Fuel costs

I worked these out, and thought they might interest some here. They are
just meant to be BOAE calculations, and you may want to design differently,
but they should be fairly realistic. They certainly support the idea that
fuel prices should be ignored as a design consideration, at least until LEO
cost gets well below $1,000/kg.

The prices are based on recent-ish NASA prices except for kero, which I just
guessed based on petrol prices. You can probably get the TSTO figure down to
about $7, but I didn't try.

Assuming Lox is $0.15/kg, LH2 is $3.25/kg, and kero is $0.30/kg; mission is
1 kg payload to a comfortable LEO; Lox/LH2 mix 5:1, Isp 350/450; Lox/kero
mix 2:1, Isp 265/330.


For an SSTO

overall mr 13; stage dry mass 6.65% of propellant; GLOW is 65kg.
60 kg Lox/LH2 propellant, 4 kg dry mass, 1 kg payload:
10 kg LH2 $32.50
50 kg Lox $7.50

Total $40.00 per kilo payload



For a TSTO

1st stage overall mr 3; stage dry mass 20% of propellant; GLOW is 45 kg.
30 kg Lox/kero propellant, 7.5 kg dry mass, 7.5 kg load:

10 kg Kero $3.00
20 kg Lox $3.00

2nd stage overall mr 5; stage dry mass 8.35% of propellant: Gross 7.5 kg.
6 kg Lox/LH2 propellant, 0.5 kg dry mass, 1kg payload:

1 kg LH2 $3.25
5 kg Lox $0.75

Total $10.00 per kilo payload





--
Peter Fairbrother

In article ,
Peter Fairbrother wrote:
The prices are based on recent-ish NASA prices except for kero...
Assuming Lox is $0.15/kg...

NASA's paying fifteen cents a kilogram for LOX? I want to be their LOX
supplier! :-)

They ought to be paying maybe a third of that.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Henry Spencer wrote:

In article ,
Peter Fairbrother wrote:
The prices are based on recent-ish NASA prices except for kero...
Assuming Lox is $0.15/kg...

NASA's paying fifteen cents a kilogram for LOX? I want to be their LOX
supplier! :-)

They ought to be paying maybe a third of that.

$0.68 per gallon is the figure I was given.

It may well be an overall cost of Lox delivered in Shuttle ET's rather than
a raw feedstock price, but unfortunately I am not in a position to check.


--
Peter Fairbrother

"Peter Fairbrother" wrote in message
...
I worked these out, and thought they might interest some here. They are
just meant to be BOAE calculations, and you may want to design
differently,
but they should be fairly realistic. They certainly support the idea that
fuel prices should be ignored as a design consideration, at least until
LEO
cost gets well below $1,000/kg.

The prices are based on recent-ish NASA prices except for kero, which I
just
guessed based on petrol prices. You can probably get the TSTO figure down
to
about $7, but I didn't try.

Assuming Lox is $0.15/kg, LH2 is $3.25/kg, and kero is $0.30/kg; mission
is
1 kg payload to a comfortable LEO; Lox/LH2 mix 5:1, Isp 350/450; Lox/kero
mix 2:1, Isp 265/330.


For an SSTO

overall mr 13; stage dry mass 6.65% of propellant; GLOW is 65kg.
60 kg Lox/LH2 propellant, 4 kg dry mass, 1 kg payload:
10 kg LH2 $32.50
50 kg Lox $7.50

Total $40.00 per kilo payload



For a TSTO

1st stage overall mr 3; stage dry mass 20% of propellant; GLOW is 45 kg.
30 kg Lox/kero propellant, 7.5 kg dry mass, 7.5 kg load:

10 kg Kero $3.00
20 kg Lox $3.00

2nd stage overall mr 5; stage dry mass 8.35% of propellant: Gross 7.5 kg.
6 kg Lox/LH2 propellant, 0.5 kg dry mass, 1kg payload:

1 kg LH2 $3.25
5 kg Lox $0.75

Total $10.00 per kilo payload

Thanks for the numbers. They certainly take a lot of the force out
of my arguments against SSTO on another thread. I hadn't realized
that dry weight and operational support was that much more expensive
than fuel.

But, I'm curious. Why did you assume a much better mr for the SSTO
than for either stage of the TSTO. ISTM that you should be able to
get similar mass ratios for the 2nd stage of a TSTO that you can
achieve for an SSTO. What am I missing here? Do "mounting brackets"
weigh that much? Can't they be part of the first stage weight?

Also, why the low ISP kero in the first stage of the TSTO? And how
much would the numbers change if you used kero for the second stage
as well?

In article om,
Perplexed in Peoria wrote:
Thanks for the numbers. They certainly take a lot of the force out
of my arguments against SSTO on another thread. I hadn't realized
that dry weight and operational support was that much more expensive
than fuel.

They certainly are. Titan IV is the *only* current US launcher for which
fuel costs aren't completely "down in the noise"; it suffers from the
relatively high costs of solid fuel and hypergolic liquids.

(Another illustration of that is that fueling the Apollo spacecraft -- CSM
and LM -- with hypergolics actually cost more than fueling the Saturn V
first stage with LOX/kerosene, despite a considerable disparity in size.)

For an SSTO
overall mr 13...
For a TSTO
1st stage overall mr 3...
2nd stage overall mr 5...

But, I'm curious. Why did you assume a much better mr for the SSTO
than for either stage of the TSTO.

He's assuming that you would relax the MR to make the TSTO stages easier
to build and operate. However, a first-stage MR of 3 is a bit ridiculous
even so.

