The SpaceX Thread

[daveklingler]

Just thought I'd post a new thread to cover the SpaceX COTS Demo 2/3 mission discussion.

The webcast hasn't started yet, but should start soon:

http://www.spacex.com/webcast/

 

[Alamout]

Caught the launch just in time. That was fun. Not sure how long I'll keep watching. edit: oh the stream stopped. I guess that answers that.

 

[kevinstout]

perfect launch

 

[Megalodon]

Gotta love those instantaneous launch windows.

 

[daveklingler]

Launch replay at http://youtube.googleapis.com/v/e2_49KPj9jE&rel=1.

"Dragon spaceship opens the navigation pod bay door without hesitation. So much nicer than HAL9000 #DragonLaunch" -
https://twitter.com/elonmusk/status/204882126884188160

 

[Paltivar]

What would it take to pick out the Dragon in the night sky?

Usually if I want to find something, I hit up Google sky to figure out when/where I should be looking, but I don't expect an update to include it. How visible is it? Binoculars sufficient?

 

[Alamout]

Well, you could just wait until it gets to the ISS--that's easier to find a location for. It seems like the problem is that both of them are moving rather fast across the sky, so it'll be hard to track them with a telescope or binoculars, but you should be able to see them with your eye (not a lot of detail, of course).

Maybe binoculars would work, if you can get out in the country with a good view.

(just going off this page and this one)

 

[Paltivar]

Well, you could just wait until it gets to the ISS--that's easier to find a location for. It seems like the problem is that both of them are moving rather fast across the sky, so it'll be hard to track them with a telescope or binoculars, but you should be able to see them with your eye (not a lot of detail, of course).

Maybe binoculars would work, if you can get out in the country with a good view.

(just going off this page and this one)

That's very useful, thank you. Got to decide if the 3-4 minute viewing between 3 and 5 in the morning is worth trying to make, but at least I can make an informed decision. Might have to hope unberthing goes as scheduled and I can catch it before it descends.

 

[kalahari bushman]

Paltivar put your location in here, and you should find any visible passes.

 

[Paltivar]

The second link from Alamount had one as well, and I trust it more. Partly because it's NASA, partly because it doesn't have passes listed for when Dragon is supposed to be berthed to the ISS.

 

[sophistry]

So I was reading about it and...what struck me was that their ambition is to make it fully reusable.

Well, it seems to me that it would very difficult to reuse rocket stages. I have no idea how they would get it back on the ground, how it would survive reentry, how it might be refurbished, what if it landed in ocean salt water, etc. Also, by all indications, they HAVEN'T been able to retrieve and reuse stages yet.

Otherwise, sending a 2 stage rocket to low earth orbit doesn't seem to be that much of a technical challenge, now is it? I mean, the private sector has boosted plenty of satellites into orbit since the 1980's.

 

[Alamout]

I mean, the private sector has boosted plenty of satellites into orbit since the 1980's.

Not so much, and those that do are nearly all using Russian rockets, not their own. The SpaceX launch is already cheaper than those efforts, anyway.

 

[mperrin]

So I was reading about it and...what struck me was that their ambition is to make it fully reusable.

Well, it seems to me that it would very difficult to reuse rocket stages. I have no idea how they would get it back on the ground, how it would survive reentry, how it might be refurbished, what if it landed in ocean salt water, etc. Also, by all indications, they HAVEN'T been able to retrieve and reuse stages yet.

They haven't done it yet, no, but give them time. I think they do have a pretty solid plan for this. (And it's kinda harsh to give them a hard time for not having done so yet - the only spacecraft reused, ever, is the space shuttle, developed at a cost of some 30x what SpaceX have spent so far...)

You're right that landing in salt water is terrible. You want to land on, well, land. Two options for that: (1) launch from Texas, east over the Gulf, and land in Florida. That's part of the motivation for their recent efforts to open a new launch facility in Texas. (Confirmed from talking to a SpaceX engineer recently at an event in DC). (2) Launch from Florida out over the Atlantic, and land on a repurposed oil platform or something like that. It'd be the inverse of how Sea Launch operates.

