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Were liquid boosters on Shuttle ever realistic?



 
 
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  #1  
Old October 21st 17, 05:36 AM posted to sci.space.policy
Fred J. McCall[_3_]
external usenet poster
 
Posts: 10,018
Default Were liquid boosters on Shuttle ever realistic?

Alain Fournier wrote:

On Oct/19/2017 at 9:48 PM, JF Mezei wrote :
On 2017-10-19 20:15, Alain Fournier wrote:

It was my claim from the very start that LH2/LOX does NOT offer better
performance for the first stage.


If discussing engine performance only, would it be correct to state that
SSMEs with higher ISP would offer better performance then RP1 engines or
SRBs since it has better Isp?


Performance is a somewhat vague term. Higher ISP shouldn't be your
measure of performance. If you measure performance by ISP, SSME
is better, but it will cost you dearly if you measure performance
by cost in dollars.


That applies to all stages, though, so if LH2/LOX has poor performance
(in dollars) on the first stage, it will have equally poor performance
on all other stages.


Better ISP means more thrust per pound of fuel, does it not?


Yes.

Or is there something magical which makes LH2 engines perform not as
well at sea level? Isn't the difference in tuning engines for sea level
vs vacuum more the engine bell shape/size?


It is very difficult to pump enough of LH2 to the combustion
chamber in a very short time. You end up needing more engines
or bigger engines to have enough thrust.


RS-68A seems sufficiently 'high thrust' at over 700,000 lbs of thrust.
Using two of them gives you F-1 levels of thrust with almost half
again the Isp. Two of them will weigh (dry weight) about half again
what an F-1 weighs, but that difference is lost in the noise when you
look at the difference in fuel weight you get from the Isp advantage.

Does 14.7 difference in PSI outdoors make a difference when it comes to
the turbines and combustion chambers which are assume are a tad more
pressuzised ?

is the only drawback of LH2 engines (from engineering) the aerodynamics
of the bigger tanks, which for the first stage, must endure max Q ?


The bigger tanks giving greater aerodynamic pressure is a problem. But
it is not as important as the problem of getting enough thrust. You
can get enough thrust, but it will cost you dearly.


Or not so much. RS-68A cost is significantly less than SSME and it's
designed to be thrown away after one use. It gives up some
performance over the SSME but the high thrust and lower cost make that
more than worthwhile. An RS-68A costs about 20% of what an RS-25
costs.

As for aerodynamics, R.H. Coates, lead propulsion engineer for SLS,
seems to disagree with you. When asked why RP-1/LOX is better for
first stage he said, "Refined petroleum is not the most efficient
thrust-producing fuel for rockets, but what it lacks in thrust
production it makes up for in density. It takes less volume of RP-1 to
impart the same thrust force on a vehicle, and less volume equates to
reduced stage size. A smaller booster stage means much less
aerodynamic drag as the vehicle lifts off from near sea-level and
accelerates up through the more dense (thicker) part of the atmosphere
near the earth. The result of a smaller booster stage is it allows a
more efficient ascent through the thickest part of the atmosphere
which helps improve the net mass lifted to orbit."

Given a choice between him as an authority and you, well, I'm going to
go with him.

From a cost point of view, would RP1 engines be much easier and simpler
to make or do they represent same level of complexity and thus cost of
manufacture ?


Bingo! Pumping a tonne of RP1 per second is much easier than pumping
a tonne of LH2 per second. Both because the LH2 will have a much bigger
volume and because it is very cold. To get the same thrust from a LH2
engine will cost you much more than from an RP1 engine.


Both F-1 and RS-68 are gas generator fed engines. RS-68A reportedly
costs on the order of $10-$12 million each. I don't find a cost for
what an F-1B (the current version) would cost, but I'd bet they're not
cheap. SpaceX Merlin, also a gas generator fed engine, but with much
less thrust than an F-1, costs around 20% of what an RS-68 costs. Yes,
an LH2/LOX engine is going to cost more than an RP1/LOX engine of
similar thrust, but don't let RS-25 cost lead you to exaggerate the
difference.


--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw
  #2  
Old October 21st 17, 11:22 AM posted to sci.space.policy
Alain Fournier[_3_]
external usenet poster
 
Posts: 548
Default Were liquid boosters on Shuttle ever realistic?

