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An SSTO as "God and Robert Heinlein intended".

Posted by: RGClark - Tue Jan 04, 2011 8:37 am
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An SSTO as "God and Robert Heinlein intended". 
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Space Walker
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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Thu Jun 16, 2011 4:30 pm
ckpooley wrote:
Rob's remarks re parallel staging being best can only apply to large rockets, where all stages deal with aerodynamics. For smaller rockets it would be better for the upper stage(s) be inside the first stage, and that to be optimized for taking the upper stages to vacuum. The upper stages being free of aerodynamic restraints can then be lighter, more efficient.
The first stage then could be design for durability, re-useability. It can be structurally heavier, as it would not be called upon to go very fast and could have a low mass ratio. MR for the Microlauncher stage one is to be about 2.5.
And, again, an engine optimized for vacuum cannot work well at sea level. The Atlas center engine was an attempt, and it was a compromise.
So, 2 stage for LEO; 3 for escape makes sense.


When I was doing my calculations I was assuming the air drag took up a relative small proportion of the required delta-V to orbit. This has been the case for most orbital rockets developed, even for the space shuttle system with its rather ungainly design at launch. See for example this list here:

Drag: Loss in Ascent, Gain in Descent, and What It Means for Scalability.
Quote:
Ariane A-44L: Gravity Loss: 1576 m/s Drag Loss: 135 m/s
Atlas I: Gravity Loss: 1395 m/s Drag Loss: 110 m/s
Delta 7925: Gravity Loss: 1150 m/s Drag Loss: 136 m/s
Shuttle: Gravity Loss: 1222 m/s Drag Loss: 107 m/s
Saturn V: Gravity Loss: 1534 m/s Drag Loss: 40 m/s (!!)
Titan IV/Centaur: Gravity Loss: 1442 m/s Drag Loss: 156 m/s

http://gravityloss.wordpress.com/2008/0 ... alability/

It is true though that smaller rockets will incur relatively more air drag loss. I don't have the computer programs to simulate this. Perhaps you do have some that can do the numerical calculations.
I would like to see what numbers you get if you used the lightweight structures of your upper stage but with the sea level-to-vacuum optimized engines for a bimese type launcher. Assume that with lightweight materials, such as composites, you are able to maintain the lightweight of your upper stages but with the strength required for the aerodynamic pressures of ground launch.


Bob Clark

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Single-stage-to-orbit was already shown possible 50 years ago with the Titan II first stage.
Contrary to popular belief, SSTO's in fact are actually easy. Just use the most efficient engines
and stages at the same time, and the result will automatically be SSTO.
Blog: http://exoscientist.blogspot.com


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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Thu Jun 16, 2011 5:47 pm
Rob:

True, for large rockets, air drag is not a major loss, but the requirement to sustain passage through air is. That requires making the stage capable of that. And, especially for small rockets that is a problem.

Better to have the upper stage(s) enclosed. The example of Centaur is not quite right here--that is enclosed because it is a hydrogen fuel stage.

Non aerodynamic stages can be lighter, more efficient if they operate only in vacuum.


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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Thu Jun 16, 2011 5:51 pm
Rob: responding to 2nd 1/2 of earlier message: I do not intend to compare the enclosed upper stages with a combination using altitude adjusting engines.

Doing that would require assumptions not to be used in Microlaunchers design.

Already decided upon is that the first stage carry the upper stages internally.


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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Thu Jun 16, 2011 6:43 pm
does your first stage separate like a payload fairing? having a hard time visualizing how you make the contained design work esp wrt fuel tanks for the first stage.

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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Thu Jun 16, 2011 6:55 pm
I am considering 2 designs.
[1] there is to be an extension of tank tubing which is to contain the upper stages, and the nose splits open in 2 or 3 longitudinal sections, like flower petals, and locks in that position (altitude >60 km). The 2nd stage is to launch out of the tube. There would be vents at the bottom of the extension for exhaust.

The locked petals would act as air brakes to effect a tail firat reentry and velocity low enough for a drogue chute.

[2] With a split tubular extension and a solid nose, a longitudinal panel would open, carrying the two stages (>60 km), and 2nd stage would launch from that.

In either case, the 1st stage is to maintain a small acceleration from a 30 km powered ascent to the staging altitude to keep propellants settled. Propellant mass to do this will be less than 1 kg.

The first version of stage one is to be parachute recovered an the launch site.


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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Thu Jun 16, 2011 8:18 pm
ckpooley wrote:
Rob: responding to 2nd 1/2 of earlier message: I do not intend to compare the enclosed upper stages with a combination using altitude adjusting engines.
Doing that would require assumptions not to be used in Microlaunchers design.
Already decided upon is that the first stage carry the upper stages internally.