As for re-entry: The first stage never gets up to orbital velocity, not even close, so re-entry heating for that stage is not that significant. For the second stage, you'd use an ablative heat shield (same as for the Dragon capsule itself). The only difference is, the heat shield is up at the top, and the stage flips upside down to re-enter, then flips right side up to land. Technically challenging for sure - but even just re-using the first stage alone would be a huge game changer.

Otherwise, sending a 2 stage rocket to low earth orbit doesn't seem to be that much of a technical challenge, now is it? I mean, the private sector has boosted plenty of satellites into orbit since the 1980's.

Sending a 2 stage rocket to LEO for a small fraction of the cost at which anyone else has ever launched a comparably sized rocket is a Big Deal. We're not talking 10% cheaper, here, we're talking 5x-10x cheaper.

 

[MilleniX]

Well, it seems to me that it would very difficult to reuse rocket stages. I have no idea how they would get it back on the ground, how it would survive reentry, how it might be refurbished, what if it landed in ocean salt water, etc. Also, by all indications, they HAVEN'T been able to retrieve and reuse stages yet.

All of the exposed surfaces are made of non-corroding materials that will be unaffected by salt water.

 

[sophistry]

Okay, so from the looks of it, this is the first commercial rocket. Commercial satellites have been launched off of space agency rockets in the past.

Unless you can control the descent of the first stage, it is nigh irresponsible to not have it land in the ocean.

the venturestar was so much more promising. Why was it cancelled?

 

[blargh]

So I was reading about it and...what struck me was that their ambition is to make it fully reusable.

Well, it seems to me that it would very difficult to reuse rocket stages. I have no idea how they would get it back on the ground, how it would survive reentry, how it might be refurbished, what if it landed in ocean salt water, etc. Also, by all indications, they HAVEN'T been able to retrieve and reuse stages yet.

Have you actually read any of the public materials about how SpaceX is planning to accomplish the reuse? It seems like you just read a headline and started wildly speculating. Yes, they're planning to land controlled, on land, tail down.

Criticizing them for not doing it yet, considering how few launches they've done at all so far, is pretty ridiculous.

 

[Megalodon]

So I was reading about it and...what struck me was that their ambition is to make it fully reusable.

Well, it seems to me that it would very difficult to reuse rocket stages. I have no idea how they would get it back on the ground, how it would survive reentry, how it might be refurbished, what if it landed in ocean salt water, etc.

Their plans have been widely publicized. The only way I can think of for you not to have any idea is if you have made no attempt to find out, as your ignorance would not have survived even a single rudimentary attempt to Google it.

Unless you can control the descent of the first stage

It's a shame you've put so much thought into this without making any attempt to access all the available information, otherwise you'd know how they plan to control the descent.

the venturestar was so much more promising. Why was it cancelled?

Turns out it wasn't that promising.

 

[pieguy3141]

So I was reading about it and...what struck me was that their ambition is to make it fully reusable.

Well, it seems to me that it would very difficult to reuse rocket stages. I have no idea how they would get it back on the ground, how it would survive reentry, how it might be refurbished, what if it landed in ocean salt water, etc. Also, by all indications, they HAVEN'T been able to retrieve and reuse stages yet.

Otherwise, sending a 2 stage rocket to low earth orbit doesn't seem to be that much of a technical challenge, now is it? I mean, the private sector has boosted plenty of satellites into orbit since the 1980's.

It's engineering, not magic. Stuff doesn't happen overnight, or even within months. This stuff (at the costs of what they operate with) takes years just to do simple tests. And the fact that they ARE ambitious is pretty remarkable for the kind of insurance/accounting-laden atmosphere that pervades our society.