On Oct/21/2017 at 12:36 AM, Fred J. McCall wrote :
Alain Fournier wrote:

On Oct/19/2017 at 9:48 PM, JF Mezei wrote :
On 2017-10-19 20:15, Alain Fournier wrote:

It was my claim from the very start that LH2/LOX does NOT offer better
performance for the first stage.

If discussing engine performance only, would it be correct to state that
SSMEs with higher ISP would offer better performance then RP1 engines or
SRBs since it has better Isp?


Performance is a somewhat vague term. Higher ISP shouldn't be your
measure of performance. If you measure performance by ISP, SSME
is better, but it will cost you dearly if you measure performance
by cost in dollars.


That applies to all stages, though, so if LH2/LOX has poor performance
(in dollars) on the first stage, it will have equally poor performance
on all other stages.


No. Assume a rocket where the first stage and the second stage are
identical except that the first stage has 9 engines and the second
stage has only 1, and that the rocket uses RP-1. Somewhat like the
Falcon 9, the Falcon 9 isn't exactly like that but assume a rocket
that is like that.

Let's divide the cost of a stage into 3 parts. E = cost of an engine,
T = cost of empty tank and F cost of fuel. For the fuel, the cost
should be in kg, the dollar cost of the fuel is not important, for
the engines and tank using dollars or weight for costs doesn't
make much difference. So the costs of the the first stage is
S1 = T + F + 9E
and the cost of the second stage is
S2 = T + F + E.

Now someone comes along and says, we could save by using LH2 on
the second stage. Now we have to see if you really save overall.
The tank would cost more, the fuel would be lighter and the engine
would cost more. So cost of stage with LH2 becomes
S2LH2 = T + delta T + F - delta F + E + delta E.
So the difference in cost between the RP-1 second stage and the
LH2 second stage is
S2LH2 - S2 = delta T - delta F + delta E.
People at SpaceX looked at that and figured it's not worth it
meaning at SpaceX they think that
delta F delta T + delta E.
Other rocket people looked at that and said yes we would save
by having LH2 for the second stage, so others think that
delta F delta T + delta E.
What is important to know is that it isn't obvious which is
true. That is because we have approximately
delta F = delta T + delta E.

Now let's look at the first stage. If we go to LH2 we get the
cost for the first stage with LH2
S1LH2 = T + delta T + F - delta F + 9(E + delta E).
And the difference between a first stage with RP-1 is
S1LH2 - S1 = delta T - delta F + 9(delta E).
We just said that we have approximately
delta F = delta T + delta E. So we have approximately
S1LH2 - S1 = 8(delta E).
And no sane person would pay 8(delta E) for nothing.

The difference comes from the fact that the first stage
has more engines because you need more thrust on the
first stage.


Alain Fournier
  #3  
Old October 21st 17, 03:56 PM posted to sci.space.policy
Fred J. McCall[_3_]
external usenet poster
 
Posts: 10,018
Default Were liquid boosters on Shuttle ever realistic?

Alain Fournier wrote:

On Oct/21/2017 at 12:36 AM, Fred J. McCall wrote :
Alain Fournier wrote:

On Oct/19/2017 at 9:48 PM, JF Mezei wrote :
On 2017-10-19 20:15, Alain Fournier wrote:

It was my claim from the very start that LH2/LOX does NOT offer better
performance for the first stage.

If discussing engine performance only, would it be correct to state that
SSMEs with higher ISP would offer better performance then RP1 engines or
SRBs since it has better Isp?

Performance is a somewhat vague term. Higher ISP shouldn't be your
measure of performance. If you measure performance by ISP, SSME
is better, but it will cost you dearly if you measure performance
by cost in dollars.


That applies to all stages, though, so if LH2/LOX has poor performance
(in dollars) on the first stage, it will have equally poor performance
on all other stages.


No. Assume a rocket where the first stage and the second stage are
identical except that the first stage has 9 engines and the second
stage has only 1, and that the rocket uses RP-1. Somewhat like the
Falcon 9, the Falcon 9 isn't exactly like that but assume a rocket
that is like that.


Yes, you can always rig the numbers by the problem statement.

