Heavy Lift Launch Vehicles VS Orbital Assembly

[Nicholander]

Which is better, designing and building expensive HLLVs, or sending whatever you want into orbit as individual parts separately, and putting them all together in orbit?

Personally, I think orbital assembly is the better option, as it doesn't involve designing and building gigantic, and thus very complex rockets, which is very costly. But, maybe there is a reason for HLLVs, as I'm just confused on why NASA is developing the SLS, instead of launching their heavy payloads in parts on Atlas Vs.

Speaking of the SLS, why is NASA still developing it, when it will be extremely costly and they already have good, dependable and cheap launch vehicles?

[Kryten]

Which is better for what?

[Mitchz95]

They're building SLS because the government is forcing them to in order to create more jobs. But that's a subject for another thread.

Orbital assembly would require smaller launch vehicles, but the assembly itself would be much more complicated and would involve leaving the object in orbit for potentially months with nobody to make sure everything's functioning properly. Launching it to orbit in one piece allows it to begin its mission immediately, and lets NASA keep an eye on it right up till launch. That's my take on it, anyway.

[fredinno]

Which is better, designing and building expensive HLLVs, or sending whatever you want into orbit as individual parts separately, and putting them all together in orbit?

Personally, I think orbital assembly is the better option, as it doesn't involve designing and building gigantic, and thus very complex rockets, which is very costly. But, maybe there is a reason for HLLVs, as I'm just confused on why NASA is developing the SLS, instead of launching their heavy payloads in parts on Atlas Vs.

Speaking of the SLS, why is NASA still developing it, when it will be extremely costly and they already have good, dependable and cheap launch vehicles?

It's technically easier to launch things from a single large payload, than from many smaller ones- orbital assembly is much more complex, less efficient (need docking post mass, extra exterior wall mass, etc.) - Look at the ISS for example.

Also, there is the thing that trying to launch a Mars mission using 18 T chunks from Atlas Vs... Mars Semi Direct used 3x 140T launchers- aka 24 Atlas V 551 launches. Using the Falcon Heavy Expendable, there would still be 10 Launches needed.

Even lunar Missions benefit- to do Apollo with Atlas V 552s or Saturn IB would mean 7 launches needed to assemble it.

[Kryten]

There will always be payloads that are simply too big for small vehicles; for example, current NASA notional Mars landing plans need SLS to get a 10m fairing to fit the heat shield. Good look fitting one of those on Atlas, or even Vulcan.

[regex]

Why not use super-heavy LVs for orbital construction?

[Nicholander]

That's a very good, point. And, I meant cheaper by 'better', sorry I forgot to say that in my post. But, I still think that orbital assembly would be cheaper because:

1. Developing launch vehicles is just simply difficult and expensive, because they're very complicated and delicate pieces of equipment, and they become more complex the bigger they are. And if you make a launch vehicle bigger, the more time and more money you will need to work out every fine little detail, in other words; HLLVs are very very expensive in the design process. (In general I think the KISS principle is very important when designing launch vehicles.)

2. If we're using a manned Mars mission as an example, you could make the spacecraft(s) your going to need modular in design, I.E. they were designed from the beginning to be easily assembled in parts in orbit.

And I also don't see how rendezvous and docking is difficult, autonomous supply vessels often dock with the ISS, and there's never been a problem with that. And I don't see a problem with the parts of the thing you're trying to make in orbit (The manned Mars spacecraft, for example.) either, as again, we've had satellites actively operating for years on end.

EDIT: Kryten, couldn't you use an inflatable heatshield instead?

[More Boosters]

I recall someone mentioning that larger vehicles are inherently more efficient though I can't say I am sure why exactly. Maybe it's that aerodynamic losses make up a greater fraction of the overall losses? Not sure, but something like the SLS is a good starting point in my opinion. I mean, we intend to eventually get something larger than a probe out there, and the SLS is a class of its own. They probably shouldn't and won't use it to place satellites one by one, maybe in bulk.

[Kryten]

EDIT: Kryten, couldn't you use an inflatable heatshield instead?

As far as I can tell nobody has done a successful re-entry with an inflatable at any scale, so quite possibly not; and certainly not without an awful lot of development funding.

[fredinno]

As far as I can tell nobody has done a successful re-entry with an inflatable at any scale, so quite possibly not; and certainly not without an awful lot of development funding.

The Mars reentry vehicle in development comes close though, it's expanded by inflatables.

