Wednesday, September 28, 2011

Elon Musk and Reusable Rockets

Musk says making a reusable rocket is "super damn hard".  But that is where the fun begins, to borrow a phrase.

progress on reusability "sucks", say Musk

Well, the fun begins, but only in thinking about it. Musk has built rockets, I can only speculate about them.  I don't want to seem too presumptuous, so let's keep it real.  There's a rival of sorts of Musk's in the UK who is building an SSTO, the Skylon,, which I'd like to speculate upon for a little bit.  A Skylon would be an Single Stage to Orbit, fully reusable, fast turnaround type of spacecraft.   Just what we want in terms of re-usability.

First, let me mention that I've looked at the Saturn IV-B from the Apollo Era to get an idea about relative masses of hydrogen and oxygen.  Let's say the ratios can be altered somehow.  If so, you can save a lot of mass if you use a lot of hydrogen and a lot less oxygen.  Instead of carrying the oxygen, pull the oxygen out of the atmosphere.  The S IV-B had a ratio of 4 kg oxygen for every kg of hydrogen.

Now, if that ratio can be reduced to something less than the Saturn, it could lead to significant mass savings.  The Skylon concept is an attempt at doing this.  I am using the Saturn rocket as an example because of sixties rocket guy said that this rocket had nearly enough thrust to get to orbit on its own.  This may not be realistic though, and the Skylon's mass is closer to the mass of the Falcon 9.

But the Skylon is horizontally launched.  My thought is, what if you can make it launch mostly vertical?  Or go vertical really soon after getting airborne? 

The idea is to take atmospheric oxygen with as little drag penalty as possible.  Use as little oxygen as you can while in the densest part of the atmosphere.  Gain enough altitude and defer forward thrust until higher in altitude.  Shift to forward thrust while still having enough oxygen to propel the spacecraft forward.  This is to avoid the use of on-board oxygen for as long as possible.  Gain as much velocity as is permissible and then convert to a conventional rocket mode only when absolutely unavoidable. 

The SR71 Blackbird could fly up to 100,000 feet, and was an air-breather.  Let's say you get to 100k, then go horizontal.  The Blackbird could go Mach 3, but we need a lot more than that.
The speculation of mine isn't likely to produce a flight trajectory chart like the one above of the Skylon, though.  So, perhaps going vertical soon after takeoff isn't feasible.  On the other hand, Elon Musk was asked about using atmospheric oxygen, and he believes that it isn't worth it because of the extra drag.  Hence the early vertical ascent.  Is it feasible to go vertical early, or is it better to wait?

The Skylon is said to be able to carry significant amount of cargo.  What if further mass saving could gained by reducing mass further?  Let's say a bare minimum for a two man crew.

Let's say you can save your use of oxygen and reduce the mass for cargo to a bare minimum.  The above discussion is only into the ascent part.  The descent can be discussed at another time, as I am out of time.


Additional thoughts about going vertical early in the trajectory:
  • throttle down on ascent so as to conserve fuel, reduce drag
  • use the nitrogen taken from the atmosphere for active cooling  
  • reverse direction of engines' thrust, use it for braking on reentry 

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