Let's divide the cost of a stage into 3 parts. E = cost of an engine,
T = cost of empty tank and F cost of fuel. For the fuel, the cost
should be in kg, the dollar cost of the fuel is not important, for
the engines and tank using dollars or weight for costs doesn't
make much difference. So the costs of the the first stage is
S1 = T + F + 9E
and the cost of the second stage is
S2 = T + F + E.

Now someone comes along and says, we could save by using LH2 on
the second stage. Now we have to see if you really save overall.
The tank would cost more, the fuel would be lighter and the engine
would cost more. So cost of stage with LH2 becomes
S2LH2 = T + delta T + F - delta F + E + delta E.
So the difference in cost between the RP-1 second stage and the
LH2 second stage is
S2LH2 - S2 = delta T - delta F + delta E.
People at SpaceX looked at that and figured it's not worth it
meaning at SpaceX they think that
delta F delta T + delta E.
Other rocket people looked at that and said yes we would save
by having LH2 for the second stage, so others think that
delta F delta T + delta E.
What is important to know is that it isn't obvious which is
true. That is because we have approximately
delta F = delta T + delta E.

Now let's look at the first stage. If we go to LH2 we get the
cost for the first stage with LH2
S1LH2 = T + delta T + F - delta F + 9(E + delta E).
And the difference between a first stage with RP-1 is
S1LH2 - S1 = delta T - delta F + 9(delta E).
We just said that we have approximately
delta F = delta T + delta E. So we have approximately
S1LH2 - S1 = 8(delta E).
And no sane person would pay 8(delta E) for nothing.


It's not 'nothing'. It's more payload to orbit, which is sort of the
goal of the thing (and what you've ignored with your rigged numbers).
Explain Delta IV.


The difference comes from the fact that the first stage
has more engines because you need more thrust on the
first stage.


And because you rigged the problem by assuming you only have a low
thrust engine that you use everywhere.


--
"Millions for defense, but not one cent for tribute."
-- Charles Pinckney
  #4  
Old October 22nd 17, 06:51 PM posted to sci.space.policy
Alain Fournier[_3_]
external usenet poster
 
Posts: 548
Default Were liquid boosters on Shuttle ever realistic?

On Oct/21/2017 at 10:56 AM, Fred J. McCall wrote :
Alain Fournier wrote:

On Oct/21/2017 at 12:36 AM, Fred J. McCall wrote :
Alain Fournier wrote:

On Oct/19/2017 at 9:48 PM, JF Mezei wrote :
On 2017-10-19 20:15, Alain Fournier wrote:

It was my claim from the very start that LH2/LOX does NOT offer better
performance for the first stage.

If discussing engine performance only, would it be correct to state that
SSMEs with higher ISP would offer better performance then RP1 engines or
SRBs since it has better Isp?

Performance is a somewhat vague term. Higher ISP shouldn't be your
measure of performance. If you measure performance by ISP, SSME
is better, but it will cost you dearly if you measure performance
by cost in dollars.


That applies to all stages, though, so if LH2/LOX has poor performance
(in dollars) on the first stage, it will have equally poor performance
on all other stages.


No. Assume a rocket where the first stage and the second stage are
identical except that the first stage has 9 engines and the second
stage has only 1, and that the rocket uses RP-1. Somewhat like the
Falcon 9, the Falcon 9 isn't exactly like that but assume a rocket
that is like that.


Yes, you can always rig the numbers by the problem statement.


Which number was rigged? The only slightly dubious thing in my
example is that I was assuming the same size for the second stage
as for the first stage, which would be unusual for a rocket. But
it doesn't change the idea of the outcome if you divide by two
the size of the tanks in the second stage of my example. It only
makes the computations a little more complicated.

Let's divide the cost of a stage into 3 parts. E = cost of an engine,
T = cost of empty tank and F cost of fuel. For the fuel, the cost
should be in kg, the dollar cost of the fuel is not important, for
the engines and tank using dollars or weight for costs doesn't
make much difference. So the costs of the the first stage is
S1 = T + F + 9E
and the cost of the second stage is
S2 = T + F + E.