Mind you, an MR of 13 with LOX/LH2 verges on fantasy. With LOX/kerosene,
okay, but not LOX/LH2. Achieving really high mass ratios with LOX/LH2 is
quite difficult, because the LH2 is so bulky, its tanks need insulation,
and the engine hardware for it is so big and heavy.

ISTM that you should be able to
get similar mass ratios for the 2nd stage of a TSTO that you can
achieve for an SSTO. What am I missing here? Do "mounting brackets"
weigh that much? Can't they be part of the first stage weight?

There is some small mass penalty because the need to carry large forces
through the interstage adapter, and then disconnect it quickly and
cleanly, tends to require narrow load paths and concentrated loads which
add structural mass. You don't get to leave all of this behind, because
the thrust loads have to be transmitted into the second stage somehow.

There can also be a mass penalty because of higher acceleration loads.
Two-stage systems with light upper stages have some tendency to have high
accelerations just before staging. (This is why the Saturn V's first
stage shut down its center engine early.)

But most of this is just the assumption that the TSTO engineers will have
room to be lazy, and will be. "You don't want happy engineers -- they do
not make competitive designs." (Max Hunter)

Also, why the low ISP kero in the first stage of the TSTO?

The first stage benefits less from high Isp and more from denser fuel.
(And also more from cheaper fuel, since it has much of the fuel mass.)

Moreover, it sounds like you've missed a subtle point. Engine performance
and stage performance are two different things. In practice, LOX/LH2
stages struggle to equal the *stage* performance of good dense-fuel
designs, because as indicated above, they pay for their high Isp with a
lot more dry mass. LOX/kerosene stages can easily have higher delta-V and
lower cost despite lower Isp; the one aspect where they are inherently at
a disadvantage is gross mass.

(Note that although gross mass is an issue for certain things, the common
assumption that cost scales directly with gross mass is demonstrably wrong.)

And how much would the numbers change if you used kero for the second
stage as well?

A thorough, unbiased assessment -- not just spreadsheet engineering --
tends to conclude that a TSTO is better with all kerosene and no LH2. And
surprise surprise, so is an SSTO. The problems and dry-mass penalties of
LH2 more than cancel out its high Isp for launch to LEO. But the blind
religious belief in the innate superiority of hydrogen -- which dates to
the days when its problems were not well understood -- is very persistent.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Henry Spencer wrote:
I hadn't realized that dry weight and operational
support was that much more expensive than fuel.

They certainly are. Titan IV is the *only* current US launcher for which
fuel costs aren't completely "down in the noise"; it suffers from the
relatively high costs of solid fuel and hypergolic liquids.

Hydrazine and tetroxide, not hypergols in general.

Nitric acid is roughly as cheap as LOX is...
And is hypergolic with stuff which isn't that much
more expensive than kerosene. And other than being
a oxidizer and corrosive, it's not chemically toxic
(though, if you spill it on organic stuff, will
produce toxic fumes under many circumstances).


-george william herbert

I hate following myself up, but:
[ nitric acid]
And other than being a oxidizer and corrosive,
it's not chemically toxic

I am of course referring to non-red-fuming nitric
acid types. Adding nitrogen dioxide/tetroxide to
the mix produces a toxic chemical, RFNA (Red Fuming
Nitric Acid).


-george william herbert

(George William Herbert) writes:

Nitric acid is roughly as cheap as LOX is...
And is hypergolic with stuff which isn't that much
more expensive than kerosene. And other than being
a oxidizer and corrosive, it's not chemically toxic
(though, if you spill it on organic stuff, will
produce toxic fumes under many circumstances).

Just curious, but where does nitrous oxide fit in with
all of this?

--
pgf

Phil Fraering pgf@AUTO writes:

(George William Herbert) writes:

Nitric acid is roughly as cheap as LOX is...
And is hypergolic with stuff which isn't that much
more expensive than kerosene. And other than being
a oxidizer and corrosive, it's not chemically toxic
(though, if you spill it on organic stuff, will
produce toxic fumes under many circumstances).

Just curious, but where does nitrous oxide fit in with
all of this?


Very low performance, not at all suitable for an orbital
launch vehicle. Moderate cost. But, N2O only acuires the
oxidizer-nature at high temperature, which means it's *safe*
whereas other oxidizers merely compete for the "less dangerous
than the other guy" label.

Good stuff for entry-level rocket builders, or for anyone who
plans on hitching a ride to orbit as a secondary payload on
someone else's billion-dollar space shuttle or comsat launch.


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*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
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In article ,
Phil Fraering pgf@AUTO wrote:
Nitric acid is roughly as cheap as LOX is...
And is hypergolic with stuff which isn't that much
more expensive than kerosene...

Just curious, but where does nitrous oxide fit in with
all of this?

Neither fish nor fowl. :-)

It's a passable oxidizer, not great but the stored energy somewhat makes
up for the relatively modest oxygen content. Cost is much higher than LOX
or nitric acid. It's not hypergolic with any normal fuel; indeed, until
you get it hot, it's essentially chemically inert, which is nice for
fault-tolerant handling but a headache for ignition. Density is a bit
low but not disastrously so.

The big problem is storage. Room-temperature N2O needs heavy high-pressure
tanks, and you can't fill them too full because its coefficient of thermal
expansion is pretty high. If you want lightweight tanks, you have to
chill it -- not nearly as much as LOX, but enough to bring in some of the
same headaches.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |