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rodion_herrera

Is the jettisoning of external fuel tanks considered staging?

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Granted, this is an aircraft. But, if you built a spaceplane that carries external fuel tanks or pods, and you jettison them (much as a fighter plane would) as you perform your ascent to orbit, are you "staging"? So a spaceplane that does this, cannot be considered an SSTO, even though technically, it is still relying on the same engines for ascent and do not rely on other "assist" engines to reach orbit?

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I'd say yes, it's staging. It doesn't matter though. SSTO has been a vastly over-hyped and over-rated concept on here for far too long. SSTO isn't necessarily even the "best', you're possibly carrying extra weight up to orbit.

SSTOs are still interesting, but the game is more interesting/complete/better/easier if you don't get too hung up on them, and they have really been done to death now. Almost-SSTO, i.e. drop tank type stuff, NASA Shuttle style with booster and (very large) drop tank may well be the better choice overall, but there's really nothing wrong in traditional staged rockets.

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I'd say yes, it's staging. It doesn't matter though. SSTO has been a vastly over-hyped and over-rated concept on here for far too long. SSTO isn't necessarily even the "best', you're possibly carrying extra weight up to orbit.

SSTOs are still interesting, but the game is more interesting/complete/better/easier if you don't get too hung up on them, and they have really been done to death now. Almost-SSTO, i.e. drop tank type stuff, NASA Shuttle style with booster and (very large) drop tank may well be the better choice overall, but there's really nothing wrong in traditional staged rockets.

I agree with many of your points. But purely for the sake of just getting a spaceplane up into orbit or another planet/moon, it is still an interesting way to get someplace. I was just curious if the term SSTO is strict enough to disallow ET-equipped craft to be considered as such.

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Not sure. In aviation, the term used when dropping tanks is "jettison" ... which to me is different than "staging". The definitions of both terms (in their respective meanings) are also different.

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An article about the proposed ROMBUS design (google it) said it was an SSTO but wasn't due to its jettisoning of 8 external tanks during ascent. So I'd say it IS an SSTO but it isn't. :)

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Besides SSTO to orbit space planes having the cool sci-fi factor, their advantage is that you recover the full cost of parts minus fuel. The cost of losing a few empty tanks really isn't much at all, and while it is a small distinction, I consider it much closer to SSTO than a traditional multistage rocket.

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If you are saving funds by having drop tanks, it's still a better design than a 'pure' SSTO. Categorization doesn't matter.

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If you look into variants on that design, there were others that retained the external tanks and used them for fine control of their descent through atmosphere. This was because the external tanks of those designs were hinged at the base, so moving the tops of the tanks in or away from the core of the ship a bit would change the aerodynamic forces on re-entry, and thus the trajectory of the craft. I'd love to be able to build a Pegasus/Ithacus/Rhombus type craft in KSP, but so far my efforts have failed. :-}

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I would consider it quarter-staging. Dropping a tank-and-engine assembly from under another such assembly is staging; firing three assemblies and dropping two radial ones is half-staging; dropping a tank off an assembly would be quarter staging.

-Duxwing

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If you have to ask, it probably isn't an SSTO.

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If you stage while getting to orbit, then your ship is not an SSTO.

BUT

If you already reached orbit and then later staged, you are still technically a SSTO because you reached orbit in one stage.

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Eh, there isn't really a clearly defined line between "staging" and "not staging" anyway. Technically the act of burning fuel could be considered staging, because you're removing infinitely many infinitely small pieces of mass from the craft over a period of time. Alternatively the jettisoning of the empty fuel tanks could be considered a form of rocket propulsion, at least if you throw the tanks backwards instead of merely releasing them. All motion is relative anyway. Did the tanks move away from the spaceship, or did the spaceship move away from the tanks? Neither: it is your reference frame that is moving. The zen of physics. :D

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The primary point of an SSTO is to save money by reusing the craft. There are other reusability systems than SSTOs, eg SpaceX's "land the first stage and let the upper stages burn up" version, or a shuttle style "land the upper stages and the solid boosters and let a big tank burn up". In career mode, SSTOs are a way to save Spesos, so if using drop tanks is cheaper than recovering those tanks do so. In sandbox mode it's more about the aesthetics, so do whatever is prettiest/coolest/etc.

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All I take away from the picture in the OP is that pilot just dropped two fuel tanks over what appears to be a suburban area.

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If anything falls off of your ship on purpose on the way up, it's not an SSTO. Atlas wasn't an SSTO for this reason - it dumped two outboard engines on the way up, but otherwise only had only the one core stage.