Now someone comes along and says, we could save by using LH2 on
the second stage. Now we have to see if you really save overall.
The tank would cost more, the fuel would be lighter and the engine
would cost more. So cost of stage with LH2 becomes
S2LH2 = T + delta T + F - delta F + E + delta E.
So the difference in cost between the RP-1 second stage and the
LH2 second stage is
S2LH2 - S2 = delta T - delta F + delta E.
People at SpaceX looked at that and figured it's not worth it
meaning at SpaceX they think that
delta F delta T + delta E.
Other rocket people looked at that and said yes we would save
by having LH2 for the second stage, so others think that
delta F delta T + delta E.
What is important to know is that it isn't obvious which is
true. That is because we have approximately
delta F = delta T + delta E.

Now let's look at the first stage. If we go to LH2 we get the
cost for the first stage with LH2
S1LH2 = T + delta T + F - delta F + 9(E + delta E).
And the difference between a first stage with RP-1 is
S1LH2 - S1 = delta T - delta F + 9(delta E).
We just said that we have approximately
delta F = delta T + delta E. So we have approximately
S1LH2 - S1 = 8(delta E).
And no sane person would pay 8(delta E) for nothing.


It's not 'nothing'. It's more payload to orbit, which is sort of the
goal of the thing (and what you've ignored with your rigged numbers).
Explain Delta IV.


In my example I was assuming the same functionality for the stages.
Meaning, the stages would put the same mass at the same altitude
and same speed whether it was the RP1 variant or the LH2 variant.

For the Delta IV, there are several variants. Let's look at the
one with one CBC as a first stage and the 5m diameter second stage.
The first stage has one RS-68A engine with a dry weight of the
engine of 6740 kg and 3137 kN thrust, the gross mass of the booster
is 226400 kg. The second stage has one RL10B-2 engine weighing
277 kg and providing 110 kN thrust, the gross mass of the second
stage is 30710 kg.

The example I gave above applies with only minor modifications.
Instead of putting 9 times more engines on the first stage, they
put only one engine as for the second stage but that engine
weighs 24 times more. Saving on the mass and cost of the RL10B-2 by
replacing it with a RP-1 engine isn't much worth the trouble even
if you include the weight saved on the dry weight of the tank.
The added weight of the fuel would approximately counter balance
your gains. Different people will come to different conclusions
on this but the gains or losses by going to RP-1 wouldn't be great.
But for the first stage, you have *much* more to gain by going
to RP-1, the engine is 24 times bigger, I don't know how many times
more expensive it is but it should also be much more expensive
than the second stage engine (which is more important than the
fact that the engine is heavier). Even if you take into account
the fact the the first stage is bigger than the second stage,
the first stage's engine is comparatively bigger. The gains you
can get by going to RP-1 would be more important on the first
stage.

Why didn't they go to RP-1 for the first stage of the Delta IV? I
don't know. But what I have said before is that if you use LH2
for the first stage, it will work but it will cost you. I
don't consider Delta IV to be a low cost launcher.

The difference comes from the fact that the first stage
has more engines because you need more thrust on the
first stage.


And because you rigged the problem by assuming you only have a low
thrust engine that you use everywhere.


No, I was assuming the same engine everywhere like on the Falcon 9.
The Merlin engine on the Falcon 9 has a thrust to weight ratio of 180
which is much more than what you have on the Delta IV.


Alain Fournier
  #5  
Old October 23rd 17, 10:01 AM posted to sci.space.policy
Fred J. McCall[_3_]
external usenet poster
 
Posts: 10,018
Default Were liquid boosters on Shuttle ever realistic?

Alain Fournier wrote:

On Oct/21/2017 at 10:56 AM, Fred J. McCall wrote :
Alain Fournier wrote:

On Oct/21/2017 at 12:36 AM, Fred J. McCall wrote :
Alain Fournier wrote:

On Oct/19/2017 at 9:48 PM, JF Mezei wrote :
On 2017-10-19 20:15, Alain Fournier wrote:

It was my claim from the very start that LH2/LOX does NOT offer better
performance for the first stage.

If discussing engine performance only, would it be correct to state that
SSMEs with higher ISP would offer better performance then RP1 engines or
SRBs since it has better Isp?

Performance is a somewhat vague term. Higher ISP shouldn't be your
measure of performance. If you measure performance by ISP, SSME
is better, but it will cost you dearly if you measure performance
by cost in dollars.


That applies to all stages, though, so if LH2/LOX has poor performance
(in dollars) on the first stage, it will have equally poor performance
on all other stages.

