sevenperforce

No, I don't think they are pretending to get reusability for the sake of secretly practicing autopilot for Mars missions that may not ever happen.

There have been a handful of threads recently about possible ways that colonizing Mars could prove economically viable. We haven't really gotten much solid consensus. In the near-term, it just doesn't seem like there is any slam-dunk reason to live on Mars. Why not look a little closer to home? Let's suppose, for the sake of argument, that some entity was willing to build a space station in orbit for long-term habitation. What, other than space tourism, would cause people to move there? On a basic economics level, people will endure a higher cost of living if the income they can receive is correspondingly greater. Living on a space station in orbit would be the extreme example. The cost of living is very, very high...to grossly understate...but if there was a way to generate correspondingly greater income, people would live there. The trouble is finding a lucrative income source in space that is not available on Earth, while also requiring you to actually be physically in space. The current jobs in in space come from NASA and other government-sponsored space agencies. That is mostly scientific research. While important, it doesn't serve as an immediate stream of revenue and so it probably doesn't fit. I could imagine other types of research that would be more lucrative. I'm sure there experiments which could only be conducted in microgravity, yet would result a lot of profit for companies here on Earth. Another opportunity would be zero g manufacturing and assembly. I don't know whether that market is big enough, or if there are aspects that would require human presence. Then there are more interesting possibilities. It might seem like a lot of trouble to set up a tax haven in space, but there may be certain business models (investment banking, stock option manipulation, encryption services, gambling servers, streaming servers, etc.) which would benefit tremendously if they could be conducted away from government oversight or control. The same goes for manufacturing of restricted substances. Finally, there's space jockey stuff -- going to repair satellites in orbit, assembling satellites, and so forth. Any other ideas?

Oh, it would definitely have to be done with something more like the dedicated Black Horse. Though the SABRE engines would help, of course.

There wouldn't be any redesign -- it would fly with the exact same first stage. The only redesign you'd need to make would be detachable nozzle extensions for the Dragon V2 and the external propellant feed lines, plus the drop tank (which could fit inside a truncated second-stage aeroshell with only the most modest of modifications).

It's been done thousands and thousands of times, and it's not terribly hazardous since you'd probably be transferring hydrogen peroxide.

What part isn't worth it?

Crunching a few numbers, just for fun, and I was looking at the SuperDraco engines on the Dragon V2. Their quoted ISP is really, really low...like, monopropellant low. If the SL impulse is 240 seconds and the SL thrust is 68,170 N, then the mass flow at full thrust is going to be about 29 kg/s. The quoted vacuum thrust is 73,000 N, so this corresponds to a vacuum specific impulse of 257 seconds. But that's crazy. The NTO/MMH hypergols used for the SuperDraco engines should have a SL specific impulse of 288 s and a vacuum specific impulse of 336 s. What gives? At first I thought that perhaps the pressure-fed SuperDraco engines were running with a lower-than-ideal chamber pressure. But they have a chamber pressure of 1000 psi, which gives the expected ~280 s SL specific impulse according to this chart. Apparently the SuperDraco engines have really, really underexpanded nozzles to allow deep throttling. This got me thinking. If SpaceX designed attachable nozzle extensions that mounted to the outside of the Dragon V2, the vacuum specific impulse could theoretically come right up to 336 s. The combined thrust from the eight SuperDraco engines would be a little over 760 kN. The thrust of the single Merlin 1D Vacuum engine in the Falcon 9 second stage is 934 kN...just 22% higher. The crewed Dragon V2 masses an estimated 9,200 kg including onboard propellant, full cargo capacity, and the trunk. The second stage has a dry mass of 3.9 tonnes, a specific impulse of 340 seconds, and 92.67 tonnes of fuel. So the second stage can deliver almost 7 km/s. Pretty impressive. But suppose we equip the Dragon V2 with nozzle extensions, raising its vacuum isp to 336 s and its total thrust capacity to 760 kN, and replace the entire second stage with a drop-away NTO/MMH tank feeding up to external couplings on the Dragon V2. We can assume a tank dry mass fraction of 4.6% for that propellant combination. If the total mass being lifted by the first stage is kept the same, then the tank will contain 92.13 tonnes of fuel and you end up with a dV of 6.74 km/s without using any of the Dragon V2's internal propellant. The first stage should easily be able to manage an extra ~200 m/s and still RTLS. So you have an almost fully reusable launch system with only a single drop tank being discarded. And sure, I know that NTO/MMH is expensive and toxic. But surely it's cheaper than throwing away a Falcon 9 second stage with each flight, right?

Still, it's a much smaller mission requirement set than the Shuttle, which had to fly crewed on every mission. Shuttle-flying the Shuttle to the ISS was like taxiing a Boeing 747 three blocks down the street to grab lunch.

Well I certainly can't do it. Can't even reach the moon, let alone do a return. If anyone can figure out a way to do it, I'd be impressed.

Skylon is also supposed to carry all kinds of cargo- even crew. It suffers most of the same complexity problems as the Shuttle, minus the crew cabin. The difference is that the Shuttle was expected to do most of those things at the same time. It had a crew cabin, a hab, a cargo bay, and an SRMS. It ran satellite servicing missions and satellite recovery missions and space station assembly missions. Skylon, on the other hand, doesn't need orbital persistence or service capability. It won't be expected to carry a crew and a satellite at the same time, and it won't be expected to return a payload to the surface.

