John:
It might be so; but adding that capability further increases the cost of
the depot, a cost which all users are going to have to pay.
Bill
On Sat, Aug 24, 2019 at 3:40 PM John's AOL <dmarc-noreply@xxxxxxxxxxxxx>
wrote:
I by no means in any kind of way are an expert or even a novice to comment
on this post but it would seem to me that a tanker/depot/whatever u call it
that is “fueled” with water and also had the capability to convert and
store some of the water as water, hydrogen and oxygen would be beneficial
to manned long term space flight but I am just a newbie.
Sent from my iPhone
On Aug 24, 2019, at 5:29 PM, William Claybaugh <wclaybaugh2@xxxxxxxxx>
wrote:
Rand:
I conclude that depots add cost to space flight because they plainly so do
compared to the existing architecture. To date, no payload has failed to
fly because the launcher couldn't carry it; arguing that depots solve a
problem that does not exist is simply specious.
I am aware that SpaceX is planning to make use of refueling for its
planned fully reusable architecture and that plan does appear to lower
their required investment as compared to building a bigger rocket. I do
not understand that their cost would be further lowered by paying the cost
of a depot; rather, it appears obvious that their costs would be higher by
the amount required to build and operate that depot.
Bill
On Sat, Aug 24, 2019 at 1:08 PM Rand Simberg <simberg@xxxxxxxxxxxxxxx>
wrote:
We are on the verge of getting a fundamentally different launch
architecture. Probably more than one. I don't understand why you think
that depots are costly.
On 2019-08-24 12:04, William Claybaugh wrote:
Henry:
I’ve reread this post more than a few times and I’m afraid I am
not able to make sense of it.
If I understand your argument, it is at heart that we should ignore
the 60 plus year existence proof that chemical rockets do not need any
of the attributes you conjecture and instead plan in future for a
fundamentally different launch architecture.
Setting aside the improbability of selling this idea to any rational
investor or government, I don’t see the point: architectures that
require refueling don’t require a depot or it’s costs.
Bill
On Fri, Aug 23, 2019 at 2:56 PM Henry Spencer <hspencer@xxxxxxxxxxxxx>
wrote:
On Fri, 23 Aug 2019, William Claybaugh wrote:
Propellant at a propellant depot costs the price of thatpropellant on the
ground plus the cost of launching it to LEO plus the pro-rataamortization
of the cost of the depot plus the pro-rata depreciation of thedepot plus
the cost of losses.ground
Propellant in an upper stage costs the price of propellant on the
plus the cost of launching it to LEO.
True if:
1. The cost of launching an extra kilogram to LEO is the same either
way.
2. There is extra room in the tanks for it.
3. The lower stage has the lift capability to carry its extra mass.
4. Reduced upper-stage launch mass doesn't confer some special
advantage,
like seriously reduced structural mass, that's very important
later.
Which is to say, if it's just a matter of whether to fill the tank a
bit
more, that does indeed usually win. Depots et al show to advantage
when
it's not that simple, when one (or more) of those assumptions is not
true.
It's appealing to think (or at least claim) that things *are* that
simple.
Trouble is, often they aren't.
For example, an important advantage of depots is that they decouple
LEO
departure mass from launcher payload mass, so violations of
assumptions 2
and 3 don't threaten to sink your project. Should we worry about
such
violations? Yes! They nearly sank Apollo -- which survived only
because
Wernher von Braun had quietly built a rather bigger rocket than
Houston's
spacecraft mass estimates would have required -- and Constellation's
troubled history conspicuously included repeated discoveries that
the
rockets were undersized. Let's see, that's 2 out of NASA's 2
previous
attempts to reach the Moon that had big trouble with those
assumptions --
should we really believe that the current attempt is immune to this?
Depots are not getting any traction because the key players--whoare at
OMB and the Space Council--know these facts.
Or because they are getting their briefings from folks who quietly
aren't
discussing the dubious assumptions underlying these "facts".
Henry
