Towards a reusable rocket system with a fast turnaround, part 4

Here's where the speculation alerts are getting closer to the ending point.  The reason is that now is the time to look at more real world attempts to get a re-usable vehicle with a fast turnaround time and could get passengers and cargo to low earth orbit.

That would be the X-33 system that was canceled in 2001.  The final product of that development would have been the VentureStar system proposed by Lockheed Martin.  It would have been the next step up from the X-33 system that was under development.  The failure point was in the composite fuel tanks.  According to the Wikipedia entry, that problem has been solved since it was canceled.

Anyway, in previous posts, there were a few details that got overlooked.  One detail was the loss of ISP at sea level.  In order to overcome that, a special rocket nozzle, called the aerospike, is necessary to get that back.  The X-33 would have had such a nozzle.

Another detail that was overlooked was the greater thrust needed at launch.  This means more engines and more mass, unfortunately.  Ultimately, this will limit how big of a machine that you can get to orbit.  Secondly, perhaps one should drop the idea of a single stage to orbit altogether.  The reason is that the VentureStar was going to be too darned big for the amount of work that it could do.   By going back to the Elon Musk model of staged re-usable rockets, a lot of that mass can be saved, and the rockets can become more modest in size.

Now, here's the proposition.  What if you could modify the existing X-33 concept to deliver a second stage with payload to a delta-v of about Mach 5 to Mach 7, and about 100, 000 feet-- then return to launch site?

The X-33 could be modified to save mass as it was designed for re-entry with an all metal fuselage.  Since this wasn't going to go orbital, this can be replaced with a lighter configuration.  If the tank problem has been solved, this will also help with mass.  Otherwise, you can use the type of tank that the Shuttle used.  The savings in mass for the fuselage can go for the tank, if necessary.

Let's look at what we've got with propulsion:

The RS-2200 Linear Aerospike Engine^[3] was derived from the XRS-2200. The RS-2200 was to power the VentureStar single-stage-to-orbit vehicle. In the latest design, seven RS-2200s producing 542,000 pounds of thrust each would boost the VentureStar into low earth orbit.

The X-33 was to use only two of these engines.  The above specification is greater than what is listed for the X-33, as can be seen.  So, what if we cut it down to just one engine?  Having two will give you the same configuration as the X-33, though.  But you would be overpowered, I would think.

With a million pounds of thrust, you could lift a half million pounds with some spare capacity.  If you were to cut that down to 1 engine only, you would have no spare capacity, and an engine loss would mean a crash.  Let's stick to two engines for this discussion.

Here's a few calcs from a spreadsheet.  Have to run, got a busy day ahead.  This is only a rough draft as I am out of time.

The spreadsheet above has an error: There's more mass fraction available to launch, about 9k kg to be exact.  This is therefore considered to be a more conservative estimate than what it appears to be

Update:

Hate to say this, but there are some more errors in this post. Back to speculation mode after all.

The ISP of the X-33 engines are not as high as I thought when I made the spreadsheet calculations above. Those are out the window now. The ISP is not as good as I thought, so can this idea still fly?

I mean, is it still worth it? Perhaps I can look at that next.

Update:

Next in Series, Part 5