 

[sophistry]

http://www.popularmechanics.com/science ... ns-6653023

The key, at least for the first stage, is the difference in speed. "It really comes down to what the staging Mach number would be," Musk says, referencing the speed the rocket would be traveling at separation. "For an expendable Falcon 9 rocket, that is around Mach 10. For a reusable Falcon 9, it is around Mach 6, depending on the mission." For the reusable version, the rocket must be traveling at a slower speed at separation because the burn must end early, preserving enough propellant to let the rocket fly back and land vertically. This also makes recovery easier because entry velocities are slower.

However, the slower speed also means that the upper stage of the Falcon rocket must supply more of the velocity needed to get to orbit, and that significantly reduces how much payload the rocket can lift into orbit. "The payload penalty for full and fast reusability versus an expendable version is roughly 40 percent," Musk says. "[But] propellant cost is less than 0.4 percent of the total flight cost. Even taking into account the payload reduction for reusability, the improvement is therefore theoretically over a hundred times."

A hundred times is an incredible gain. It would drop cost for Musk’s Falcon Heavy rocket—a scaled-up version of the Falcon 9 that’s currently rated at $1000 per pound to orbit—to just $10. "That, however, requires a very high flight rate, just like aircraft," Musk says. "At a low flight rate, the improvement is still probably around 50 percent. For Falcon Heavy, that would mean a price per pound to orbit of less than $500." Falcon Heavy is particularly amenable to reuse of the first stage—the two outer cores in particular, because they separate at a much lower velocity than the center one, being dropped off early in the flight.

Read more: Elon Musk on SpaceX’s Reusable Rocket Plans - Popular Mechanics

Basically, for the Falcon 9 to really be innovative the first stages need to be reusable so they can achieve a quick turnaround. Musk wants to turn around in a couple of days, and to do that you'd have to figure out how to make a very controlled landing of a rocket stage. Maybe with a combination of parachutes + secondary rockets it could work, but inherently I think that it would be difficult to control and meet his ambitions.

I mean, it is impressive that Space X looks like the first real private space craft to make real progress, I just don't think that it is feasible to really reuse those first two stages in a quick manner. I think a reusable spacecraft like Musk wants would have to be a single stage machine like the Venturestar, that has an airfoil to make a controlled landing with the entire thing.

http://www.youtube.com/watch?v=sSF81yjVbJE

in this video, it shows the first stage returning on its own to the launchpad.

If they can accomplish that, it would truly truly be extraordinary. If not, it is at best an evolution of the same rockets we've used since the 1960's.

 

[Bad Monkey!]

the venturestar was so much more promising. Why was it cancelled?

Like many government projects, it suffered from a combination of NIH and malicious neglect due to not fitting the current administration's goals.

 

[Megalodon]

Basically, for the Falcon 9 to really be innovative the first stages need to be reusable so they can achieve a quick turnaround.

I'm not sure why the definition of "innovative" ought to be burdened with your expectations, which don't appear to be well formed.

Even the expendable Falcon 9 does pretty well:

-Post-Apollo western launchers seem to be trapped in this death spiral where they try to use LH2 because of the amazing specific impulse, but then have to use solid boosters because of the crappy thrust to weight ratios. Which means you have mid-60s Soviet technology competing successfully on today's launch market simply because none of this new stuff actually does better than a mediocre performance hydrocarbon rocket.

-Falcon 9 has excellent performance in a number of metrics, just not the headlining stuff you're asking for, such as the mass fraction. Some might disagree that that's important, but again, listening to that crowd has us struggling to compete with Soyuz.

-Even if SpaceX don't fly a reusable vehicle anytime soon, the new stretched Falcon 9 is almost certainly going to be competitive with anything in the global market at similar sizes, and Falcon Heavy is probably going to be competitive with any current or planned launcher of any size from any country. The only thing in the works that might be bigger is SLS, but that'll be on the order of 30% more payload at multiple billions per launch. So let's please not under-estimate the accomplishment of meaningfully improving access to space just because it doesn't get your sci-fi juices flowing.

Innovation isn't SpaceX's problem. If anything, the biggest criticism is that they don't have a good cadence going with actually launching stuff.

