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Please edit, if 'engines' is not the correct term (layman here).

As I understand it, you have your SRB's, then they detached, and the External Tank supplied the liquid oxygen and hydrogen fuel to the main engines of the Orbiter. Then, after main engine cutoff, it is detached.

Why didn't the External Tank just have its own engines/boosters? The Orbiter maintained a bit of fuel for orbital maneuvering, but otherwise didn't need to be responsible for the thrust provided by the ET.

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Come to think of it, I suspect it was because it was not reusable, but then, we have precedent for non-reusable rockets with their own engine. – Mikey Jan 29 at 20:32

As a design decision, if you throw away the External Tank and it has engines attached, you are throwing away the engines. Since the Orbiter was returning for sure anyway, the decision was to leave the engines on the orbiter. Once the ET is done its job, the engines are not needed to make orbit. The OMS pods provide enough punch for the needed orbital manuevers.

The Russians on the other hand, with Energia, which was the booster for the Buran space shuttle, had liquid fueled side boosters, and the engines were on the main body of the vehicle. (No longer a tank, it is now the core booster).

They were ok with throwing away all the engines on each flight, contemplating wings on the side booster to try and recover them.

The SSME's were however so expensive that throwing them away seemed to defeat the point of a reusable space ship.

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They weren't okay with throwing them away. They just scratched the whole Buran program after considering the costs and proving "they can do it too." :) – SF. Jan 29 at 23:18
    
@SF I think that's unfair to the Buran programme. The Soviet Union had just been dissolved and Russia was in major political and economic upheaval at the time. If the country as a whole had been more stable, I think the programme would have lasted a lot longer. The final nail in the coffin was the destruction of the hangar where the Buran orbiter was stored in a storm (admittedly after having not flown for over a decade). en.wikipedia.org/wiki/Buran_(spacecraft) – Level River St Jan 30 at 0:05
    
@steveverrill: Buran suffered from the exact same flaw as the Shuttle: a very poor dead weight to payload ratio. Russians had construction of non-reusable spacecraft to the dot, Soyuz is still the safest manned design out there and you can launch five of them for the cost of one Shuttle launch. The romantic in me loves the Buran, but I need to admit to myself their single-use spacecraft is, and was at that time vastly superior. They didn't need Buran for any other purpose than for show. – SF. Jan 30 at 2:45

Reusability.

The whole idea of the shuttle was to discard all the parts that are simple, cheap and easy to replace and recover everything expensive, complex and hard to replace.

Of course the reality, involving meddling by parties other than NASA, never mind failures in the process the shuttle was designed (not so much the design itself as the process of designing it; as detailed in the Feymann Report) made the effect a mockery of the intention, with costs of refurbishing, replacing and repairing returned reusable parts overshadow what building single-use ones would be, but the initial concept was sound: throw away the simple tank, keep the complex, expensive engine with the orbiter. SSME was (despite some construction/safety flaws) one of the most efficient engines in history of rocketry, but that was at expense of complexity - it was a true miracle of technology. It was designed to be reusable and far too expensive to be disposable. And the Shuttles were the first big step towards reusability.

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Which makes one wonder how feasible the SLS is, since it throws away four SSMEs on every launch. Can we make those for much cheaper now than used to be the case? Seems plausible, but the SLS sure seems likely to have learned the wrong lessons, if it learned anything at all, from the Shuttle. – CBHacking Jan 30 at 2:09
    
@CBHacking: Considering the average lifetime of SSME fell nearly an order of magnitude below the projected value and rarely lasted more than 5 missions without major repairs, they really had to get the part of manufacturing them streamlined if they had to build roughly ten times as many of them as they had intended after all ;) I'm not holding my breath for the economical feasibility of the SLS anyway. – SF. Jan 30 at 2:35
    
Ah, I hadn't realized their reusability was that bad. Was that true right up to the late-model engines? I thought they'd improved the lifetime by a bunch towards the end, but I may be mistaken. In any case, yeah, SLS doesn't sound economical, for many reasons including the expending of 4 SSMEs per launch, but I don't know how bad that aspect of it is. – CBHacking Jan 30 at 2:39
    
@CBHacking: I don't know about the later improvements, I've just read the Feymann Report, so I cited the SSME state shortly after Challenger disaster. Still, the Columbia disaster was sourced to exactly the same management and safety procedures problems as Challenger, and exactly the same that the Rogers Commission had ordered to fix - and weren't fixed. Sorry I know very little about SLS, – SF. Jan 30 at 3:00

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