The J-2 engines (using cryogenic H2/O2) were a huge advance over anything that had been done before using H2 as a fuel; even so, they only generated 110,000 lbs of thrust vs 1.5 million for the F-1. The SSMEs were, in turn, a huge advance over the J-2s, and even they only produced 400K lbs or so. If Apollo had needed H2/O2 engines for the first stage, it likely never would have made it to the moon.

If I understand matters correctly, it's also true that the specific thrust advantage of H2/O2 engines mattered less at lower speeds and denser atmospheres.

The shuttle SRB's provided most of the thrust at launch. But the RS-25 liquid engines were used because they could be throttled. The RS-25 liquid engines were started about 3 seconds prior to igniting the solid boosters. This allowed the liquid engine turbopumps to spool up and produce thrust. If you watch a video of the RS-25 engines starting up, you'll see an initial spray of liquid propellants just before the engines ignite.

And the engines needed to be throttled down soon after launch to avoid exceeding max Q or max structural dynamic pressure loads on the STS.

To do this, they took the shuttle main engines down to 67% thrust in the lower atmosphere and then back up to 104% when the atmospheric density reduced sufficiently.

The solid boosters were more sophisticated than I imagined. They each had an APU and vectored thrust nozzle.

Have a look at this superb footage of a launch. The Shuttle main engines start first - to make sure they are all healthy - before the boosters are started. You can see (and hear) the whole STS angle over with the force, and then the boosters are lit.

Awesome !!

https://m.youtube.com/watch?v=Lq_shHu4lAs

If you're referring to around T-7, that's the sound suppression system, which is a water blanket designed to protect the shuttle stack from damage due to noise reflecting from the concrete.

The SRBs also 'throttled down' at that point by a change in the shape of the propellant at that stage of the burn.

Pedantically, the boosters were lit when the shuttle returned to the vertical after the SSME ignition pushed the stack forward and it swung back.

Also check out this film, also available as an iPad app (NASA Ascent), 45 minutes of gorgeous slow motion launch photography from the numerous cameras on and around the pad narrated by two guys who ran the operation.

https://www.youtube.com/watch?v=vFwqZ4qAUkE

One great geeky detail of the SSME start process was that the engine bell vibrated quite markedly as the engine lights up. The flight position of the two lower engines (no. 2 and 3) placed the bells very close together and there was a chance they could collide during the start so they were held apart during light up and then gimballed back into position once they are up and running. Easily visible on close ups of engines during launch.

Another vote for Riding Rockets by Mike Mullane, great behind scenes look at the operation, warts and all. One thing that is very clear from the book is that the astronauts themselves were under no illusions about how dangerous and marginal the vehicle was and the risks they were taking with every launch.

Disconnect?
 

I think GordonR missed his boat or flight. Space Cadet?
The original question said NOTHING about the Saturn V. Not a single word. Focus, please...:p

:)

Invoice for keyboard on its way.

Somewhere I found that the last check before the SRB's are lite off all three of the main engines test their gimbaling and only after being successful on all three engines do they fire the SRB's or do an abort if any one fails.

The whole American space program was mind blowing. I have visited KSC twice, and I spent ages just looking at the Saturn V and all the engines in awe.

Another awesome fact is that the Shuttle's rate of climb is 2000 feet per second. I will write that again, 2000 feet per second.

The Airbus I fly can climb at 2000 - 2300 per minute with normal loading........

The more rapid liftoff for the shuttle was due to higher initial thrust-to-weight ratio, but this changed vs the Saturn V during the ascent.

From a thrust standpoint, the Saturn V sea-level liftoff thrust on Apollo 15 was 7.823 million lbf (34.8 MN), which increased with altitude to a peak of 9.18 million lbf (40.8 MN) at T+135 seconds. The engines were not throttled; this was due to increasing nozzle efficiency as ambient pressure dropped:

Image:SaturnVThrust2.jpg - from the Schools Wikipedia

The shuttle liftoff thrust was about 6.78 million lbf (30.16 MN). The SRBs underwent significant throttling over their approx. 120 sec firing time, as can be seen in this graph. This throttling was not dynamic but designed in by controlling propellant surface area:https://en.wikipedia.org/wiki/Space_...Srbthrust2.svg

Like the Saturn V, the shuttle SRB and SSME engines improved with altitude, but they both throttled back around Max Q, roughly T+60 sec. Despite improving nozzle efficiency with altitude, the SRB throttle schedule probably meant the peak vehicle thrust was at liftoff not at altitude like the Saturn V. So comparing peak thrust to peak thrust, it was about 6.78 million lbf vs about 9.18 million lbf.

Despite the slower start, at higher altitudes the Saturn V accelerated more rapidly, reaching a peak of 3.8 g just before 1st stage cutoff.

Of course the ultimate goal is delivering a payload. On Apollo 15 the Saturn V delivered a payload of 140,930 kg (310,697 lbs) to low earth orbit. The heaviest shuttle payload was about 23,586 kg (52,000 lbs).

It would have taken six shuttle launches to deliver to LEO the same useful payload as a single Saturn V.

Getting the desired time/thrust curve is quite the science for solid rockets. Unlike the simple 'end burning' black powder rockets, most AP based solid rocket motors are 'core burning' and are ignited at the front. The shape of that core cavity determines the time/thrust relationship. IIRC, the Shuttle boosters had a 'star' shaped core to get the desired characteristics.