I mean, it is impressive that Space X looks like the first real private space craft to make real progress, I just don't think that it is feasible to really reuse those first two stages in a quick manner. I think a reusable spacecraft like Musk wants would have to be a single stage machine like the Venturestar, that has an airfoil to make a controlled landing with the entire thing.

The problem with SSTO is that the mass fraction is incredibly challenging, and if you miss your targets even slightly the whole vehicle becomes impractical. It's awesome if it works, but the more likely outcome is that it ends up tens of billions over budget and then canceled by the next administration. It's not an idea that has a plausible path to actually existing in the real world.

If they can accomplish that, it would truly truly be extraordinary.

Well, Musk recently tweeted that the demonstrator for the vertical landing technology is going to be ready to fly soon, so they're going to be routinely doing the landings long before they actually attempt an orbital vehicle.

 

[longhornchris04]

I think 1st stage reuse is a major driver behind SpaceX's decision to put a launch pad in south Texas. For GTO entries, i.e. most commercial satellite launches, its almost as good as Florida from a latitude perspective, and you have both Florida and Puerto Rico downrange that would make good landing sites for a 1st stage booster. Until ITAR is modified, they have to recover on US soil.

After all, if you only have to slow the rocket down for landing, rather than fully reverse its course, it will take less fuel.

For COTS launches to the ISS, Texas won't work because the launch inclination will take it over populated areas. I think Wallops, or perhaps Maine would be valid land recovery sites, but it would require cross ranging. Of course, the hit to payload capacity may mean that recovering the 1st stage may not be economical, but I'll let SpaceX prove me right or wrong.

 

[jbode]

Okay, so from the looks of it, this is the first commercial rocket. Commercial satellites have been launched off of space agency rockets in the past.

Sealaunch?

[/quote]
Unless you can control the descent of the first stage, it is nigh irresponsible to not have it land in the ocean.
[/quote]

The plan is for a controlled, powered descent on a single engine. I'm skeptical, personally, but I'm not a rocket engineer.

the venturestar was so much more promising. Why was it cancelled?

Too many problems, both in the tech and program management.

 

[Megalodon]

So that happened.

 

[jbode]

the venturestar was so much more promising. Why was it cancelled?

Like many government projects, it suffered from a combination of NIH and malicious neglect due to not fitting the current administration's goals.

"Current" administration? It was cancelled in 2001.

X-33/VentureStar - What really happened:

The X-33 VentureStar was highly criticized as a chunky flying fuel tank with little to offer once the satellite launch business diluted between several regular and proven launch systems around the world.

But for those that worked on the X-33, the pure complexity of the new system – and the chain reactions felt from issues and bad decisions such as the heavy engine ramps to the resulting need for the low weight of the LH2 tank – to the lack of options open to use a solution for the LH2 tank failures, turned her from a potential leap in space vehicle technology, to one that became a $1.5 billion white elephant to the tax payer.

If SpaceX can make truly reusable first and second stages for the Falcon and still keep launch costs in the hundreds of dollars per kg range (big ifs, I'll admit), then much of the impetus behind SSTO disappears.

 

[Sputnik]

I mean, it is impressive that Space X looks like the first real private space craft to make real progress, I just don't think that it is feasible to really reuse those first two stages in a quick manner. I think a reusable spacecraft like Musk wants would have to be a single stage machine like the Venturestar, that has an airfoil to make a controlled landing with the entire thing.

The problem with SSTO is that the mass fraction is incredibly challenging, and if you miss your targets even slightly the whole vehicle becomes impractical. It's awesome if it works, but the more likely outcome is that it ends up tens of billions over budget and then canceled by the next administration. It's not an idea that has a plausible path to actually existing in the real world.

Skylon

the venturestar was so much more promising. Why was it cancelled?

Like many government projects, it suffered from a combination of NIH and malicious neglect due to not fitting the current administration's goals.

"Current" administration? It was cancelled in 2001.

Meaning contemporary

 

[Megalodon]

The problem with SSTO is that the mass fraction is incredibly challenging, and if you miss your targets even slightly the whole vehicle becomes impractical. It's awesome if it works, but the more likely outcome is that it ends up tens of billions over budget and then canceled by the next administration. It's not an idea that has a plausible path to actually existing in the real world.

Skylon

Like I said, plausible.

It'll never be funded at a high level, it would never stay on budget if it did, and the idea is worse than VentureStar even if it had someone to pay the bills that didn't care.

Setting aside financing, I'll try to explain the challenges of a concept like this and why it's so unlikely to work.

Some of the challenges here are:

-SSTO pays a huge mass penalty for hauling all the extra mass to orbit.
-Using hydrogen pays a huge penalty because the mass fraction of the tankage sucks
-hydrogen engines have a poor thrust to weight ratio
-air breathing engines have a terrible thrust to weight ratio
-heat shields have a huge mass penalty, particularly big ones.
-gravity losses are made worse by poor thrust to weight ratios and mass penalties.

So basically all the engineering challenges for Skylon are multiplicative, they compound with each other. Every aspect of the problem makes every other aspect harder. VentureStar is almost as bad, but at least it didn't try to use air breathing engines.

The SpaceX concept might seem unwieldy, but it's extremely clever because the design decisions are synergistic, they make each other's jobs easier.

-Hydrocarbon fuel allows high mass fractions.
-Hydrocarbon fuel allows extremely high T/W engines
-Not doing SSTO leaves unnecessary mass behind
-And allows reduced thermal protection on the first stage
-And allows the only thing that needs a full heat shield, the upper stage, to be small.

It's still incredibly difficult, but it's not obviously going to fail for known reasons, which is an improvement in this industry.

 

[Hop]

The SpaceX concept might seem unwieldy, but it's extremely clever because the design decisions are synergistic, they make each other's jobs easier.

-Hydrocarbon fuel allows high mass fractions.
-Hydrocarbon fuel allows extremely high T/W engines
-Not doing SSTO leaves unnecessary mass behind
-And allows reduced thermal protection on the first stage
-And allows the only thing that needs a full heat shield, the upper stage, to be small.

It's still incredibly difficult, but it's not obviously going to fail for known reasons, which is an improvement in this industry.

The part I'm most pessimistic about is recovering the 2nd stage. The 8 km/s re-entry subjects it to extreme conditions.

If there were orbital propellant, some of the re-entry delta V could be accomplished with reaction mass. We wouldn't have to rely solely on aerobraking. And thus wouldn't have to endure the extreme temperatures re-entry presently entails.

If Planetary Resources succeeds in bringing back water rich asteroids, I believe recovery and re-use of upper stages will become much more doable.

 

[Megalodon]

Welp, it's all over but the splash.

We wouldn't have to rely solely on aerobraking. And thus wouldn't have to endure the extreme temperatures re-entry presently entails.

I can't see how the propellant would be worth it once there's already a full heat shield. It'd take a hell of a lot of propellant to reduce peak heating even a little.

What's probably worse is the first stage surviving without the full heat shield. That'll also be going a hell of a lot slower. Might make sense there.

 

[Hop]

We wouldn't have to rely solely on aerobraking. And thus wouldn't have to endure the extreme temperatures re-entry presently entails.

I can't see how the propellant would be worth it once there's already a full heat shield.

I remains to be seen if SpaceX can make a "full heat shield" that will keep the second stage intact during re-entry. I do not give them even odds.

It'd take a hell of a lot of propellant to reduce peak heating even a little.

Atmospheric abuse is roughly proportional to v³.

Let's say the second stage takes enough propellant to slow it 1 km/s. So it enters the atmosphere at 7 km/s rather than 8 km/s. (7/8)³ = ~.67.

Or if 2 km/s is accomplished with reaction mass, the re-entry delta V via aerobraking is 6 km/s. (6/8)³ = ~.42.

