Comments on Rocketpunk Manifesto: Halfway To Anywhere

Rick: "So I remain broadly agnostic in pr...

2011-08-11T12:23:03.404-07:00
Rick:

"So I remain broadly agnostic in principle - though as a practical matter I'd guess that early generation interplanetary craft will be single-use."

One has to retain one's agnosticism WRT the midfuture when so much is unknown. My prior comments about the advisability of reusing hab or any other mission modules are strictly in the context of current or foreseeable technologies. Develop a 1 kg/kw power supply and hook it up to a big enough gang of electric motors (no need to wait for VASIMR or whatever) and you have a real spaceship worth reusing.

Short answer, I don't know. But the first an...

2011-08-11T11:07:14.766-07:00
Short answer, I don't know.

But the first and foremost reason for expending vehicles today is that we would have to add Lots of Cost and Mass to get them back for re-use.

If the mission architecture of an early interplanetary mission calls for the crew to leave the transfer hab and use an aerobraked capsule for Earth return, then obviously we'll kiss off the hab. This is what I'd expect for a chemfuel mission.

On the other hand, if a 'fast' electric transfer vehicle is used, the Earth approach speed may be dangerously high for capsule aerobraking, while decelerating the transfer craft is less costly. So there's a greater likelihood that the transfer craft will end up in Earth orbit.

In that case re-use of the hab is more viable - depending on a host of secondary factors.

So I remain broadly agnostic in principle - though as a practical matter I'd guess that early generation interplanetary craft will be single-use.

Rick: "I'm not sure that's the ca...

2011-08-10T12:12:12.505-07:00
Rick:

"I'm not sure that's the case. Judging from the ISS, the useful service life of a hab structure is on order of a decade or two. I think the same would be true of a hab that is safe enough for interplanetary missions at all.

Yes there are counter-arguments: Mission requirements will evolve, and it could end up simpler to configure Mars II to meet its mission requirements than to reconfigure Mars I.

But I don't see either one as a given."

No, nothing is a given. But if you're going to reuse just the transit hab portion, perhaps it's better to launch the hab into a cycler orbit and just use it as a transit shelter. Stopping it at Mars and Earth on every round trip doesn't make a lot of sense. (You do get bonus points for reusing shielding materials as well.) If you need orbital facilities at Mars, you put a hab in the most appropriate Mars orbit. When we're talking about reusing interplanetary spacecraft, we're at that stage of investment, right?

I'm not sure that's the case. Judging from...

2011-08-10T11:44:37.334-07:00
I'm not sure that's the case. Judging from the ISS, the useful service life of a hab structure is on order of a decade or two. I think the same would be true of a hab that is safe enough for interplanetary missions at all.

Yes there are counter-arguments: Mission requirements will evolve, and it could end up simpler to configure Mars II to meet its mission requirements than to reconfigure Mars I.

But I don't see either one as a given.

Ferrell: Your "core" should be just as ...

2011-08-09T21:07:18.176-07:00
Ferrell:

Your "core" should be just as expendable as anything else. There's nothing special about hab modules. Even though you could find nutty astronauts that would do it, no space program manager in his right mind is going to take the risks of reusing flight hardware on interplanetary missions. There's just not that much savings compared to the risks involved.

Ok, let me clairify this since people seem to have...

2011-08-09T19:47:57.188-07:00
Ok, let me clairify this since people seem to have missunderstood the intent of my design.
The core (i.e. the crewed portion) of the ship is reusable and does not land; the mission module can be manned and/or cargo landers; the propulsion module would be swapped out after each mission. All the modules would be sized to be launched into orbit by a heavy LV. The ship could be used to travel to NROs, planets, the Asteroid Belt, where ever. The "support" function would be to transport the people and equipment from Earth orbit to, for example, Mars orbit; that some of the transported equipment would be landers is just part and parcel of the mission; also, as new propulsion methods become avaliable, they could be used just by swapping out those modules.
I hope this clears up any of your misunderstanding.

Ferrell

Monte Davis: "If it's "orbit to ...