No. Assume a rocket where the first stage and the second stage are
identical except that the first stage has 9 engines and the second
stage has only 1, and that the rocket uses RP-1. Somewhat like the
Falcon 9, the Falcon 9 isn't exactly like that but assume a rocket
that is like that.


Yes, you can always rig the numbers by the problem statement.


Which number was rigged? The only slightly dubious thing in my
example is that I was assuming the same size for the second stage
as for the first stage, which would be unusual for a rocket. But
it doesn't change the idea of the outcome if you divide by two
the size of the tanks in the second stage of my example. It only
makes the computations a little more complicated.


Why does the first stage need nine engines?

Let's divide the cost of a stage into 3 parts. E = cost of an engine,
T = cost of empty tank and F cost of fuel. For the fuel, the cost
should be in kg, the dollar cost of the fuel is not important, for
the engines and tank using dollars or weight for costs doesn't
make much difference. So the costs of the the first stage is
S1 = T + F + 9E
and the cost of the second stage is
S2 = T + F + E.

Now someone comes along and says, we could save by using LH2 on
the second stage. Now we have to see if you really save overall.
The tank would cost more, the fuel would be lighter and the engine
would cost more. So cost of stage with LH2 becomes
S2LH2 = T + delta T + F - delta F + E + delta E.
So the difference in cost between the RP-1 second stage and the
LH2 second stage is
S2LH2 - S2 = delta T - delta F + delta E.
People at SpaceX looked at that and figured it's not worth it
meaning at SpaceX they think that
delta F delta T + delta E.
Other rocket people looked at that and said yes we would save
by having LH2 for the second stage, so others think that
delta F delta T + delta E.
What is important to know is that it isn't obvious which is
true. That is because we have approximately
delta F = delta T + delta E.

Now let's look at the first stage. If we go to LH2 we get the
cost for the first stage with LH2
S1LH2 = T + delta T + F - delta F + 9(E + delta E).
And the difference between a first stage with RP-1 is
S1LH2 - S1 = delta T - delta F + 9(delta E).
We just said that we have approximately
delta F = delta T + delta E. So we have approximately
S1LH2 - S1 = 8(delta E).
And no sane person would pay 8(delta E) for nothing.


It's not 'nothing'. It's more payload to orbit, which is sort of the
goal of the thing (and what you've ignored with your rigged numbers).
Explain Delta IV.


In my example I was assuming the same functionality for the stages.
Meaning, the stages would put the same mass at the same altitude
and same speed whether it was the RP1 variant or the LH2 variant.

For the Delta IV, there are several variants. Let's look at the
one with one CBC as a first stage and the 5m diameter second stage.
The first stage has one RS-68A engine with a dry weight of the
engine of 6740 kg and 3137 kN thrust, the gross mass of the booster
is 226400 kg. The second stage has one RL10B-2 engine weighing
277 kg and providing 110 kN thrust, the gross mass of the second
stage is 30710 kg.

The example I gave above applies with only minor modifications.
Instead of putting 9 times more engines on the first stage, they
put only one engine as for the second stage but that engine
weighs 24 times more. Saving on the mass and cost of the RL10B-2 by
replacing it with a RP-1 engine isn't much worth the trouble even
if you include the weight saved on the dry weight of the tank.
The added weight of the fuel would approximately counter balance
your gains. Different people will come to different conclusions
on this but the gains or losses by going to RP-1 wouldn't be great.
But for the first stage, you have *much* more to gain by going
to RP-1, the engine is 24 times bigger, I don't know how many times
more expensive it is but it should also be much more expensive
than the second stage engine (which is more important than the
fact that the engine is heavier). Even if you take into account
the fact the the first stage is bigger than the second stage,
the first stage's engine is comparatively bigger. The gains you
can get by going to RP-1 would be more important on the first
stage.


And an RP1/LOX engine with equivalent thrust will weigh about the same
as the LH2/LOX RS-68A, so you save nothing so far as weight goes. The
fuel to get equivalent performance will weigh much more, so your
RP1/LOX engine needs higher thrust because it has to lift more mass in
fuel.


Why didn't they go to RP-1 for the first stage of the Delta IV? I
don't know. But what I have said before is that if you use LH2
for the first stage, it will work but it will cost you. I
don't consider Delta IV to be a low cost launcher.