Exhaust velocity of Lox/Kerosene is about 3.3 km/s.

Propellant mass/dry mass = e^dV/Ve - 1.

Shedding 1 km/s and thus losing nearly a third of the re-entry abuse would take propellant massing ~36% of the second stage dry mass.

Shedding 2 km/s and thus losing more than half of the re-entry abuse would take propellant massing ~83% of the second stage dry mass.

Shedding .5 km/s and thus losing 17% of re-entry abuse would take propellant massing 16% of the second stage dry mass.

 

[Megalodon]

I remains to be seen if SpaceX can make a "full heat shield" that will keep the second stage intact during re-entry. I do not give them even odds.

They did it for Dragon, which is the same diameter, and there's pretty good evidence that that a) works and b) is launchable on Falcon 9.

 

[sophistry]

I have always wondered why horizontal launch systems aren't used for space craft. Vertical launch seems to burn a lot of fuel + dangerous vibration for accelerating from early stages of earth gravity. Any ideas?

 

[Megalodon]

I have always wondered why horizontal launch systems aren't used for space craft. Vertical launch seems to burn a lot of fuel + dangerous vibration for accelerating from early stages of earth gravity. Any ideas?

It's been done, by Orbital's successful Pegasus for example.

Generally speaking:

-A rocket has high loads to deal with, but it helps that they're pretty much all along one axis. Horizontal takeoff adds an axis.

-You invariably end up wasting a decent amount of delta-v transitioning from horizontal to vertical flight.

-It can help at small sizes, but it's really hard to scale. The Stratolaunch carrier will be the biggest aircraft in the world, yet only big enough to carry a shrunken 4-engine SpaceX rocket, when even the full-sized Falcon 9 isn't considered that big.

-The atmosphere is shallow and the speeds possible inside it are slow. Even if you get everything you can for free in the atmosphere, you still have almost all your work ahead of you to get to orbit. Better to just minimize gravity losses by having a powerful first stage.

tl;dr It's been done for small launchers, but it's not as helpful as you think and it doesn't scale.

 

[Christopher James Huff]

The part I'm most pessimistic about is recovering the 2nd stage. The 8 km/s re-entry subjects it to extreme conditions.

If there were orbital propellant, some of the re-entry delta V could be accomplished with reaction mass. We wouldn't have to rely solely on aerobraking. And thus wouldn't have to endure the extreme temperatures re-entry presently entails.

If Planetary Resources succeeds in bringing back water rich asteroids, I believe recovery and re-use of upper stages will become much more doable.

That's a lot of propellant used just to salvage an upper stage. Probably more effective to equip it with heat shielding made from orbital materials...foamed slag or something. You're also missing one other option: if Planetary Resources succeeds, you needn't necessarily bring second stages back to Earth's surface to reuse them! This is especially the case if they develop and start using the Raptor engine (as LH2 is relatively easy to make from orbital resources...liquid hydrocarbons that might substitute for RP-1 are doable given a source of carbon, though, or the Merlin engines might be adapted for LH2).

It would also be possible to use PR tugs to deliver the upper stages to an orbital facility to extract the engines and package them up with heat shields. Stack the tanks together, add temperature control systems, and use them in orbit for storage of propellant and other liquids/low-pressure gases, send back the relatively expensive but easily-returned engines.

 

[Sputnik]

The problem with SSTO is that the mass fraction is incredibly challenging, and if you miss your targets even slightly the whole vehicle becomes impractical. It's awesome if it works, but the more likely outcome is that it ends up tens of billions over budget and then canceled by the next administration. It's not an idea that has a plausible path to actually existing in the real world.

Skylon

Like I said, plausible.

Maybe you have different definition of plausible.

the idea is worse than VentureStar even if it had someone to pay the bills that didn't care.

That is great. It means this discussion is going nowhere because you are a non-"believer".

 

[Hop]

The part I'm most pessimistic about is recovering the 2nd stage. The 8 km/s re-entry subjects it to extreme conditions.