2011-08-07T18:46:38.580-07:00
Monte Davis:

"If it's "orbit to orbit" -- i.e. the human crew's role will be to aim instruments at a planet -- then why have a human crew?"

As a proof of concept and flight hardware test, it makes sense, a la Apollo 8. Otherwise it has no real justification.

What Ferrell is missing is that we just haven't achieved the level of reliability to safely reuse flight hardware. Even if we get to a place where we know enough, and it makes economic sense, I doubt that we'll reuse interplanetary flight hardware more than five or six times before retiring it. It's the same thing as launch vehicles -- just too many dynamic stresses and too much riding on your equipment reliability.

(Sorry, sent the last one too soon) As always, ge...

2011-08-07T09:47:42.126-07:00
(Sorry, sent the last one too soon)

As always, getting out of and into gravity wells is the hard part. You toss in "it would be used to support manned missions," but it's not clear to me why making this "support" function the next step for the 2010's, as you propose, would be preferable to beefing up the manned mission itself.

[obRick'sPostTitle] When we get a lot better at that first hard step to orbit, then everything beyond opens up. Until then, anything beyond tends to be forced into the Apollo mode of highly mission-specific, git-'er-done hardware that doesn't make sense for recurring operations.

No one disputes the case for orbit-to-orbit craft, free of the constraints of launch and landing. But the critical path to those, and to everything else we'd like to see, goes through CATS.

Each mission would last months or years, but sh...

2011-08-07T09:17:19.648-07:00
Each mission would last months or years, but should yeld massive amounts of data and discovery, if robotic missions are used as examples.

If it's "orbit to orbit" -- i.e. the human crew's role will be to aim instruments at a planet -- then why have a human crew?

Thinking about this, maybe a 'space cruier'...

2011-08-07T08:19:51.052-07:00
Thinking about this, maybe a 'space cruier' orbit to orbit interplanetary space craft would be a good next step; the central structure could be five 'standard' (i.e. ISS modules) modules arranged three parallel connected by two others so it can rotate freely; on the ends of the central module, are the propulsion module and a mission module that can be swapped out to match whatever mission profile is needed. The central crew structure would have both physical and magnetic radiation shielding, long-term life-support, and a relitively large crew; it would be used to support manned missions, recover failed robotic missions, be on-orbit C&C for probes and rovers, or whatever other mission. It would park next to the ISS(or its successor) for reconfiguration, refurbishment and resupply, and to swap crews, before going out on another mission. Each mission would last months or years, but should yeld massive amounts of data and discovery, if robotic missions are used as examples. Seven heavy cargo launches and a heavy crew launch initially, with just two additional cargo launches per mission, plus a crew launch.

I think that we could at least one of these ships operational by the end of this decade.

Ferrell

Most gasses "solidify" under extreme con...

2011-08-05T18:54:02.082-07:00
Most gasses "solidify" under extreme conditions, if at all. Dry ice (frozen CO2) is one of the few exceptions I am aware of. This may be less extreme than "metallic" solids or metastable forms of matter, but would still make a materials science and engineering headache that rocket engineers (and engineers in general) probably want to avoid.

The only way to have high energy density for your spaceships (or avoid the use of huge fuel/oxidizer or remass tanks, which is essentially the same thing) is to go to some form of nuclear energy, with all the various issues that brings.

All is not entirely lost, very compact and powerful nuclear thermal engines have been demonstrated in prototype form, and ISPs of up to 1200 seconds are achievable (with 800-1000 seconds being the norm) without any mind boggling science or handwaving.

How difficult is it to use solidified gasses in sp...

2011-08-02T08:35:05.109-07:00
How difficult is it to use solidified gasses in space as propellant for a nuclear thermal drive?
I have been looking into reducing the volume of tankage on my warship designs as any decent mass ratio with LH2 just leads to a balloon capped by the payload and drive on the other end. Then I tried water (much denser, but hurts a lot in terms of exhaust velocity, and has to be kept liquid). And methane (not too much hurt EV wise, but has to kept liquid again and still has 0.5kg/L). Nowhere have I found desnities for stuff like solid CH4...it seems much easier to store (thin walled tanks) and residual waste heat can warm it to slush for use as propellant.

