Moon. Oops.

Sacrificial fluids: During World War 2, Wernher Von Braun came up with the idea of using rocket fuel itself as the sacrificial fluid cooling medium. Cold rocket fuel is circulated through channels in the rocket exhaust nozzle, then this pre-heated fuel is injected into the combustion chamber. It's called regenerative cooling. Every cryogenic rocket engine today already does this.

The solution for human access to space is a reusable spacecraft. DreamChaser is one option, Dragon v2 is another. I would like to see both fly. Let's see which works better.

Antius · 2014-11-06 11:53:06

RobertDyck wrote:

Sacrificial fluids: During World War 2, Wernher Von Braun came up with the idea of using rocket fuel itself as the sacrificial fluid cooling medium. Cold rocket fuel is circulated through channels in the rocket exhaust nozzle, then this pre-heated fuel is injected into the combustion chamber. It's called regenerative cooling. Every cryogenic rocket engine today already does this.

Slightly different idea to regenerative cooling. Sacrificial fluids actually fall under ablative cooling. We are essentially doing two things at once: (1) Cooling the surface; (2) Thickening the boundary layer, effectively pushing it away from the chamber using high pressure phase change.

The rate of cooling permissible by doping this is an order of magnitude greater than regen cooling, as you are able to take full advantage of the latent heat of evaporation of the fluid. In addition, ablatively cooling an engine in this way does require heat transfer through the chamber walls, which is problematic if heat fluxes get too large.

For small engines, such a cooling mechanism would consume too much propellant. As the engine gets larger, the mass penalty becomes more tolerable. Useful for lower stages, where engines are large and chamber pressures (and reynolds number) are high.

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#26 2014-10-20 13:27:10

Re: Habcrafts and Cyclers

#27 2014-10-20 14:59:34

Re: Habcrafts and Cyclers

#28 2014-10-21 05:36:38

Re: Habcrafts and Cyclers

#29 2014-10-21 06:13:58

Re: Habcrafts and Cyclers

#30 2014-11-06 11:53:06

Re: Habcrafts and Cyclers

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