So why aren’t all rockets given super acceleration like the second example? The main reason is to minimise stress on the payload. The payload (satellite or whatever) may be damaged by high acceleration.

In fact, the rocket has to throttle down (reduce thrust) once the propellant mass gets below a certain level to keep the payload from being crushed by the acceleration, and I think the F9 actually shuts down two engines near the top of the climb for this reason.

Although I think the story will be slightly different with the V1.1 F9; the Merlin 1Ds are supposed to have a greater throttle range than the 1Cs if I remember correctly.

Yes, the Saturn V could and Falcon 9 can operate after losing engines, but not always.

What it boils down to is fuel efficiency – for a given amount of fuel, the greater the thrust, the further you go. Remove an engine and you’ll get proportionately less use from the fuel.

A rocket’s acceleration is calculated by dividing thrust by mass. If the result is greater than 1, the rocket can accelerate, and the higher it is the faster it accelerates. If the number is less than 1, the rocket slows down.

If the rocket loses an engine, its thrust is reduced (by 1/5 in the case of the Saturn V 1st or 2nd stages, or by 1/9 in the case of the Flacon 9 first stage). But mass isn’t reduced. This means acceleration is reduced. That is, lose an engine and the rocket accelerates more slowly.

(Another factor to consider is that as a rocket uses up fuel, it gets lighter. This means that even with the thrust remaining the same, the rocket’s acceleration increases while the engines are burning that fuel. So the number we’ve calculated above is constantly changing.)

In the case of the Saturn V, at launch its thrust was about 7,500,000 pounds, while the mass was about 6,200,000 pounds. Divide the first by the second, and the result is about 1.2. This is not much above 1, and means that the Saturn V initially accelerated very slowly. This is why it took so long to clear the launch tower. Now consider what would happen if we removed one of the five engines on the Saturn V: thrust would reduce by 1/5 to 6,000,000 pounds. Divide this by the mass of 6,200,000 pounds and the result is less than 1. In other words, the Saturn V could not lift off if it only had four engines. In fact, the Saturn V didn’t get its engine-out ability until about 20 seconds into the flight – by which time it had burned enough fuel to lighten the load.

To take your question more generally, if the rocket can operate without one engine, why not take it out, let’s go back to the calculation of dividing thrust by mass. If that result is just over 1, the rocket accelerates very slowly, while if the number is higher, the acceleration is higher.

Consider an imaginary rocket with a thrust to mass ratio barely above 1. That rocket is going to accelerate very slowly, and take hours to reach orbit. Burning the engines for hours is going to take huge amounts of fuel.

Now consider another imaginary rocket with a thrust to mass ratio of, say, 10. That rocket is going to shoot up into the sky, and reach orbit in under 10 minutes. That’s going to take a lot less fuel than the previous example.

Go to http://phet.colorado.edu/sims/lunar-lander/lunar-lander_en.html to try out its lunar landing simulator. Once you’re familiar with it, try this experiment. Burn the lander’s fuel until you have only 50 kilograms left, with the lander sitting on the Moon. Then set thrust to maximum and see how high you reach (I made it to 110 metres). Reset and again burn the fuel down to 50 kilograms, with the lander sitting on the Moon. Now set the thrust to the lowest possible that actually lets the lander lift off, and see how high you reach this time (I made it to 3 metres). What this experiment shows is that the higher the thrust for a fixed amount of fuel, the greater the distance you travel.

So why aren’t all rockets given super acceleration like the second example? The main reason is to minimise stress on the payload. The payload (satellite or whatever) may be damaged by high acceleration. Lower acceleration places less stress on the payload. As launch into orbit is a one-off stress in the satellite’s life, it makes more sense to reduce the launch stress than to over-engineer the satellite for an event which represents so little of its working life.

So the design of rockets and payloads for each other is a balancing act: too little thrust and you need more fuel; too much thrust and you might damage the payload.

I think it was also reflected in the fact that it was a test satellite, not the fully-equipped model they will send up later.

Unfortunately, the engine malfunction placed the Falcon 9 upper stage in a slightly different approach to the International Space Station. That new approach caused the stage to violate a set of conditions known as a “safety gate”; there was no way the satellite could ascend to its intended 350km x 750km orbit without crossing the ISS orbit, and no time to check to make sure that no collision would occur. The second stage’s flight control software automatically cancelled its second burn, leaving the satellite in a much lower (203km x 323km) orbit than intended. Engineers from Orbcomm and Sierra Nevada Corporation, the manufacturer, are deciding what can be done, and both SpaceX and Orbcomm are being noticeably cagey about their press releases.

As a secondary payload, Orbcomm was exposed to more risk; hopefully that was reflected in the contract and the pricing.

I’m sure one of the virtues of having nine stage 1 engines is that losing one isn’t a huge deal. Didn’t Apollo 13 lose its stage 1 center engine in flight? As I recall, the word from mission control was “meh, we’ll just burn the others for a bit longer.” I imagine this is also a failure that NASA insisted SpaceX to be able to cope with before they’d release cargo to fly with them.

On a more PR side, I’m also not really surprised that this wasn’t really announced over the net that we heard. If it’s no big deal, then there’s no reason to use scary phrases like “engine out/cutoff/shutdown” or “We’ve lost pressure in the engine” or “abnormal condition” or whatever phrase they use on the net that the entire world is watching. That was probably announced to the flight conductor on a closed net. Put yourself in a customer’s shoes. You’re not a rocket scientist or an afficionado of all things space, you’re a businessman. Are you really going to contract your multi-million dollar satellite (or whatever) launch to a company that LOST AN ENGINE IN FLIGHT? OMG, OUR SATELLITE IS GONNA CRASH AND BURN AND WE’RE GONNA BE OUT MILLIONS!

“Yeah, let’s just keep that stuff to ourselves for now.”

It was NASA who said don’t do the re-light, not SpaceX

My interpretation of events is that the second stage had to make up for some delta-V lost by the first stage after the engine shutdown, which is why it didn’t have enough propellant to meet NASA’s safety margins.

If anyone has a link to a statement claiming otherwise, I’d be glad to see it.

@Chris:

I take it you’re speaking from experience. Care to share with us other code monkeys exactly what is involved with writing rocket guidance and control software? I’ll admit I haven’t done any real embedded or avionics work, and very little realtime work, so I’m sure there are issues I’m not aware of. But I’m also pretty sure that, compared to most desktop systems, the code is relatively straightforward, at least in terms of overall design. The hard bit is meeting the performance and fault-tolerance requirements, and just slogging through the process.

Bater

Messier Tidy Upper – LOL, I remember Apollo 13′s launch pretty well. I saw it in person, as I did every Saturn V that ever flew. It was nice living in Cocoa Beach, FL as a kid and having family members who worked at the Space Center….

Ah some people have all the luck, you lucky ..! (Envious sigh.)

Oh well. Do you happen to recall when /if you were told by NASA / Apollo launch commentators of engine failures and other problems at the time for comparison porpoises?

And what’s with this disdain towards programmers? After all, it was IBM that designed the Saturn V guidance computer. And what do you call the people who wrote the Falcon 9 guidance code? Not programmers?

To handle an engine-out situation, you’d first gimble the engines to compensate for the unbalanced thrust vector. Then, it’s just a matter of computing a new trajectory to orbit — one with a different ascent profile, longer burn time and higher overall fuel consumption.