NASA looked at repurposing spent stages on-orbit d...

2011-07-29T09:12:00.591-07:00
NASA looked at repurposing spent stages on-orbit during their "wet workshop" studies for both Apollo S-II and Shuttle ET launch vehicle components. Aside from the residual contents problems*, the big showstopper was that they couln't figure out how to make a strucutre both safe enough to use as a propellant tank and accessible enough to insert life support and other compnents on-orbit. It turns out that enclosed volume per se is not all that valuable -- it has to be reasonably usable.

*Which weren't all that big a deal, because you can effectively helium-purge and LH2 tank, and a LOX tank contains a cabin atmosphere component anyway.

=Milo= Rick: "those tanks (other than ...

2011-07-27T10:11:35.796-07:00
=Milo=

Rick:

"those tanks (other than O2 tanks) had previously carried rather dangerous stuff"

Pure oxygen is dangerous too. See: Apollo 1.

(SA Phil) Good point Rick, You don't need al...

2011-07-26T20:03:50.820-07:00
(SA Phil)

Good point Rick,

You don't need all those hunks of metal until you have an orbital industry capable of processing them.

But once you have an orbital industry not using those pieces once the are in orbit would be extremely wasteful.

I remember that at one time there was a lot of tal...

2011-07-26T19:45:47.428-07:00
I remember that at one time there was a lot of talk about re-using top stage fuel tanks, including as habitat, etc. My impression is that this talk faded once people concentrated on the fact that those tanks (other than O2 tanks) had previously carried rather dangerous stuff.

Yes, once exposed to vacuum you'd expect the nasty stuff to rather quickly disappear, but it may be more complicated in the real world.

There is also the more mundane question of how you actually use an ex fuel tank. Cutting a hatch in it, in space, is not like doing the equivalent on the ground.

Eventually we will probably do this stuff, but it will be a slow development curve.

Just a couple thoughts. In regards to turbo pumps...

2011-07-25T22:44:52.572-07:00
Just a couple thoughts.

In regards to turbo pumps being complicated, finicky pieces of kit I do find the XCOR developments in cryogenic piston-pumps quite interesting and I find myself wondering if they could be scaled up or at least scaled up how far. Their rocket and pump designs so far seem to be rugged, reliable and have demonstrated being refueled and restarted multiple times in one day. Granted this is with the engine in a rocket plane and not in a launch vehicle, but still interesting.

Moving on…

No matter what one thinks of the shuttle it and all of its assorted hardware are a sunk cost at this point, and as such I’ll come out strongly in favor of shuttle derived launch vehicles if for no other reason than to keep the factories going, the politically connected contractors placated and all of the experienced engineers onboard.

Looking at a couple figures on Wikipedia and scratching out some numbers on the back of a napkin here, the shuttle weighs 68,585 kg without the SSME on it, 78,018 kg WITH the SSME installed, and a gross liftoff weight of 109,000 kg. So switching out the orbiter with a cargo pod like the Shuttle-C or some such should buy us an extra 60 or so tons of delivered payload. That strikes me as…useful.

To continue SA Phil’s comment on reusing upper stages in orbit, my understanding of it is that the shuttles fuel tank get something in the area of 98% of orbital velocity anyway and (someone correct me if I’m mistaken) they have to waste a bit of fuel maneuvering to dump it into the Indian ocean. While I’m not the first person by any means to have this idea it always felt to me that if you were going to try and reuse upper stages then taking the shuttle tank to orbit would seem like a logical place to start.

So lets say we add this all up to a shuttle tank modified slightly with an airlock in the intertank area and brackets on the interior of the tank to bolt stuff to along with a Shuttle-C cargo pod loaded with all of the stuff needed to outfit the interior the tank.

Later on a different launch will send up the first crew to start assembling everything.

…

I guess what I’m getting at is that while the shuttle never really impressed me the bits-n-pieces that make it up are useful. Maybe I’m dreaming in Technicolor here but I’d like to think such a plan might allow for us to do a bit more Cool Stuff (TM) in space on more or less the same budget and operational tempo.