But it's comparable to most other launchers of its generation. Both
Atlas V and Delta IV cost about $13k/kg to get stuff into orbit. Atlas
V is RP1/LOX while Delta IV is LH2/LOX for the 'core stage'.

The difference comes from the fact that the first stage
has more engines because you need more thrust on the
first stage.


And because you rigged the problem by assuming you only have a low
thrust engine that you use everywhere.


No, I was assuming the same engine everywhere like on the Falcon 9.


Which rigs your result.


The Merlin engine on the Falcon 9 has a thrust to weight ratio of 180
which is much more than what you have on the Delta IV.


And it has a lower Isp and lower thrust. So what? Engine weight is
trivial unless you rig the numbers.


--
"False words are not only evil in themselves, but they infect the
soul with evil."
-- Socrates
  #6  
Old October 24th 17, 01:20 AM posted to sci.space.policy
Alain Fournier[_3_]
external usenet poster
 
Posts: 548
Default Were liquid boosters on Shuttle ever realistic?

On Oct/23/2017 at 5:01 AM, Fred J. McCall wrote :
Alain Fournier wrote:

On Oct/21/2017 at 10:56 AM, Fred J. McCall wrote :
Alain Fournier wrote:

On Oct/21/2017 at 12:36 AM, Fred J. McCall wrote :
Alain Fournier wrote:

On Oct/19/2017 at 9:48 PM, JF Mezei wrote :
On 2017-10-19 20:15, Alain Fournier wrote:

It was my claim from the very start that LH2/LOX does NOT offer better
performance for the first stage.

If discussing engine performance only, would it be correct to state that
SSMEs with higher ISP would offer better performance then RP1 engines or
SRBs since it has better Isp?

Performance is a somewhat vague term. Higher ISP shouldn't be your
measure of performance. If you measure performance by ISP, SSME
is better, but it will cost you dearly if you measure performance
by cost in dollars.


That applies to all stages, though, so if LH2/LOX has poor performance
(in dollars) on the first stage, it will have equally poor performance
on all other stages.

No. Assume a rocket where the first stage and the second stage are
identical except that the first stage has 9 engines and the second
stage has only 1, and that the rocket uses RP-1. Somewhat like the
Falcon 9, the Falcon 9 isn't exactly like that but assume a rocket
that is like that.


Yes, you can always rig the numbers by the problem statement.


Which number was rigged? The only slightly dubious thing in my
example is that I was assuming the same size for the second stage
as for the first stage, which would be unusual for a rocket. But
it doesn't change the idea of the outcome if you divide by two
the size of the tanks in the second stage of my example. It only
makes the computations a little more complicated.


Why does the first stage need nine engines?


You should ask SpaceX why they put nine engines on their first stage.

Let's divide the cost of a stage into 3 parts. E = cost of an engine,
T = cost of empty tank and F cost of fuel. For the fuel, the cost
should be in kg, the dollar cost of the fuel is not important, for
the engines and tank using dollars or weight for costs doesn't
make much difference. So the costs of the the first stage is
S1 = T + F + 9E
and the cost of the second stage is
S2 = T + F + E.

Now someone comes along and says, we could save by using LH2 on
the second stage. Now we have to see if you really save overall.
The tank would cost more, the fuel would be lighter and the engine
would cost more. So cost of stage with LH2 becomes
S2LH2 = T + delta T + F - delta F + E + delta E.
So the difference in cost between the RP-1 second stage and the
LH2 second stage is
S2LH2 - S2 = delta T - delta F + delta E.
People at SpaceX looked at that and figured it's not worth it
meaning at SpaceX they think that
delta F delta T + delta E.
Other rocket people looked at that and said yes we would save
by having LH2 for the second stage, so others think that
delta F delta T + delta E.
What is important to know is that it isn't obvious which is
true. That is because we have approximately
delta F = delta T + delta E.

Now let's look at the first stage. If we go to LH2 we get the
cost for the first stage with LH2
S1LH2 = T + delta T + F - delta F + 9(E + delta E).
And the difference between a first stage with RP-1 is
S1LH2 - S1 = delta T - delta F + 9(delta E).
We just said that we have approximately
delta F = delta T + delta E. So we have approximately
S1LH2 - S1 = 8(delta E).
And no sane person would pay 8(delta E) for nothing.