If there were orbital propellant, some of the re-entry delta V could be accomplished with reaction mass. We wouldn't have to rely solely on aerobraking. And thus wouldn't have to endure the extreme temperatures re-entry presently entails.

If Planetary Resources succeeds in bringing back water rich asteroids, I believe recovery and re-use of upper stages will become much more doable.

That's a lot of propellant used just to salvage an upper stage. Probably more effective to equip it with heat shielding made from orbital materials...foamed slag or something.

Yes, given an asteroid in high lunar orbit, more robust TPS also becomes an option, importing the shield materials from an asteroid as you say.

You're also missing one other option: if Planetary Resources succeeds, you needn't necessarily bring second stages back to Earth's surface to reuse them!

That is one of my favorite daydreams, in fact. Leaving the upper stages in orbit. Given propellant at various places, a ship could move between orbits indefinitely. Between LEO and GEO for example. Or between EML1 and GEO.

I believe Planetary Resources have such vehicles in mind when they write: "In Earth orbit, water from asteroids can also be converted and used to refuel satellites, increase the payload capacity of rockets by refueling their upper stages, reboost space stations, supply propellant needed to boost satellites from Low Earth Orbit to Geostationary Orbit, provide radiation shielding for spaceships, and provide fuel to space tugs that could clean up space debris."

This is especially the case if they develop and start using the Raptor engine (as LH2 is relatively easy to make from orbital resources...liquid hydrocarbons that might substitute for RP-1 are doable given a source of carbon, though, or the Merlin engines might be adapted for LH2).

Besides possible abundance of water, there are other reasons hydrogen could make a good fuel for moving about in cislunar space. Ships moving between orbits do not have the gravity loss penalty incurred during ascent, so thrust to weight ratio isn't a concern. And exhaust velocity for kerosene vs hydrogen is 3.3 vs 4.4 km/s.

I hope SpaceX develops a hydrogen/lox engine. Another possibility is ULA providing the upper stages using hydrogen fuel. As ULA points out when touting the ACES propellant depots, they already have hydrogen experience from the Centaurs and Delta IVs.

 

[sophistry]

I don't quite understand what you mean by planetary resources making it possible to leave a stage in orbit. You're sort of envisioning some sort of waypoint system, I take it? where a stage stays in a certain orbital path and boosts up incoming spacecraft?

Knowledge about the real complexities of space flight kinda make me like mass effect less, where real complexity is glossed over in favor of space magic.

 

[Hop]

I don't quite understand what you mean by planetary resources making it possible to leave a stage in orbit.

You misread me. Leaving stages in orbit is already possible. In fact it has been done. But once propellant's used up, it's dead. A spent rocket just adds to the growing cloud of space debris unless they can get perigee low enough to cause re-entry.

Given orbital propellant, a space craft could move back and forth between orbits many times. In which case leaving an upper stage in orbit could be a good thing.

 

[Megalodon]

Like I said, plausible.

Maybe you have different definition of plausible.

Appearing worthy of belief. Seeming reasonable or probable.

That it doesn't appear to be in gross violation of known physical law doesn't enter into it. It's not going to be funded at the level they claim to need, and couldn't be completed for that much even if it was. It might be physically possible to build, but is virtually certain not to be.

That is great. It means this discussion is going nowhere because you are a non-"believer".

What I lack in faith I make up in predictive power.

 

[sophistry]

I don't quite understand what you mean by planetary resources making it possible to leave a stage in orbit.

You misread me. Leaving stages in orbit is already possible. In fact it has been done. But once propellant's used up, it's dead. A spent rocket just adds to the growing cloud of space debris unless they can get perigee low enough to cause re-entry.

Given orbital propellant, a space craft could move back and forth between orbits many times. In which case leaving an upper stage in orbit could be a good thing.

How far up does the first stage usually take? And what percentage of fuel? I would think that catching a 2nd stage in orbit would almost be like catching a bullet.