(SA Phil) You could make both the stages of a two...

2011-07-25T15:51:57.291-07:00
(SA Phil)

You could make both the stages of a two stage system reusable in a sense.

Stage 1 reusuable as a launch vehicle after some fashion. Even if it just deploys Parachutes and is recovered.

Stage 2 recyclable in SPACCEEE!!
Heck you have a big honking piece of metal and rocket engines .. you could find something to do with it if you were creative with the design.

You could even make a space station with all the stage 2 parts you have lying around.

Jetliners marginalized prop airliners within a few...

2011-07-25T15:42:40.272-07:00
Jetliners marginalized prop airliners within a few years of the 707's commercial debut in 1958, suggesting that they were more economical nearly from the outset.

I would guess that what mattered to the airlines was that jets were more productive: They could make a transcontinental round trip - combining flight time and ground servicing time ('block speed') - in less time than the prop planes.

I'd further guess that the underlying economics were established by the B-52. The B-47 was a tricky plane to fly, and likely a maintenance hog as well.

The 'conceptual simplicity' of rockets was a frequent argument in books I read as a kid. Heinlein's The Rolling Stones, IIRC, has a wonderful little passage where the (22nd century?) protagonist describes a 20th c. automotive engine as fantastically complex compared to a nuke thermal rocket engine.

In principle this is actually true, but as noted above the devils are in the details. And, turbopumps etc. aside, rocket engines have enormous power density - on order of 1 MW/kg (!) - which pretty much means that they are always just short of melting or disintegrating.

The simple fact is that no matter how many engines...

2011-07-25T15:03:47.196-07:00
The simple fact is that no matter how many engines a launch vehicle uses, and no matter how economical their reuse might seem, you have to convince the customer, and by more than price, that those engines are reliable enough for his payload. That's a curve I don't think is realistically climable.

Each Falcon 9 will use 10 Merlin engines, and a Fa...

2011-07-25T14:02:41.661-07:00
Each Falcon 9 will use 10 Merlin engines, and a Falcon 9 Heavy will need a total of 19 Merlins. Even at a low launch rate that is a lot of engines, plus development engines and perhaps more for Falcon 1 rockets if SpaceX is planning to enter the small launcher market.

Four Falcon 9 launches a year is 40 engines, which is fairly high rate production for rocket engines. If the Falcon architecture supports a third stage for interplanetary launch, then I don't see why their orbital transfer stage would not use a Merlin engine as well, increasing the economies of scale.

That should qualify in terms of rocket production.

Monte Davis: "For sure: like many innovat...

2011-07-25T13:51:29.219-07:00
Monte Davis:

"For sure: like many innovations it served the military and then high-end (=price-insensitive) civil markets first. But those did provide the scale and experience that permitted the "massification" of air travel in following decades -- as well as funding ongoing R&D at RR, GE, and P&W, so eventually the masses could fly on high-bypass, ducted fans, etc."

And things turned very quickly. My parents met in Honolulu, Hawaii in 1960, both having gone there on DC-6s. When they moved to San Diego in 1961, they flew on a jet, though they were lower middle class at best.

"I guess my question (and Elon Musk's :-) ) is: how many rocket engines do you have to build and fly per year before that virtuous circle begins to spin? And at what angle does that cut across the question of transition from one-shot to reusable engines? It's got to be of some value to design/mfg teams to *see* the damn things after use, not just pore over telemetry."

Well, the designers see a lot of hardware after ground tests. That's how they developed the RP-1fuel specifications -- common jet kerosene was not refined enough to keep from gumming up their engines. Aside from that, I'm not sure I believe in reusable engines as an acceptable commercial choice for customers. Given the scope of the dynamic stresses inflicted on a launch vehicle, As long as the vehicle is going to be expended too -- and I don't see how it can't be, given current technology -- I want my mission flying on new engines with only a ground test or two behind them.

people were willing to pay higher air transport...

2011-07-25T11:36:07.467-07:00
people were willing to pay higher air transport prices for getting there in a few hours rather than ten or twelve.