It's not 'nothing'. It's more payload to orbit, which is sort of the
goal of the thing (and what you've ignored with your rigged numbers).
Explain Delta IV.


In my example I was assuming the same functionality for the stages.
Meaning, the stages would put the same mass at the same altitude
and same speed whether it was the RP1 variant or the LH2 variant.

For the Delta IV, there are several variants. Let's look at the
one with one CBC as a first stage and the 5m diameter second stage.
The first stage has one RS-68A engine with a dry weight of the
engine of 6740 kg and 3137 kN thrust, the gross mass of the booster
is 226400 kg. The second stage has one RL10B-2 engine weighing
277 kg and providing 110 kN thrust, the gross mass of the second
stage is 30710 kg.

The example I gave above applies with only minor modifications.
Instead of putting 9 times more engines on the first stage, they
put only one engine as for the second stage but that engine
weighs 24 times more. Saving on the mass and cost of the RL10B-2 by
replacing it with a RP-1 engine isn't much worth the trouble even
if you include the weight saved on the dry weight of the tank.
The added weight of the fuel would approximately counter balance
your gains. Different people will come to different conclusions
on this but the gains or losses by going to RP-1 wouldn't be great.
But for the first stage, you have *much* more to gain by going
to RP-1, the engine is 24 times bigger, I don't know how many times
more expensive it is but it should also be much more expensive
than the second stage engine (which is more important than the
fact that the engine is heavier). Even if you take into account
the fact the the first stage is bigger than the second stage,
the first stage's engine is comparatively bigger. The gains you
can get by going to RP-1 would be more important on the first
stage.


And an RP1/LOX engine with equivalent thrust will weigh about the same
as the LH2/LOX RS-68A, so you save nothing so far as weight goes.


Four Merlin 1D engines will give more thrust (4x845 = 3380 kN) than one
RS-68A (3137 kN) but they will weigh 1880 kg which is much less than
the 6740 kg of the RS-68A. But that is not really very important here.

The
fuel to get equivalent performance will weigh much more, so your
RP1/LOX engine needs higher thrust because it has to lift more mass in
fuel.


Yes. The fuel will weigh much more. You don't use RP1/LOX engines
instead of LH2/LOX engines to save weight. I'm glad to see that we
now agree on this.

Why didn't they go to RP-1 for the first stage of the Delta IV? I
don't know. But what I have said before is that if you use LH2
for the first stage, it will work but it will cost you. I
don't consider Delta IV to be a low cost launcher.


But it's comparable to most other launchers of its generation. Both
Atlas V and Delta IV cost about $13k/kg to get stuff into orbit. Atlas
V is RP1/LOX while Delta IV is LH2/LOX for the 'core stage'.

The difference comes from the fact that the first stage
has more engines because you need more thrust on the
first stage.


And because you rigged the problem by assuming you only have a low
thrust engine that you use everywhere.


No, I was assuming the same engine everywhere like on the Falcon 9.


Which rigs your result.


Hum! are you claiming that SpaceX designed their Falcon 9 that way
to rig the results of this conversation? I can assure you that I
did not collude with SpaceX on that (nor on anything else for that
matter).

The Merlin engine on the Falcon 9 has a thrust to weight ratio of 180
which is much more than what you have on the Delta IV.


And it has a lower Isp and lower thrust. So what? Engine weight is
trivial unless you rig the numbers.


Engine weight isn't the important thing here. I'm glad to see we agree
on this. But the cost of the engines is important. If you need nine
on the first stage against only one on the second stage, you can see
that saving on engines on the first stage is more important than on
the second stage.

And if you have a design such as the Delta IV, it isn't as obvious but
the same argument applies. The RS-68A on the first stage weighs 24 times
the RL10B-2 on the second stage. Also the RS-68A has 28.5 times more
thrust than the RL10B-2. I don't know what's the price tag of neither
the RS-68A nor the RL10B-2, but I would expect it to cost many times
more. So once again, you can see that saving on the first stage engines
is more important than on the second stage engines.

This argument still holds even if you take into account that the second
stage is smaller than the first. The second stage of the Falcon isn't
nine times smaller than the first stage and the second stage of the
Delta IV isn't 28 times smaller than the first. You want to have
proportionally more thrust on the first stage than on the second stage
and you have to pay for that one way or another.


Alain Fournier
 




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