OK. But let me illustrate how much more payload you can carry with bimese staging with cross-feed fueling. Note this still works without altitude compensation engines. Those just improve the performance.
The first post in this thread was on getting a SSTO from the Delta Thor first stage by swapping out the original low efficiency RL-27 engine for the high efficiency NK-33.
I'll do the calculation without using altitude compensation. A problem with doing these payload to orbit estimates is the lack of a simple method for getting the average Isp over the flight for an engine, which inhibits people from doing the calculations to realize SSTO is possible and really isn't that hard. I'll use a guesstimate Ed Kyle uses, who is a frequent contributor to NasaSpaceFlight.com and operator of the Spacelauncereport.com site. Kyle takes the average Isp as lying 2/3rds of the way up from the sea level value to the vacuum value. The sea level value of the Isp for the NK-33 is 297 s, and the vacuum value 331 s. Then from this guesstimate the average Isp is 297 + (2/3)(331 - 297) = 319.667, which I'll round to 320 s.
The gross mass for the Delta Thor first stage was 84,067 kg with an empty Mass of 4,059 kg, giving a propellant mass of 80,008 kg. We're using an engine in the NK-33 that is 200 kg heavier so the empty mass is increased to 4,259 kg, and the gross mass to 84,267. Take as the delta-V required for orbit as 8,900 m/s, approx. 300 m/s less than that needed for a hydrogen fueled rocket since dense propellant rockets incur less gravity loss. Calculate first the payload this could carry as a single stage to orbit:

320*9.8ln(1 + 80,008/(4,259 + 700)) = 8,909.6 m/s. So the payload possible is 700 kg.
Now calculate the payload possible with bimese staging with cross-feed fueling. Estimate the payload as 6,200 kg:

the first stage delta-V will be: 320*9.8ln(1 + 80,008/(4,259 + 84,267 + 6,200)) = 1,920 m/s. Note that the 84,267 gross mass of the second stage is in the ending mass for the first stage burn because the entire propellant load is retained for the upper stage, as both engines get their fuel from only the first stage tanks while they are connected together,
And the second stage delta-V will be: 331*9.8ln(1 + 80,008/(4,259 + 6,200)) = 6,999 m/s, for a total delta-V of 8,919 m/s.
So you get a 9 times greater payload to orbit by using the bimese staging with cross-feed fueling.


Bob Clark

_________________
Single-stage-to-orbit was already shown possible 50 years ago with the Titan II first stage.
Contrary to popular belief, SSTO's in fact are actually easy. Just use the most efficient engines
and stages at the same time, and the result will automatically be SSTO.
Blog: http://exoscientist.blogspot.com


Last edited by RGClark on Fri Jun 17, 2011 8:30 am, edited 1 time in total.



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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Thu Jun 16, 2011 9:20 pm
Bob, what you're saying makes a lot of sense as soon as you replace "SSTO" with "highly efficient". You keep using the term SSTO however, and that misleads your readers, because it makes them think that this is about the disadvantages of staging. At least this reader does...

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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Fri Jun 17, 2011 9:22 am
Lourens wrote:
Bob, what you're saying makes a lot of sense as soon as you replace "SSTO" with "highly efficient". You keep using the term SSTO however, and that misleads your readers, because it makes them think that this is about the disadvantages of staging. At least this reader does...


SSTO is not a bad word. It is a very good word. It is my contention that the reason why launch costs are so high, the reason why we don't have passenger access to space as routine as say trans-Pacific flights is that the idea has been promulgated that SSTO is impossible. That is not the case. In fact it is easy, IF you do it in the right way. The right way is summarized in that one simple sentence at the end of my sig file.
We all know that to get a good payload to space you want a high efficiency engine. And we all know we want to use lightweight structures so the weight savings can go to increased payload. So you would think it would be obvious to use both these ideas to maximize the payload to orbit, right?
And indeed both have been used together - for upper stages. Yet this fundamentally obvious concept still has not been used for first stages. It is my thesis that if you do this then what you wind up with will automatically be SSTO capable. This is true for either kerosene fueled or hydrogen fueled stages.
Part of the misinformation that has been promulgated is that mass ratio for SSTO's is some impossible number. This is false. We've had rocket stages with the required mass ratio's since the 60's, nearly 50 years, both for kerosene and hydrogen fueled. Another part of the misinformation is that it would require some unknown high energy fuel and engine to accomplish. This is false. The required engines have existed since the 70's, nearly 40 years, both for kerosene and hydrogen fueled.
What has NOT been done is to marry the two concepts together for first stages. All you need to do is swap out the low efficiency engines that have been used for the high mass ratio stages and replace them with the high efficiency engines. It really is that simple.
This makes possible small, low cost orbital vehicles that could transport the same number of passengers as the space shuttle, about 7, but would have a comparable cost to a mid-sized business jet, a few tens of millions of dollars.
Then once you have the SSTO's they make your staged vehicles even better because you can carry greater payload when they are used for the individual stages of the multi-staged vehicle.