For sure: like many innovations it served the military and then high-end (=price-insensitive) civil markets first. But those did provide the scale and experience that permitted the "massification" of air travel in following decades -- as well as funding ongoing R&D at RR, GE, and P&W, so eventually the masses could fly on high-bypass, ducted fans, etc.

I guess my question (and Elon Musk's :-) ) is: how many rocket engines do you have to build and fly per year before that virtuous circle begins to spin? And at what angle does that cut across the question of transition from one-shot to reusable engines? It's got to be of some value to design/mfg teams to *see* the damn things after use, not just pore over telemetry.

Monte: "It's my impression that one b...

2011-07-25T11:00:10.362-07:00
Monte:

"It's my impression that one big reason for the relatively fast transition to jets in the 1950s and 1960s was that the engines proved to need significantly *less* maintenance per passenger-mile than the high-performance radials of the 1940s had."

That's true, as far as it goes. But it's also a fact that people were willing to pay higher air transport prices for getting there in a few hours rather than ten or twelve. There's no analogous situation in launch vehicle economics. You pay the minimum you can to get there in pretty much the same amount of time, determined by astrodynamics, combustion physics, and materials technology. While consideration of launch site availability, ground and vehicle services, and some other things do influence decisions, launch vehicles within a certain class are pretty much fungible. Even class doesn't mean much if appropriate packaging can be arranged. Iridium used three different rockets in two different classes to launch their constellation.

"I've seen it argued that rocket engines are inherently simpler than turbojets and therefore shouldn't be so much more expensive. Is that handwaving, or does it take into account turbopumps, gimbaling, weight-pared thrust and cooling/preheat structures, and everything else around then "simple" engine..?"

I think there's a desire to look at rockets as being conceptually simpler, but that's not really true. Rockets need turbined to compress and deliver both propellant components to the combustion chamber. Turbojets only use turbines -- albeit with many more discs per shaft -- to compress the oxidizer. And turbojets are simple, combustion driven devices. Rockets use precombustion, or separate stream, much lower energy combustion products to spin their turbines. Oh, and did I mention that ROckets run so hot they need to be actively cooled, while turbojets are passively cooled? Yeah, rockets are simple all right...

I'd love to see historical data on the cost/pe...

2011-07-25T05:07:55.925-07:00
I'd love to see historical data on the cost/performance learning curve for jet and turbojet engines starting from B-47 days: where were the "knees" at which economies of scale and of production experience began to tell? IOW, what counts as "mass production?"

It's my impression that one big reason for the relatively fast transition to jets in the 1950s and 1960s was that the engines proved to need significantly *less* maintenance per passenger-mile than the high-performance radials of the 1940s had. I've seen it argued that rocket engines are inherently simpler than turbojets and therefore shouldn't be so much more expensive. Is that handwaving, or does it take into account turbopumps, gimbaling, weight-pared thrust and cooling/preheat structures, and everything else around then "simple" engine..?

Comments on Rocketpunk Manifesto: Halfway To Anywhere

Rick: "So I remain broadly agnostic in pr...

Short answer, I don't know. But the first an...

Rick: "I'm not sure that's the ca...

I'm not sure that's the case. Judging from...

Ferrell: Your "core" should be just as ...

Ok, let me clairify this since people seem to have...

Monte Davis: "If it's "orbit to ...

(Sorry, sent the last one too soon) As always, ge...

Each mission would last months or years, but sh...

Thinking about this, maybe a 'space cruier'...

Most gasses "solidify" under extreme con...

How difficult is it to use solidified gasses in sp...

NASA looked at repurposing spent stages on-orbit d...

=Milo= Rick: "those tanks (other than ...

(SA Phil) Good point Rick, You don't need al...

I remember that at one time there was a lot of tal...

Just a couple thoughts. In regards to turbo pumps...

(SA Phil) You could make both the stages of a two...

Jetliners marginalized prop airliners within a few...

The simple fact is that no matter how many engines...

Each Falcon 9 will use 10 Merlin engines, and a Fa...

Monte Davis: "For sure: like many innovat...

people were willing to pay higher air transport...

Monte: "It's my impression that one b...

I'd love to see historical data on the cost/pe...