[SemiaCloud]

Orbital Assembly is good if you don't have access to heavy lifters. Also, we must remember, you can only build rockets so big.

[fredinno]

That's a very good, point. And, I meant cheaper by 'better', sorry I forgot to say that in my post. But, I still think that orbital assembly would be cheaper because:

1. Developing launch vehicles is just simply difficult and expensive, because they're very complicated and delicate pieces of equipment, and they become more complex the bigger they are. And if you make a launch vehicle bigger, the more time and more money you will need to work out every fine little detail, in other words; HLLVs are very very expensive in the design process. (In general I think the KISS principle is very important when designing launch vehicles.)

2. If we're using a manned Mars mission as an example, you could make the spacecraft(s) your going to need modular in design, I.E. they were designed from the beginning to be easily assembled in parts in orbit.

And I also don't see how rendezvous and docking is difficult, autonomous supply vessels often dock with the ISS, and there's never been a problem with that. And I don't see a problem with the parts of the thing you're trying to make in orbit (The manned Mars spacecraft, for example.) either, as again, we've had satellites actively operating for years on end.

EDIT: Kryten, couldn't you use an inflatable heatshield instead?

Docking isn't difficult- designing a spacecraft to be built from tiny chucks is, or at least more so than a single stack launch. Main problem though, is efficiency and complexity.

[sgt_flyer]

@kryten actually, nasa sucessfuly did some subscale tests of inflatable heatshields (even at 7600mph of speeds) as part of the ongoing HIAD research.

http://www.nasa.gov/home/hqnews/2012/jul/HQ_12-250_IRVE-3_Launch.html

[Frozen_Heart]

Why not both? Design a massive super heavy launch vehicle and then send up 100 ton modules to be assembled in orbit!

[GoSlash27]

They're building SLS because the government is forcing them to in order to create more jobs. But that's a subject for another thread.

Orbital assembly would require smaller launch vehicles, but the assembly itself would be much more complicated and would involve leaving the object in orbit for potentially months with nobody to make sure everything's functioning properly. Launching it to orbit in one piece allows it to begin its mission immediately, and lets NASA keep an eye on it right up till launch. That's my take on it, anyway.

I would think it has to be something like this.

Otherwise... why would we *not* assemble in orbit? Lifting an entire ship in one go severely limits it's size and mass, while assembling in orbit removes the bottleneck.

Best,

-Slashy

[Bill Phil]

If you already have a super heavy launcher, use it. But if you don't, use orbital assembly. It's cheaper than spending over 20 billion on the SHLV, then spending another 20 for the development of payloads.

[RuBisCO]

Orbital assembly is better only if you can get many loads to LEO cheaper then one big load.

The Saturn V per lbs to orbit and in adjusted dollars could get cargo to LEO at half the price of the space shuttle.

[Kibble]

Orbital assembly vs. heavy lift single-launch are useful for different tasks. Space Station was perfectly suited for orbital assembly because it can be used almost immediately after the very first couple parts are on orbit, and slowly improved while also constantly requiring launches for resupply and astronauts. Apollo was perfectly suited to heavy-lift since the destination was high-energy, so they could use hydrogen fuel for TLI.

[G'th]

A mix of both would be the most efficient out of a choice of either or. There's purposes on both sides that the other can't provide efficiently, and they both can complement each other pretty well.

You wouldn't want to construct a super large space station with medium lift vehicles (as it'll take years if not decades and very possibly be more costly depending on how you do it), but you also don't want to use heavy lift if you're not fully utilizing its capabilities with every launch.

[Bill Phil]

Orbital assembly is better only if you can get many loads to LEO cheaper then one big load.

The Saturn V per lbs to orbit and in adjusted dollars could get cargo to LEO at half the price of the space shuttle.

The Shuttle isn't a good comparison. How about Proton or Atlas V?

[prophet_01]

Actual construction in space (not just docking modules that were specifically designed for the task) is very complex and requires specialised personal, materials and tools that you don't even need for the actual mission. So you suddenly need a lot more heavy equipment and people in orbit than you might think which drastically increases the costs. Assembling something like 20 small pieces of a craft in space is a huge task and would very likely require the work of half a dozen astronauts over many months, likely even longer and with crew rotations. The initial construction of the ISS is a pretty good example of how much time, equipment and personal is needed for such things.