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Strictly, no it isnt an SSTO. But why does it matter? What authority are you trying to impress? May very well be that its cheaper to drop some tanks than bringing then to orbit and back.

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Strictly, no it isnt an SSTO. But why does it matter? What authority are you trying to impress? May very well be that its cheaper to drop some tanks than bringing then to orbit and back.

Cheaper, or possibly desirable in other ways. E.g. dropping the tanks might be just enough to give you that little bit extra that you need later on.

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I think in spirit it is not staging, even though it uses the KSP staging controls. You would not say the plane in the picture was dropping a stage.

I take it this important to determining if something is an SSTO?

My understanding is the spirit of an SSTO, is a plane that takes off from a runway, reaches orbit and lands on the runway. I know technically, this is not what it means. A rocket which does not jettison any tanks, and returns by parachute is technically an SSTO. But my impression from players of KSP, SSTO design, is all about designing planes.

But hey, it only really matters to pedants, and we shouldn't be kow-towing to their kind. :wink:

Edited by Tourist
bad grammer. Its still bad, but at least I tried.

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Eh, there isn't really a clearly defined line between "staging" and "not staging" anyway. Technically the act of burning fuel could be considered staging, because you're removing infinitely many infinitely small pieces of mass from the craft over a period of time. Alternatively the jettisoning of the empty fuel tanks could be considered a form of rocket propulsion, at least if you throw the tanks backwards instead of merely releasing them. All motion is relative anyway. Did the tanks move away from the spaceship, or did the spaceship move away from the tanks? Neither: it is your reference frame that is moving. The zen of physics. :D

For a liquid oxygen + liquid hydrogen rocket, the smallest 'stage' would be one molecule of oxygen reacting to hydrogen. Something like: O2 + 2H2 > H2O.

The Space Shuttle's external fuel tank hold's 630,000 kg of liquid oxygen. Liquid oxygen has a molar mass of ~32. So, the number of moles of oxygen is 630,000,000 grams / 32 = 19,687,500. Multiply by Avogradro's constant, 6.022 x 1023 x 19,687,500 = 1.1855812e+31 molecules of O2.

Therefore, the Space Shuttle's external fuel tank isn't 1 stage, but 11,855,800,000,000,000,000,000,000,000,000 stages.

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I think in spirit it is not staging, even though it uses the KSP staging controls. You would not say the plane in the picture was dropping a stage.

I take it this important to determining if something is an SSTO?

My understanding is the spirit of an SSTO, is a plane that takes off from a runway, reaches orbit and lands on the runway. I know technically, this is not what it means. A rocket which does not jettison any tanks, and returns by parachute is technically an SSTO. But my impression from players of KSP, SSTO design, is all about designing planes.

But hey, it only really matters to pedants, and we shouldn't be kow-towing to their kind. :wink:

It doesn't have to be the runway. I've had a SSTO plane which took off from the launch pad and returned to the runway, because the runway is too short for heavy transports with a full load. I've done SSTO rockets as well, with a vertical parachute landing of a HUGE 3.75m lift stage, which got to circular orbit, dropped its load, and returned to land at KSC.

SSTO isn't a precise thing, it's a general concept.

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For a liquid oxygen + liquid hydrogen rocket, the smallest 'stage' would be one molecule of oxygen reacting to hydrogen. Something like: O2 + 2H2 > H2O.

The Space Shuttle's external fuel tank hold's 630,000 kg of liquid oxygen. Liquid oxygen has a molar mass of ~32. So, the number of moles of oxygen is 630,000,000 grams / 32 = 19,687,500. Multiply by Avogradro's constant, 6.022 x 1023 x 19,687,500 = 1.1855812e+31 molecules of O2.

Therefore, the Space Shuttle's external fuel tank isn't 1 stage, but 11,855,800,000,000,000,000,000,000,000,000 stages.

cryogenic engines burn hydrogen-rich to get their high ISPs. derived from the Space Shuttles external tank, there was roughly a oxygen-hydrogen ration of 1:2.5 (molar). since a ratio of 1:2 is required for the reaction, that leaves another 0.5 mol hydrogen per mol oxygen unburnt.

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If anything falls off of your ship on purpose on the way up, it's not an SSTO.

The though of unintentional falling off got me to stitches :D

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I don't know that it is "staging," but I think a ship that did this would NOT be what most people on this board mean when the say "SSTO."

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