The shuttle was a tremendous technological achievement, but it failed miserably at it's primary goal of reducing the cost of inserting payload into orbit by at least an order of magnitude. By most measures, even accounting for inflation, the cost of placing a pound of payload into orbit was not meaningfully different between the Space Shuttle and Saturn V. About the only meaningful capability we gained with the shuttle was the ability to bring large objects back from orbit.

I find it both amusing and pathetic that NASA is now spending billions of dollars to recreate the capability we already had 45 years ago with the Saturn V :ugh:

Wow.

So does that mean that the Shuttle and its fuel tank could have used the Saturn V first stage instead of the boosters?

I presume the boosters were much easier and vastly cheaper to manufacture?

I didn't know the booster thrust varied during the launch - very clever.

Indeed. The amount of expertise around here is truly overwhelming at times.

The Saturn V S-IC first stage was considered as a shuttle booster:

https://joema.smugmug.com/Aerospace/...er/i-TmrdjpS/A

However the initial goal was a fully-reusable two-stage vehicle. Unfortunately the optimum staging velocity for lowest total mass is around Mach 12, which means a reusable first stage would essentially be a hypersonic transport in the 3-4 million pound gross weight class. This would have entailed extremely high development cost and risk. Former shuttle program manager Robert F. Thompson said even had they been given the money, he didn't think it would have been possible.

This iteratively drove the design process to an expendable booster (the above Saturn S-IC was one of many concepts), which then led to an orbiter with external propellant, which then led to an expendable tank with semi-reusable solid rocket boosters.

See "The Space Shuttle Decision": The Space Shuttle Decision: Chapter 6

It was understood early on the shuttle would likely not be economical to operate. This was covered in the 1972 GAO report to Congress: http://archive.gao.gov/f0302/096542.pdf

There was also a RAND study in that period which had similar statements about projected per-flight cost, also reviewed in the Jenkins book, "Space Shuttle: History of the National Space Transportation System", p. 173. It was known in the early 1970s even at 60 (!) flights per year, the shuttle payload cost per pound would only be about 38% cheaper than expendable boosters. If that flight rate could not be achieved it would be (and was) more expensive.

The vehicle was never designed to reach that flight rate, e.g, the maximum production rate of external tanks using three shifts at the Michoud facility was only 24 tanks per year.

The shuttle was an amazing vehicle but in hindsight there was no possible design in the early 1970s that would have permitted achieving the conflicting performance, safety, cost and flight rate goals.

I get the sense everybody knew this was a bad idea, but did it anyway. Gotta love the government.

Not so much a bad idea as ahead of its time - the technology just wasn't ready yet. I was only a teenager when NASA abandoned the fully reusable 'two stage' shuttle concept - but even then I knew that basically meant throwing out the baby with the bathwater.

NASA has long been accused of being more interested in "sexy" than in functionality, and the shuttle was "sexy". There have long been advocates of "Big Dumb Rockets" - simple, (relatively) cheap, not man rated so reliability was not as critical. One proposal was for a Saturn 1B sized rocket based around a single F-1 engine, million plus pound launch weight that could put the same payload as the shuttle into orbit but for much lower cost (there was even talk of recovery and reuse of the F-1 engines but I don't know how viable it was). But that NASA wasn't interested because such a rocket was boring and the shuttle was sexy :rolleyes: Although to be fair, boring doesn't get as much funding as sexy :E

My personal vision has long been for a two stage shuttle - horizontal takeoff, the first stage being air breathing using hydrocarbon liquid fuel and some combination of turbojets/ramjets/scramjets, and a pure rocket H2/O2 orbiter. Of course it would cost a large fortune to develop, but per launch costs would basically be fuel and maintenance. Sadly it won't happen in my lifetime. Neither the money or the technology are there to make something like that viable.

It still seriously ticks me off that all that engineering that went into the Saturn V was lost - I still consider the Saturn V/Apollo (along with the associated moon landings) to be the greatest technological achievement of the 20th century and we literally threw that away.

One way of thinking of the acceleration the crew of the orbiter experience is to imagine doing 0-60 in just under two seconds. But continuously for 8.5 minutes...

The fundamental problem with the space shuttle program was the same basic problem that NASA had post-Apollo 11; there simply was no appetite on the part of either the public nor the political leadership for any potential follow-on missions that would justify the kind of funding required for successful completion of those programs.

As the document referenced in post #57 made clear, the design of the shuttle system - a re-usable vehicle with a pretty small payload, launched with the aid of two massive solid-rocket boosters and a disposable tank - was chosen because it was the cheapest to develop at a time when NASA budgets were being pared back significantly in real terms. They knew at the time it wouldn't be the cheapest to operate, but they weren't given the money to develop a better or more sustainable program.

To give one example of the choices forced on NASA after Apollo 11, the last three moon missions were cancelled, even though the hardware was built, and even though the cost of launching those missions was a tiny fraction of what had been spent on the space program, which had been driven entirely by the goal of getting to the moon. Yet NASA cancelled almost 1/3 of the moon missions - although hundreds of billions in current dollars had been spent to develop the capability to launch those missions - simply to free up some cash for shuttle development, because they weren't allocated the money to do both.

The ultimate irony of the shuttle program was that it was most designed to build, service and re-supply a space station - which was, due to budgetary constraints, abandoned pretty early on in shuttle development. It became a machine with the sole mission of flying people into space to do - what?

No doubt NASA made some bad decisions post-Apollo. But the fundamental decision that crippled the space program was the decision by the political leadership and the American people not to have an Apollo-scale space program at all.

History was rather more complex than usually made out. See, for example http://history.nasa.gov/SP-4221/ch9.htm