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Comparisons - expend./reusable,heavy/light,theory/practice

Posted by: Ekkehard Augustin - Thu Mar 30, 2006 8:38 am
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Comparisons - expend./reusable,heavy/light,theory/practice 
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Post    Posted on: Wed Jul 05, 2006 9:04 pm
publiusr wrote:
Winged Saturns were being considered, after all:
http://www.up-ship.com/apr/apr.htm
Unfortunately, when I go to that page, all the Saturn links in it say they are "Not Currently Available". :(


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Post    Posted on: Thu Jul 06, 2006 5:27 pm
An interesting way to look at the problem is to postulate a vehicle (for example an SSTO) with a constant dry mass and a constant volume for tanks. Assumes that tank mass is closely related to volume, regardless of propellant density, which appears to be the case. Also that engine mass will be about the same, or roughly cancelled out by other factors such as insulation.

Apply different propellant combinations to the total tank volume at the appropriate O/F ratios to get individual volumes, then calculate the mass of each from the volume & density. The total propellant mass plus the dry mass divided by the dry mass gives the Mass Ratio, which with the ISP of the propellants can be plugged into the Rocket Equation to get the deltaV.

Interestingly, a LOX / RP-1 vehicle with an MR of 34, ISP 350 would have 30% more deltaV than a LOX / LH2 vehicle with an MR of 12, ISP 400, OF EXACTLY THE SAME SIZE. It would also weigh 3 times as much at liftoff, yet the propellant costs would be slightly lower. Alternately the vehicle could have 90% greater dry mass ( MR = 18 ) and still have equivalent performance to the LH2.

Propellant density is very important.


Last edited by WannabeSpaceCadet on Fri Jul 07, 2006 4:42 am, edited 1 time in total.



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Post    Posted on: Thu Jul 06, 2006 6:01 pm
WannabeSpaceCadet wrote:
Interestingly, a LOX / RP-1 vehicle with an MR of 34, ISP 350 would have 30% more deltaV than a LOX / LH2 vehicle with an MR of 12, ISP 400, OF EXACTLY THE SAME SIZE. It would also weigh 3 times as much at liftoff, yet the propellant costs would be slightly lower. Alternately the vehicle could have 90% greater dry mass (MR = 18) and still have equivalent performance to the LH2.
Since the RP-1 fueled rocket weight 3 times more, it needs three times more engine power to lift off. That means either making each engine three times bigger and heavier or just tripling the number of engines. Since about 90% of the DRY weight of any well designed rocket is the engines, adding engines, or enlarging the engines, will severely increase the dry weight and degrade the mass ratio. I can do some numerical examples if you like, but I am 100% sure that no MR=1 rocket could perform as well as a MR=12 rocket based only on a 350 vs 450 ISP difference.


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Post    Posted on: Fri Jul 07, 2006 5:35 am
That was MR = 18, the forum turned my "8 )" into a smiley. :roll:

The DRY weight of a rocket includes the empty tanks. My research suggests that rocket engines would be approximately 25% of the mass that reaches orbit for an SSTO. Slightly less for dense fuels.

Also, LOX / LH2 engines are generally twice as heavy for the same thrust as RP-1 engines. (Even though the mass flow rate is a bit lower, the volume flow rate is much higher because of the low density. So the combustion chamber & nozzle have to be much bigger)

So yes the RP-1 vehicle might need engines a little heavier to develop the same liftoff acceleration, but it could also afford to develop less, since the total weight will decrease rapidly as fuel is burnt. Maybe 1.2 or 1.3 G's compared to 1.5 or more for LH2. The fuel tank doen't need insulation, but the thrust structure needs to be stronger. All these factors roughly balance out. The dry masses wouldn't differ by much, and the RP-1 vehicle has a substantial margin to play with.

Gravity losses are also reduced because the weight decreases faster.
This is why high density/lower ISP fuels are preferred for first stages. As I read recently:

"An SSTO is a first stage!"


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Post    Posted on: Fri Jul 07, 2006 7:00 am
So obviously it is far from clear that Heavy Lifters are totally better than, superior to or more economical than other rockets or vehicles.

But I am far from ready with looking into and looking for functions - I already identified two more at least which I still need to think about.



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Post    Posted on: Fri Jul 07, 2006 1:08 pm
WannabeSpaceCadet wrote:
That was MR = 18, the forum turned my "8 )" into a smiley. :roll:
That makes a lot more sense!

