This should be the last of the series, as I have probably gone far enough with this idea.
Incidentally, the idea does not seem so bad. The basic idea was that all attempts at Single Stage to Orbit are too ambitious. In the end, there'll be too much mass being carried to orbit, so the procedure of staging is actually pretty sound.
The problem to overcome is to make the two stages re-usable. The use of higher ISP engines should give enough margin to do that. For example, the Shuttle External Tank achieved near orbital velocity from the ground until main engine cutoff. The Shuttle itself weighed in at over 250k pounds loaded. The external tank fully loaded with fuel along with the shuttle put the launch weight ( not including external boosters) at nearly 2 million pounds. The Shuttle, therefore, was too heavy.
On the other hand, the Shuttle's would-be replacement, the VentureStar, was also a pretty heavy bird. It appears to have suffered the same malady as the Shuttle, too much ambition. The VentureStar was to be a single stage to orbit vehicle. But it could only deliver a little more mass than a Dragon Capsule even though its launch weight was as much as the Shuttle and fueled External Tank together on the launch pad.
In trying to get to some solution to this problem with equipment already produced or had been produced in the recent past, this is probably something that's totally impractical. Yet a configuration with 2 shuttle main engines in the first stage and 1 main engine in the second stage yields some interesting numbers.
For instance, the mass to thrust ratio is about the same with both stages and with the shuttle at liftoff. There's plenty enough thrust to get off the ground, but acceleration may be so ferocious that it may need to be throttled down the entire time it is firing. I don't know if the engine is capable of this. It can be throttled down, that much I do know.
The mass is about the same for both stages. This is a bit odd and it may be a problem. I can't answer that question myself.
There's plenty of wiggle room on the second stage. I showed two possibilities-- a dragon capsule and a dream chaser. In either case, the second stage has enough leeway to add stuff to make it back from orbit for re-usability. The Dragon and Dreamchaser are already re-usable. If, by any chance, this actually could work, you'd have plenty of extra mass available for some add-ons. For instance--- extra fuel, wings, landing gear, and so forth.
Well, here it is
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| Errata: Final velocity: 17644 mph. Lifting mass is more than 100k in the rectangular box. It actually came out to 48k kg in the bottom box. Note how much mass available at the bottom. It can go back to the first stage or stay on the second stage. The layout of this spreadsheet leaves something to be desired. |
Update:
This is getting pretty obsessive. I just did some more work on this thing. Surprise, surprise, surprise. Using the same performance obtained from the Shuttle flights and mapping it onto the two engines in the first stage and the single engine in the second stage, the numbers work out beautifully. So, a little over 8 minutes and you're in orbit.
In addition, I fiddled around with the propellant numbers and so forth. It is now staged out at about mach 7 or so. Two and a half minutes after launch, the first stage shuts off, and the second stage takes over for almost 6 minutes.
To top it all off, I started calculating volumes and so forth in order to get the dimensions of the stack. Maybe a 150 foot stack would do the trick.
Don't bet the farm on all this, though.
Update: 7/29/2012
Sorry, I know I said that I was done with this, but wait just one more little update. The shuttle's main engines and external tank was all you needed to get to orbit with a small payload. I worked out the numbers, so it's true. As mentioned, the main engines fired from launch until 98% orbital velocity anyway. The reason you needed solid rocket boosters was to lift the shuttle itself into orbit as well. Take about that 250,000 lbs and replace it with a payload comparable to a Dragon capsule, and you'll get to orbit with room to spare.
To repeat, in order to have a chance at re-usability, you must break up the launch vehicle into stages. The mostly empty fuel tanks and rocket motors are too much mass to lift together. By splitting it up, you have enough wiggle room to make the rockets re-usable.
For instance, consider the JDAM bombs, which are launched from planes flying at 30,000 feet or more. They can glide to their targets for 20 miles or so. If you make a glider out the the first stage, it can come back that way. The amount of fuel needed to decelerate- and re-accelerate towards the launch site- isn't as much as you would think. That's because it isn't carrying all that much weight anymore since the payload and second stage is off towards orbit.
The second stage may be a challenge. But consider that the shuttle came back as a glider, so if you can arrange that with the second stage, it could be done.
Now for the main point of all this. It seems that the Shuttle program, with its stated goal of re-usability was a very limited success. However, it failed if the mission was to make low-cost and frequent access to space possible. It failed because it was too ambitious. If the goal was to get people to orbit, or a small payload, then it would have been successful. But it was asked to get a payload, plus astronauts, plus all of the other re-usability and low-cost access type of goals, and all that was just too much to ask.
This is what the system produced. The system was to blame. It failed and it failed the nation. That's the whole point.
Update: ( Jan 7, 2016 Several years later ):
Musk has landed his first stage rocket on a pad near where it launched. As of this writing, it probably has undergone a lot of testing and looks good. By and large, Musk has achieved his goal.
So, who am I to criticize? The next thing he wants to do is to send 100 people at a time to Mars.
Time for more speculation about how he will do that.
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