From: spgreg@mars.lerc.nasa.gov (Greg Macrae)
Subject: My final word on Ion to Pluto (long)
News-Software: VAX/VMS VNEWS 1.41    
Organization: NASA Lewis Research Center
Distribution: usa
Date: 29 Sep 1992 19:49 EDT  

In article <1992Sep24.045007.10724@news.Hawaii.Edu>, 
tholen@galileo.ifa.hawaii.edu (Dave Tholen) writes...
> Greg MacRae (me) writes:
DT>>> Lots of interesting information, but one very important, extremely crucial
DT>>> item was missing:  cost, including launch.  From the content of your
> 
me>> Balderdash!  The information I posted was for comparison of trip times and
me>> payload fraction purposes.  There have been innumerable claims that ion 
me>> propulsion is slow.  Beyond the Mars orbit, that is incorrect; the numbers 
me>> prove it.
> 
>(a) You're evading the issue.  (b) You're putting words into my mouth.
> 
>(a) The issue is cost.  Please tell me how much it would cost to develop
>and flight-qualify an ion propulsion system appropriate for putting, for
>example, 100 kg on a 7 year trajectory to Pluto.  Remember, these costs

Your issue is cost.  Yet you continue to avoid presentation of any cost 
analysis.  I admit that I fully understand why no one in his right mind would 
attempt to present a cost analysis as justification for any project.  The few 
analyses I have worked on have taught me how to find holes big enough to drive 
a $10,000 toilet seat through in any 'a priori' and many 'a posteriori' cost 
analyses.  I will not do a cost analysis for you, it is a waste of my time.  

>are going to be included in the mission cost.  Yes, we can argue that they
>shouldn't be, but that's life.  I wholeheartedly agree with the proposition
>that technology development be decoupled from any one specific mission, but
>unfortunately you and I don't make these decisions.  For costing purposes,

Your ignorance is hanging our here.  Huges Research Company developed and 
flight qualified an ion thruster system, including power processors, for use
in near earth orbit.  To my knowledge, there is still no user for the system.
The point is not that this system is appropriate for Pluto.  The point is 
that development money is more often decoupled from specific missions than 
you realize, especially when a research center like Lewis is pushing the 
project rather than a development center like Marshall or Johnson.  

>(b) I never said ion propulsion was slow.  

Correct, you didn't.  I did not accuse you of that particular faux pas.  I don't
recall who or how many did, and I don't really care.  The claim 
occurs rather frequently.  Analysis has shown that it is incorrect.  I 
posted on this subject in the hope that I can erase some misconceptions.  I 
am more opposed to misinformation than I am to you personally :).
> 
>Theoretically, ion is great for fast Pluto missions.  I don't recall anybody
>claiming that it was not appropriate.  Practically speaking, however, the
>technology demonstration has yet to be done, and that is a very valid
>objection.

More phantom claims that the technology has not been demonstrated...  We have 
demonstrated >10,000 hr life, characterized operation on different fuels, 
demonstrated 88:1 throttling, built thrusters up to 50 cm in diameter, 
designed for zero performance variation over the thruster life, etc.  
Is there something that must yet be demonstrated that is generic to all (or 
even many) missions?  This is a genuine question that we have here.  We want 
to know if we are overlooking something big that we shouldn't overlook.  

me>> There are valid objections to ion propulsion.
me>> I like to think that I am aware of most if not all of them.  I have seen 
me>> none posted here so far.
> 
>So why don't you, if you know what they are?  Let the reader determine if
>any of the objections are applicable to a fast Pluto mission.
> 
me>> If you have some that I am unaware of, let's hear 
me>> them...
> 
>I'm not in the mood to play
>50 questions.  When the Outer Planets Science Working Group had its
>presentation from the ion drive folks, the cards were laid out on the table
>and the group collectively assessed the situation and felt that the time was
>not yet right, while at the same time thinking highly of the prospects.  

I am getting tired of your raising the OPSWG as a method of validating your 
claims.  I have heard enough blathering to conlude that you either did not 
attend the propulsion sessions or you slept through them.  The biggest 
detraction that ion thrusters currently face is the power supply requirements.  
For Pluto missions, PV arrays obviously will not suffice.  That pretty much 
leaves nuclear sources.  Furthermore, the low mass efficiency of RTG's 
combined with the high power requirements drive you to a nuclear reactor as 
the power source.  The biggest issue facing ion thrusters for interplanetary 
missions is qualifying a nuclear reactor for flight.  Even that is more of a 
political issue than a cost issue.  Although the political aspect tends to 
drive the cost far higher than is technically necessary.  This is the ONLY 
issue that I have seen that is big enough to kill ion for Pluto without 
any further analysis required.  

In all fairness to Mr. Tholen, ion seems to get a bad rap largely because it 
has been in development since the mid 1950's and the only real ion mission
so far has been SERT-2. The thruster was radically different from modern 
thruster technology.  The common assumption is that the technology 
must be immature for it not to have flown.  It tends to be dismissed almost
out of habit when missions are being planned, and it has never recieved the 
focussed monetary attention that a major mission can offer.  It is probably 
inapropriate for a near term mission to Pluto (unless someone is willing to 
fly a Topaz reactor:) ).  It is also very difficult to optimize a low thrust 
trajectory, whereas any high school graduate can solve the two body problem 
on a hand calculator for a reasonable impulsive mission approximation.  This 
makes it more time consuming and expensive to compare electric propulsion in 
any meaningful fashion.  Also, fewer people can do it.  This often leads to an 
inadequate basis for comparison.  

This is my bitter response to what I see as uninformed ion bashing.  I 
apologize if it is too close to a flame.  Judgeing from the recent decline 
in ion posts, interest is waning.  If anyone has further questions for ME, 
about ion, please E-mail them.  

Greg
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     MacRae                  |       Friend, that open mouth
                             |     Reveals your whole interior
  spgreg@mars.lerc.nasa.gov  |         Silly hollow frog!
                             |                      -Anon.
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