The internationally-funded, multiple-PhD paper I linked above said that you can run on water on the moon, barely. Not place, throw. But yes. Yes, it does. Have you ever skipped a rock across a lake? Hopping in a very frog-like fashion is exactly what happens. Are you just trying to troll us?

You're...wronger and wronger. What is an equal-and-opposite force if not a push? And what do you imagine "gliding against water" means? Do you have any background in basic physics, free-body diagrams, anything like that? Because it's really easy to see. Take the example of the rock skipping across the lake. The rock does not remain in contact with the water; it bounces. What is pushing the rock back up, if not the surface of the water?

Definitely.

Colonizing beyond Terra is the only way to continue existing if something happens to Terra.

Absolutely valid points. With the Shuttle program, the amount of reconditioning, rebuilding, and refurbishing between each flight was simply astronomical. Extreme sink of money and time. This was partly because it was intended to do so much; it was a launch vehicle that was also a crew ferry that was also a flying lab that was also a Space Station Construction system that was also a cargo carrier system that was also a satellite delivery system that was also a satellite recovery system that was also a plane that was also a launch engine return system. Altogether there were so many systems that refurbishing between flights was an extreme nightmare. Skylon's engine system is more complex, but as a concept it is much simpler: a plane that flies to orbit, drops a payload, and comes back.

Improving our chances of enduring as a species?

And how, pray tell, do you imagine they "throw a lot of mass"? And what do you think "pushes the rocket in the opposite direction"? The expanding fluid coming out of the nozzle choke pushes against the exhaust bell, and the exhaust bell pushes back against the fluid. Water cannot support your weight, but it has inertial mass just like anything else, so you can run on water if you can push down on it hard enough and fast enough and keep going.

You can't push against water? You can most certainly push against water. Pushing against a fluid is how every rocket works. It's even easier with an incompressible fluid like water.

According to this paper linked in one of Randall's What-Ifs, yes.

This is the part where I wish I had a really good flow modeling software package, because I suspect that the airflow through the center will have some pretty specific effects that can potentially be tuned to do exactly that. Just like the atmosphere functions as the opposite side of the exhaust bell in an aerospike nozzle, the central flow functions as a virtual aerospike to hold the exhaust against the traditional exhaust bell. At subsonic speeds near sea level, the low pressure at the inlet caused by the compressor will tend to pull air through and into the bypass, so that it is coming by with a pressure equal to ambient but a rearward velocity relative to the engine. This will cause minor thrust augmentation, though not much. There is no significant benefit to reheat at this stage. Once the stable supersonic flight regime is reached, however, the flow through the actual inlet is choked while the spill through the central bypass is unchoked. Spill air is higher pressure than ambient, so it has a net positive thrust (or at least reduces back pressure like the ramjet burners on the SABRE). However, it's necessarily lower pressure than the choked air at the terminal shock in the inlet, so whatever the pressure in the combustor is, it will expand against the spill air stream rather than pushing back against the terminal shock and unstarting. This at least eliminates the need for an adjustable exhaust nozzle and an adjustable inlet spike.

That is part of the point of having spill air bypass through the center, rather than being redirected outside the engine entirely. You don't need a variable area nozzle, because the nozzle opens against the stream of spill air.

And unstart your engine. Or have a bypass area large enough to accommodate any amount of spill.

Sure, the incoming air is ram-compressed. The question is, when your thermal load is too great to apply any additional turbocompression, can you make any use of the ram-compression you still have? Not with the SABRE. That's kind of splitting hairs; the afterburner is obviously upstream of the final expansion nozzle, but it is downstream of the main combustion region, which was all I was trying to say. And sure, there will be an optimal length to allow more complete mixing. But for an engine like the SABRE, there's an advantage in moving the air injection downstream of the initial combustion chamber, because it allows it to accept a greater range of air pressures than if it had to be injected straight into the combustion chamber. With injection directly into the combustion chamber, you HAVE to use turbocompression; otherwise, your chamber pressure will have to be kept exceedingly low to prevent unstart. Such designs are therefore limited by the thermal capacity of the turbocompressor. Downstream compression, on the other hand, cannot unstart and thus can accept air at speeds greater than the capacity of the turbocompressor. The shock cone intake is still there. It's just rearranged. Here's the basic geometry for a shock cone inlet, in a simple ramjet sort of arrangement: The cone compresses the air into the intake while some air spills around the edges; controlling the position of the cone can adjust how much air enters the intake. Reducing by symmetry, we're looking at this basic shape: This shape can be taken through various transformations to produce different engines with the same basic geometry. For example, translation, forming an inlet ramp: Or rotation about the cone axis, forming a typical shock cone inlet: Or, by rotation around an alternate axis, which is what I'm proposing: It's the same shape and the same function. The only difference is where the spill air goes. Supersonic fighters often use the spill air to help generate lift, while in this particular instance, spill air travels through the open center. It could therefore be used to potentially augment thrust, where it would be wasted in the former case.

Dual orbit rendezvous would have been even better, but they didn't have the experience, capacity, or launch capability to pull off the EOR side of things. Lunar orbit rendezvous would have been even better if they could have gotten away with an unmanned command module but they needed someone in orbit to do the docking.

Here is a less expensive option that may interest you...