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Dumb Questions?

December 28, 2012


Watching many new launch systems coming on-line seems to suggest that vertical landing is the most accepted path to reusable space launch systems.  There must be some math behind these assumptions being made by such well-funded and educated ventures.  One must assume that an empty booster requires a fraction of the fuel required for the fully loaded launch phase of its flight.  That must be a much lower mass penalty than any form of wing or lifting body shape to these designers.  So it seems to be an attractive means of booster and orbiter recovery.

Watching the testing being done with these vertical landers is not without mishap.  Spacex has flown a large booster without an accident now.  Other ventures have had some “interesting data points” in their testing.  With a single engine it seems that each of these ventures has demonstrated some success though.  As testing progresses, I expect the data will begin to encourage the builders to press on with confidence.

As launch systems approach the real market, larger boosters are needed to reach LEO with any useful payload.  Perhaps they need not meet the needs of the Space Launch System, but they could expect to match the Atlas 5 or Falcon Heavy design.  The Falcon family may be the first to test the vertical landing idea on a full-sized booster.

When such a vehicle is launched the systems are operating in some extreme performance realms.  We have seen occasional engine outages with extra capacity available from the remaining engines to complete the mission.  The Falcon 9 was mostly successful in its primary mission in such a performance drop.  The Falcon heavy may have more engine redundancy to cover such needs in the future.  It will feature twenty-seven engines instead of just nine if I understand correctly.

Watching a modern rocket launch vertically is a wonder of stabilization and control.  They rise slowly, dancing on the thrust of gimballed engines, responding to the slightest directional deviations.  This is like lifting a broom balanced on the palm of your hand.  It requires a computerized system of balance and mechanics for direction and acceleration to reach orbit.  If you operate with multiple engines, it must also react to variations in the thrust of each engine.  This is all amazing to me now.

When a heavy booster has delivered its mission, it may be turned around for a powered vertical landing.  I am assuming the engines must be re-started at some point, not just throttled back.  This will introduce a second ignition sequence and more systems to the operation.  The entire vehicle will be slowed to reenter without burning up in the atmosphere.  It will be guided to a controlled landing at a predetermined landing site.

Now we are orchestrating a symphony of many engines in an encore performance, a second display of electro-mechanical athleticism.  It will be impressive to see such a landing, right out of science fiction.  And it can and probably will be done.  I expect to see this done, and possibly in the near future.  It will probably become part of our regular space operations, as did expendable rockets and the space shuttle system.  I hope it proves to be more economical because we need successful ventures for affordable space access.

I would like to compare the math of these proposals to winged access methods.  There may be a case for extra fuel for reentry as opposed to the mass penalty of wings.  There may be other mathematical issues to factor in though.  Statistics is a field of math that adds up to an estimate of potential failure modes.

When one starts with nine or twenty-seven engines there are individual moving parts in each engine.  There are multiple systems for fuel and computer controls in these compound systems.  When adding a second ignition sequence there are more systems for that operation, and additional operation time on each engine.  If reentry is also controlled as a vertical power dance, there is more demand on the guidance computers and servos.  Statistics may begin to gather the number of systems operating and then suggest the odds of a small glitch.  One may identify how many ways a rocket might cascade into a large glitch from one or more small ones.

We are seeing more success with vertical hops, but we still see problems with the launch phase of rocket powered flight.  Simple rockets with one or two engines may still have issues.  Statistical math may further identify the potential cost of such issues if multiplied over both launch and the landing phase of a rocket flight.  An alternative reentry system may be needed as a back up to a powered vertical landing.  A capsule ballistic reentry can slow enough to allow a parachute deployment.  Perhaps this is good to slow a capsule partially for a powered vertical landing.  A parachute could be the backup system then.  A booster is not an ideal reentry vehicle for this shielded ballistic reentry.

A booster that is slowed but crash lands may be a threat to earth if it is not burned up on reentry.  A manned vehicle that is unable to make a soft landing has a bigger impact on the space program.  Any human loss shuts programs down for years.  If it seems hard to get past liability issues now, see what an accident will do to new space ventures and FAA regulations.

Winged access and reentry is not without failure modes either.  While the wing is not a moving part, the control surfaces are.  And computer or pilot systems are still a potential source of error.  If winged access depends on new technology for propulsion or thermal protection the cost and risk goes up.  But one math works in its favor.  The passage of time is bringing the vertical landing proposals into the spotlight first.  If the suggested statistical issues add up they may provide an open door to alternative methods.

The fabric covered biplanes landed the first mail contracts, but not the future of aviation.  Commercial aviation was waiting for the metal monoplane to deliver the accepted solution.  Perhaps the march of time will meet the laws of physics and statistics to shape a predictable future.  It is possible that different technologies will find niche markets as fixed wing and rotary wing aircraft do.  Even the biplane is revered and preserved in some capacities today.

I would like to hear from anyone who does these kinds of trade or statistical studies.  It may be interesting to project the possibilities with a little math behind it.



  1. christopher wilkinson permalink

    IIRC, Clark Lindsey posted a link on his RLV site for a U.S. Air Force analytic comparative study of reuseable launch vehicles within the past 10 years. It compared VTHL, HTHL and VTVL and concluded that VTHL (Vertical Takeoff Horizontal Landing)provided the best weight combination for lifting itself and the fuel mass off the ground and returning, based on then-current technology. This was after DC-X but before Grasshopper. I recall that horizontal takeoff required large wings with commensureate weight penalites as a result. Teh conclusion was that vertical takeoff was the way to go, and SpaceX (Grasshopper) and Blue Origin seem to agree.

  2. We will see studies change as new solutions like the HILLS system are analyzed. These studies of the past indicated advantages for horizontal landing which leaves doubt about vertical landing. The statistics of complexity are a math these studies have not addressed. If they prove to be an issue when a vertical landing mishap occours, Space Dev will be in a good position. They will still not offer a path to reusable boosters. Blended wing bodies with in line staging have less mass penalty and retain full reusability. Previous horizontal takeoff studies missed this because they do not get lift from both stages wings, and require a larger booster/carrier aircraft. Is mass penalty more important than the cost of the whole booster loss and the safety issues?

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