Quote:
My research suggests that rocket engines would be approximately 25% of the mass that reaches orbit for an SSTO.
Yeah, I did some checking and found that too. I guess I was thinking of how light weight the tank alone is on vehicles like Atlas or even Falcon and just assuming all the other mass was engines. My bad.


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Post    Posted on: Mon Jul 10, 2006 11:03 am
Regarding the table or propellants demonstrating that there it includes a funtion linking the Isp to Klt something essential has to be added - the table is valid under the following conditions:

Code:
external pressure                 100 kPa (Sea Level)
burning chamber pressure   7000 KPa (68 atm)
chemical equilibrium
adiabatic burn
isotrope tension release of gases


These are translations from German into English and may be wrong partially at least.

Because some of the propellants are solid ones while others are liquid ones and a third group gases that have to be liquified three groups have to be distiguished - which in principle seems to be a further step towards practice but from my point of view is a property of those propellants which forces differences of tanks and engines. The gases to be liquified require cryogenic technologies and/or pressurization while the might be problems regrading solids that don't exist regarding liquids that don't need cryogenic technologies but can flow.

So there seems to be a function linking propellants to the kind of engine. These kinds may be of different weight or different levels of weight Klr. So once again a possible requirement to keep vehicles and rockets in separate groups seems to show up - which complicates once more the question if preferences for HLLV or LLLV are justified or even possible...



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Last edited by Ekkehard Augustin on Wed Jul 12, 2006 8:30 am, edited 1 time in total.



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Post    Posted on: Mon Jul 10, 2006 1:03 pm
Ekkehard, it seems to me you are considering too many variables at the same time. To compare heavy to light, you should keep all variables except weight the same. To compare reusable to expendable, you should keep all other variables except number of uses the same. Adding different engines, fuels, pressures and so on will just confuse the issue and make any comparisons meaningless. It will allow people to say that the light rocket is not better because it is light, it is better because of the engine or type of fuel or whatever.


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Post    Posted on: Mon Jul 10, 2006 1:22 pm
Hello, Peter,

In the HLLV-thread as well as in a few other threads nearly all what you said seems to be ignored completely - some or a few posters are talking as if it all were very simple.

And what I am doing here - regarding theory - is to consciously look and partially work out that is NOT that simple.

To keep a lot of variables constant to simplyfy comparisons is an artificial simplification which can be - and I am suspecting by a few is - abused to manipulate opinions in favor of personal interests to the diadvantage of all others.

The criterion for what variables to keep constant and what variables not objective properties of the vehicles and rockets should be chosen.

This required to look into all this.

At present I am not doing comparisons but look for the functions etc. which can be used to do the comparisons. - that's the Theory as base, as fundamentation of comparisons.

Within these functions, within this theory it later can be tried to find the vectors of numbers that are the reality, real vehicles, real rockets - which means that then comparisons are possible.

I am not consideraing the variables/constants yet - I am looking for them. And some of the variables I already made constants - they are part of the Ziolkovsky-formular.

I already can see a concept forming as if a crystal is growing.



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Post    Posted on: Tue Jul 11, 2006 9:15 am
Ekkehard,

I think the problem is too complex and the variables too interrelated to define a single equation. I suspect the best approach would be to build a computer model as follows:

Engineering:

There are many engineering parameters that can be considered, such as:
Propellant ISP
Propellant Density
Propellant Temperature
Pressure fed or pump fed engines
If pump: pressure or turbo driven, closed or open cycle etc
Tank Pressurization: Vapour, He or N2, blowdown or maintained
Tank construction materials and techniques
Engine operating pressure
Engine cooling: ablative, radiative, regenerative
Number of engines
Safety margins
Unit construction time
Expendable or re-useable
If re-useable, recovery method, and refurbishment requirements and time

And all these and more may be repeated for 2 or more stages, with staging point(s) specified.

All these could be fed into a program that calculated development and unit costs as a function of payload mass to orbit. It could even vary some of the factors to produce multiple solution sets. Other factors have discrete values that limit other factors to limited ranges of values.

Regulatory:

What the government will let or make you do and how much it will cost.

Financial:

The market then has to be determined as a function of unit mass to orbit.
As do fixed and unit launch infrastructure costs. Also the method of financing the project wil need to be included.

Only then can the lowest cost point be determined for a PARTICULAR engineering model. The same process would have to be run for every combination of factors under consideration.

