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Comment by nawitus | Hacker News Reader

The problem with single-stage-to-orbit (SSTO) is mainly weight. I think using composite tanks is enough to make a SSTO rocket. X-33 was supposed to use a composite tank, but the fuel tank failed during testing in 1999. Recently, NASA has succesfully tested a composite tank[1].

1. http://nasawatch.com/archives/2013/07/composite-cryog.html

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You don't need composite tanks for SSTO. There have been normal rocket stages that have had high enough mass ratio and specific impulse big enough for SSTO, like in Titan, but they weren't used as SSTO.

One of the secrets is dense propellants which increases mass ratio considerably. It also makes higher thrust to weight ratio engines easier to implement (because pumps primarily pump volume rather than mass), and with identical initial acceleration, the dense propellant higher mass ratio lower impulse vehicle gets lighter faster and thus has higher acceleration towards the end of the flight, meaning less gravity losses. (Also air drag is less with a smaller vehicle, but it's not a big effect, though it is proportionally bigger for smaller vehicles.)

Hydrogen has the best specific impulse but it is non-dense. It is also hard cryogenic, meaning potential complexity in the vehicle, ground infrastructure and operations.

Also, big vehicles might be more expensive, even if their contents (the propellants) don't weigh much.

I was confused by this video [1], which looks like it shows three stages. All three stages are able to land on their own. Can you explain?

[1] http://www.youtube.com/watch?v=abLC1l3loFA

The Dragon capsule is more "payload" than "stage". The Falcon 9 will have two stages (hopefully both eventually landing themselves). The Dragon (not always used. Sometimes they may just stick a satellite or something on top) will also be able to land itself.

Sort of like how the Saturn V is considered a 3-stage rocket, even though during the later Apollo missions it had on top a service module, a command module, and a (two stage!) lunar module.

So what modules does the video show?

Also, if the mission is something like transport astronauts to the ISS, why eject any stages at all? Why not just make the rocket capable of flying to the ISS, let the whole thing dock to the ISS, exchange astronauts, then detach and land back on Earth?

(Of course, if there are tons of missions happening around the same time, all requiring a first stage rocket, and each mission has a period that doesn't require any rocket, then I see the point of letting the rocket go back and be used for other things.)

>Also, if the mission is something like transport astronauts to the ISS, why eject any stages at all?

Because that requires a single-stage-to-orbit vehicle, which has never been done before. Yes, in the future that might be possible.

The video shows the lower stage and upper stage of a Falcon 9 taking off, then landing. These two stages are shown launching a Dragon spacecraft. The Dragon spacecraft is shown docking with the ISS. The pressurized half of the Dragon spacecraft is then shown landing (the unpressurized trunk section, with the solar panels attached, burns up in the atmosphere (though this is not shown in the video)).

Staging is done because not doing it ("SSTO" or "single stage to orbit") is extremely hard. In fact, it has never been done before from earth. When that second stage burns, it is only accelerating the mass of itself and the Dragon spacecraft on top. Having it accelerate the mass of the lower stage as well would be massively inefficient, requiring much more fuel to be used. So much more fuel that there most likely would no be any mass left over for the Dragon spacecraft. Without staging we wouldn't be able to put things in orbit at all (today and in the past anyway), let alone reuse rockets.

Thanks. I assumed the section with the solar panels somehow retracted the panels before re-entering the atmosphere, but they skip that part, and I now see that the part re-entering is actually a bit smaller.

Yup, that part burns up.

It's not a complete waste though. I believe at the ISS they usually pack those sorts of parts (parts that burn up that is. Other examples being the Russian "Progress" spacecraft or the ESA's ATV) full of garbage before sending them off. Intact reentry volume/mass is valuable (currently only the Soyuz and the Dragon can do that) so they just let all of their trash burn up. Here's a neat picture of the first ATV, carrying a bunch of ISS trash, burning up: http://en.wikipedia.org/wiki/File:Jules_Verne_Automated_Tran...

> Also, if the mission is something like transport astronauts to the ISS, why eject any stages at all? Why not just make the rocket capable of flying to the ISS, let the whole thing dock to the ISS

Essentially, Physics. To a lesser extent, practicality and cost.

See The Tyranny of the Rocket Equation[1] or the "Model Rocketry" entry of xkcd What-If[2]

[1] http://www.nasa.gov/mission_pages/station/expeditions/expedi...

[2] http://what-if.xkcd.com/24/

The delta-v the first, more massive, stage has to deal with is much smaller than the one the second-stage has to deal with. The second stage is more or less on LEO when the Dragon is released and uses atmospheric braking just like most returning spacecraft do. Braking a single stage like that would require lots of structural reinforcements that would increase vehicle weight.

Also, the lower stage engines are optimized for working in the low atmosphere while the second stage only needs to operate in a vacuum and therefore, has a different design.

Several folks have answered already, but here's another attempt: getting stuff into orbit takes so much fuel that any launch vehicle must devote something like 90%+ of its mass to fuel tanks. Once a fuel tank is empty, it's worthless, dead weight. If you tried to carry all that dead weight into orbit, you would never make it; it would pull you down. So you drop the empty tanks as you go, until the part that finally reaches orbit is only a small fraction of the original launch vehicle.

An SSTO--single stage to orbit--vehicle would be safer, more reliable, and above all cheaper to operate than a multi-stage vehicle, but every attempt to make something light enough that it can reach orbit with all its fuel tanks still aboard has failed. A two-stage rocket where the stages return to the pad under their own power is still a big improvement over the traditional model, with three or more stages that are simply dropped into the ocean to be recovered later.

I feel like a lot of good technology was lost with the cancellation of the X-33/VentureStar. They were very close, and a few test failures caused the entire program to be shut down. It's really a shame.

Well... even more bad technology was lost with the cancellation of the X-33. It was a horrifically over-complicated design which had everything including the kitchen sink thrown into it, strictly to maximise its potential for congressional pork. I'm serious. I was actually at a Lockheed-Martin briefing prior to the X-33 competition, wherein they presented a slide which looked like this:

  TECHNICAL FEATURES
  * Linear aerospike engine [first ever tried]
  * Conformal composite LH2 tank [first ever tried]
  * Integrated advanced metallic thermal protection [first ever tried]
  * Subcontractors in 38 states and 122 congressional districts

The level of complexity in the Venturestar design meant that it could never have achieved the lightweight mass fractions needed for an SSTO. If there is ever to be an SSTO, it will be simple and elegant. But "simple" and "infinitely divisible into a large number of congressional districts" are mutually incompatible, which is why the US government chose the Lockheed Martin design over the vastly more realistic Macdonald Douglas DC-Y: http://www.astronautix.com/lvs/dcy.htm

It's a shame that they decided to run back this same approach for the F-35.

A leap too far. You could test and mature those technologies further independently (in the lab, at test stands, flying tests with existing vehicles) at much lower cost before trying to integrate them into a design that costs billions.

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