From: tholen@galileo.ifa.hawaii.edu (Dave Tholen)
Subject: Re: My final word on Ion to Pluto (long)
Organization: Institute for Astronomy, Hawaii
Distribution: usa
Date: Sat, 3 Oct 1992 08:01:28 GMT

Greg Macrae writes:
 
>> (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.

When I want to do some science that requires something not already in hand,
I approach one of our instrument builders and ask them what it would
cost to build what I need.  They do the cost analysis.  I write the proposal
to get the funding, and if the proposal is successful, but at a reduced
funding level, then I go back to the instrument builders and ask them for
compromises that fit within the funding limitations, with a goal of minimizing
the impact on the science that can be done.  The Pluto mission has been
through a lot of design and redesign, with each redesign in response to the
likely funding scenario.  The spacecraft designers at JPL are the ones doing
the cost analysis in response to the scientific priorities agreed to by the
scientists.

> 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 justification for this project is the science that we want to do, not the
cost analysis.  Just because something can be done with a limited budget
doesn't mean that we should do it, if it isn't good science.

> 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.  

You say that ion propulsion is better than chemical propulsion.  We say that
we are excited by the prospects for ion propulsion, but have a tight cost cap
imposed by NASA of $400 million, including launch.  So we naturally have to
ask the question of how much it's going to cost to use ion propulsion.  You
say you won't tell us.  Sorry, but we're not in a position to hand out blank
checks.

>> 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.

But we are looking for a propulsion system for use in deep space, not near
Earth orbit.

> To my knowledge, there is still no user for the system.

Why is that?  It sounds like you are perplexed as to why ion hasn't been
embraced.  May I suggest that you put yourself in the customer's shoes and
ask yourself why you didn't choose ion technology.  You say there are no
valid objections, but the potential customers out there must have some, or
else they would have chosen it.  Right?

> 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.  

This is good news.  Let's hope work on ion technology continues without the
funding from a specific mission.

>> (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 :).

Likewise!  I have other outer Solar System interests, like Chiron, Pholus,
1991 DA, and 1992 QB1.  Who knows, maybe someday we'll be collaborating on
an ion-driven spacecraft to one of these objects.  No sense starting off on
the wrong foot!

>> 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,

Sounds like an automobile dealership advertising something for "under $10,000",
and it turns out to be $9,999.  Just how much longer than 10,000 hours is the
rated life?  10,001?  20,000?  100,000?  10,000 hours is just 417 days, and
I've yet to see a claim that we can get to Pluto in 417 days.  What fraction
of the time are you coasting?  You did quote from an internal study, that an
ion-driven spacecraft could deliver twice the payload INTO ORBIT around Pluto
in half the travel time.  So half the rated lifetime would be used to
decelerate.  What about the storage lifetime?  If the propulsion unit is to
go unused for a long stretch of time, how reliably can it be restarted?  If
the unit is to be used in a pulsed mode with a low duty cycle, does the
10,000 hours represent the total lifetime or just that fraction when it's
actually thrusting?

How many times has >10,000 hour life been demonstrated in real-life
conditions, as opposed to laboratory conditions?  With just one prototype
unit?  How is reliability assessed?

> 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.  

Sufficient lifetime to do a Pluto mission; 10,000 hours may not be enough.
Also, reliability.  How much of a track record does ion have?  Just SERT-2?

>> 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'll take OPSWG over sci.space any day.  NASA hasn't put fools on this working
group.  I have a great deal of respect for these people.

> I have heard enough blathering to conlude that you either did not 
> attend the propulsion sessions or you slept through them.

Hold on there.  The difference between a response and a flame is the usage
of words like "blathering".  And your conclusion is quite false.  I was both
there and awake.  And I continued the discussion with the presenter in an
informal way during dinner that evening.  I summarized the situation as a
Catch 22.  Nobody wants to be the first to use ion because of the risks
involved, so the funding to finish the development of the technology to
eliminate those risks hasn't been forthcoming.  He indicated that that was
a reasonably accurate summary of the situation.  Note that what you and we
consider to be "technology development" may not reflect the same level of
completion.  For example, I'm often hearing of technology demonstrations by
software vendors (IBM or Microsoft, for example), but a lot of fine-tuning
is still needed before such products are actually released for real-world
usage.  Apparently the presenter agreed that ion still needed some work
before it could be realistically utilized with a high degree of confidence.
This presentation was a year and a half ago, however.  If the situation has
changed signficantly, it's time for a updated presentation.  Note that when
I say just ion, I'm referring to the whole package, including the power
source.  For example, the folks at Sun may consider their Sparcstations
to be developed technology, but if the intended use is far from any AC
outlet, the customer will go looking elsewhere.  Ion may be ready, but if
there isn't a practical power source, what good does it do?

> 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.  

Even RTGs trigger the political aspect, so there must be more to it than
just that.  One issue that was raised with regard to RTGs was that unlike
Voyager, whose RTGs were boom mounted to get them as far away as possible
from sensitive instrumentation, the Pluto spacecraft is quite small, with
RTGs body mounted.  Depending on the type of instrumentation, more shielding
may be required, which adds mass, which translates into cost.  The nuclear
reactor you're talking about will also add mass.  At what tradeoff?  Smaller
payload?  Is the mass of ion's propellant, compared with chemical, less by
enough to compensate for the mass of the reactor?  Do you lengthen the travel
time or use a more powerful launch vehicle?  All of these issues eventually
translate into higher cost, which is what I've been saying all along.

> 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.

Again, I suggest putting yourself into the shoes of the customer to try and
determine why it hasn't been chosen for flight.  If the customers' reasons
aren't valid, then somebody isn't doing their job right.

> 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.  

I hope you haven't concluded that I've been bashing ion.  I said it before
and I'll say it again:  I think it's great technology, and that further work
shouldn't be tied to the "focussed monetary attention that a major mission
can offer".  (By the way, Pluto is not considered a major mission; NASA has
its different categories based on cost; Cassini is a flagship mission, for
example, but Pluto is not; it is above the Discovery class, however.)  I have
merely been defending my colleagues who have decided that chemical propulsion
is good enough to get the job done, and at lower cost.  We have no doubt that
we CAN fly a better mission, but "better" almost certainly means higher cost.
I'm not naive enough to claim that we've perfectly optimized the mission
(especially when there are conflicting scientific views as to what are the
most important measurements to make), but with the threat of not having a
mission at all if we come in too expensive, we're trying our very best.
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