Oh, and I'm sure that I have left out many other factors, both engineering and financial. Also many factors I have included need a great deal of research to determine with any level of accuracy. Minor errors in a few could lead to order of magnitude differences in the final results. (See the Space Shuttle :cry: )

In Summary:

What you build, how you build it, how big you build it, where you build it, why you build it and who pays for it, are all going to have a massive influence on the final cost per unit mass to orbit. I doubt you can work backwards to find the best solution. All you can do is model a lot of them, and pick a good one. :D


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Post    Posted on: Tue Jul 11, 2006 9:40 am
Hello, WannabeSpaceCadet,

I don't have in mind a single equation - I never had. From Economics I am used to having several equations in parallel and put into a system of equations - and the term equation can be replaced by the term functions also. I am used to handle such systems even if they tend to be complex - that's required in Economics to do analyses, investigations, studies and the like. To do it is part of exams - without it students of Political Economics couldn't get their Diploma when I got mine. And in Enterprise Economics it's a base of Operations Research and to find possible decision-alternatives. The systems are modified by a perticular method called Linear Programming or LP in short - and there also is Non-Linear Programming.

I don't have in mind to do LP here or to apply the methods of Economics - I only want to clarify that I know to handle the complexity.

In so far you are correct regarding the computer model - you have a model in mind that's that large that it wouldn't fit into the Technology section now longer.

I am not looking for costs here - I have in mind a simple technological comparison here only.

And the thread is not about construction - but about structuring all theoretical possible vehicles and rockets and later all vehicles and rockets existing in practice in the past and the presence or under development or design currently.

I simply suspect that limitations and restrictions of considerations to vehicles of a particular capacity are misleading and have a look into the problems of such restrictions and limitations.

Like I already said
Quote:
I already can see a concept forming as if a crystal is growing.
- and that concept isn't that complex. It's a kind of system of families of vehicles.

The concept might be considered to be distantly compared to a study what's offered at the market - but distantly only. It's not a market study.

The structuring seems to be required urgently.


...





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Post    Posted on: Wed Jul 12, 2006 6:52 am
Ok, sounded like you were trying to come up with only 1 equation.

By "structure" do you mean to compare different vehicles? If so what is your "figure of merit" to grade them by, if not cost?

Max Payload Mass / Empty Mass?
Max Payload Mass / Gross Liftoff Mass?
Max Payload Mass / Propellant Mass?
Payload Mass Launch Rate? (e.g. tonnes per year)

It would be easy to give examples of vehicles that rate very high in the first 3 but low in the last, and vice versa.

The generally acknowledged standard is Dollars per kg to Low Earth Orbit.
Any other measure of efficiency can be misleading, unless there are special circumstances. (e.g. Must fit into a missile silo or submarine, or must launch on very short notice)


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Post    Posted on: Wed Jul 12, 2006 8:27 am
If SpaceX manage to get their systems sorted out and fly Falcon 1.1 then proceed on to fly their 9 and 5 birds in various configurations over the next few years, we'll have a real life example of the latest costs of a modern private company associated with different vehicle configurations.

Assuming they fly them all then we'll have F1, F5, F9S5, and F9S9. Then there's 3 engines - Kestral, Merlin and Merlin2 as well as the another big engine in the works. Should be lots of interesting info' available if SpaceX survive.
It will then be interesting to see how close you manage to come using you estimation models Ekkehard.

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Post    Posted on: Wed Jul 12, 2006 8:36 am
Hello, WannabeSpaceCadet,

the structure will distinguish several families or kinds of vehicles. At present it seems to me that at least one of the criterions will be the share of the payload mass of the total mass at or before launch and to be accelerated to that constant velocity chosen as Kv.

But I am still developeing that. Previously I will modify the table I have posted recently because those propellants include an inherent structure I already described rawly a bit. This tructure needs to be applies.



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Post    Posted on: Fri Jul 14, 2006 8:48 am
Ekkehard Augustin wrote:
...At present it seems to me that at least one of the criterions will be the share of the payload mass of the total mass at or before launch and to be accelerated to that constant velocity chosen as Kv....

That's what I was afraid of. It seems to me that what we all want is much less expensive and much larger volume, access to space. The Payload Effciency (payload mass / total mass) is completely irrelevant to this goal.

Current vehicles are hugely expensive, partly because they try to maximize Payload Efficiency.


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