On modules, you always end up with a craft that isn't as stable, but still a lot heavyer compared to one that has been launched in one piece. You end up with duplicate and thus redundant systems on every piece and have to rely on docking ports and multiple smaller fuel tanks. That might not be much of an issue for a space station, but when it comes to interplanetary missions this won't make things easyer since you have to carry all of this dead weight through half the solar system...

The ISS cargo flights aren't an example of modular construction since they don't have to be a permanent part of the structure and aren't nearly as complex or bulky as modules. For resupply missions you can also use the same design for on every flight. Designing 10 very different modules that have to reach the construction site on it's own is difficult and it's not efficient to have dozens of dead and useless engines aboard. If you use a tug to deliver the modules (russian style) you cut even more into your payload per launch and need more launches in the end. Still, this is one of the best tradeoffs between complexity, mass and design costs. For stations this is a proven and fairly efficient concept.

Edited October 7, 2015 by prophet_01

[Nibb31]

There are three methods of orbital assembly:

- Mir-style, where every module is an independent spacecraft with its own propulsion, avionics and power. This makes the hardware more complex,

- Shuttle-style, where modules are "dumb canisters" that need to be brought to the station and assembled with an arm. This requires hardware that is much more complex, but is reusable.

- There is a hybrid-style form of assembly, where an orbital tug picks up a dumb canister and docks it to the station, then detaches for the next job. This has never been done yet (the Russians used expendable tugs for Pirs and Poisk, as well as Kvant-1 on Mir), so we don't really have any idea of the economics.

So to answer the OP, which one is cheaper depends on the cost of the assembly hardware and the number of times it's used. It makes no sense to develop an entire reusable infrastructure for a handful of modules (like Mir or ISS), but if you are going to have sustained assembly activity (which we aren't) then it might be more economical.

I voluntarily exclude EVA assembly, because any method that requires EVA is going to be way over the top in terms of cost.

[GoSlash27]

Orbital assembly is better only if you can get many loads to LEO cheaper then one big load.

The Saturn V per lbs to orbit and in adjusted dollars could get cargo to LEO at half the price of the space shuttle.

Well... it's cheaper, but not necessarily better. It's only better if your entire mission package can fit in a single launch.

Otherwise modular assembly is the way to go.

Best,

-Slashy

[Jouni]

In short, payloads are complex and expensive, while rockets are simple and cheap. The best option is usually to launch the payload in as few launches as possible, using as large rockets as you can justify developing.

It really boils down to R&D costs vs. production costs. Most systems in the payload are (almost) unique. Every system you can eliminate will save you a lot in R&D costs. Rockets, on the other hand, need only minor improvements after the first few launches, and the marginal costs of launching yet another one can be quite low. If you design a rocket with a significantly higher payload capacity than anything you already have, and get enough launches with large payloads for it, the R&D savings from simpler payloads will outweigh the R&D costs from developing the rocket. I believe the break-even point is at around 10-20 launches.

[Rune]

Actually this is all a question of budget, and flight rate.

Let's clarify this. We seem to be discussing SLS vs. the EELV cadre (and I throw Falcon 9, Arianne V, and Proton in there for good measure). Then, the obvious answer, for any imaginable payload mass, is that the EELVs will be cheaper. Why do I say that with such confidence? Easy, NASA's budget is not enough (and most likely, never will be), to fly the SLS, and pay for its payloads, more than perhaps once a year at most. That will translate into huge fixed costs that bloat the launch cost into shuttle territory (on the order of a billion dollars/launch). You can see how that quickly gets you nowhere.

EELVs, on the other hand, would see their fixed costs diluted into nothingness if they were built at even higher rates, so they will only come down in cost if more money is funnelled their way, meaning more money for payloads, meaning you actually can launch hardware up there and do things instead of waiting for your budget to magically double over night.

Of course the other edge of the sword is that more launches are inherently more inefficient mass-wise, on account of all the docking hardware involved. A tug-based architecture like the one Nibb31 alludes to can somewhat alleviate this issue, but it is real nonetheless: Progress has a pitiful payload faction, because Soyuz (the rocket) is barely capable of flinging a functional active-docking capable autonomous spacecraft into orbit. Even with an infinitely reusable tug you are increasing maneuvering fuel used and connection hardware (I.E: plumbing). Conclusion? Fly the biggest rocket that you can afford to fly often.

Rune. The last part is crucial, but it's way easier to sell people the part about "biggest".