Bob Clark

_________________
Single-stage-to-orbit was already shown possible 50 years ago with the Titan II first stage.
Contrary to popular belief, SSTO's in fact are actually easy. Just use the most efficient engines
and stages at the same time, and the result will automatically be SSTO.
Blog: http://exoscientist.blogspot.com


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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Fri Jun 17, 2011 2:29 pm
Rob's June 16 post:

"OK. But let me illustrate how much more payload you can carry with bimese staging ..."

The post misses the point I was trying to make. Any parallel staging requires more than one stage engine operate at launch, and exposes more than one stage to aerodynamic loads--neither being good for small designs, and not for Microlaunchers.

A LEO launcher can be 2 stages, escape 3. Only the 1st needs to start at low altitude and deal with aerodynamic loads. With the upper stages being operable only in vacuum, the engine(s) can be optimized for vacuum only and will be more efficient than any compromise design engine, or (immature technology) altitude compensating engines.

The Q at 60 km will be low enough so only a very thin shield mounted on 2nd stage will be needed to protect the 3rd stage.

By placing the heavy duty loads on the first stage, and dropping those as early as possible, the overall capability is going to be greater than that of any other design.

Also, with a 1st stage velocity being low, launch site recovery of this heaviest part should be easy.


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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Fri Jun 17, 2011 6:49 pm
The cross-fed bimese staging bothers me a bit because one tank will be full and the other one empty at first-stage burnout. This moves the Cg over to the full stage instead of being between the two halves of the rocket. The 6,200 Kg payload is also sitting on this side, which makes matters even worse. It seems like this will create some unusual stresses on the rocket.

A cross-fed trimese configuration may be better. This will make things symmetrical, and it increases the payload capacity by more than 70% to 10,750 Kg. Using Bob's numbers, but adding another rocket to the first stage, I get a delta-v of 2,930 m/s from the first stage, and 5,986 from the second stage, for a total delta-v of 8,916 m/s.


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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Fri Jun 17, 2011 8:28 pm
complete side note, didn't heinlein have to invent his "isotope fuel"/"X-fuel" or whatever to get SSTO to work in his books?

i love cross-feed fueling, SSTO while attractive seems fundamentally impractical using traditional chemical fuels. i don't see how this difficulty will ever be overcome. there's not really any reason other than integration costs for having an SSTO versus a reusable staged design anyways. after all, how will you possibly get the entire body to be able to re-enter successfully? unless you're building a space station out of spare fuel tanks, there's no point in SSTO unless you can recover it.

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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Sat Jun 18, 2011 10:08 am
RGClark wrote:
SSTO is not a bad word. It is a very good word. It is my contention that the reason why launch costs are so high, the reason why we don't have passenger access to space as routine as say trans-Pacific flights is that the idea has been promulgated that SSTO is impossible.

So, are you saying that using more than one stage will be much more expensive than SSTO? Why? If you do it like SpaceX, with two very similar stages, the only difference is an additional bulkhead and a separation mechanism. Will that make things significantly more expensive? I don't know where to start calculating on that, but I suspect that with the extra payload that the staging gives you, it may actually lower your cost per pound to orbit.

I'm not saying that SSTO is inherently a bad thing, just that it doesn't look like the best tool for the job.

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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Sat Jun 18, 2011 1:01 pm
I don't think Bob is promoting SSTO in his most recent postings. His description of bimese staging is a 2-stage rocket. He showed that SSTO alone does not yield a very large payload. It's just a question of whether you stack the stages or put them side-by-side.


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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Sat Jun 18, 2011 2:47 pm
DaveHein wrote:
I don't think Bob is promoting SSTO in his most recent postings. His description of bimese staging is a 2-stage rocket. He showed that SSTO alone does not yield a very large payload. It's just a question of whether you stack the stages or put them side-by-side.


Actually, I prefer to say I am promoting SSTO's. I'm saying with current technology that is the surest way to promote payload costs to orbit at the few hundreds of dollars per kilo range, and passenger costs to orbit at the few tens of thousands of dollars a ticket range.
Small reusable, SSTO's could be used for passenger flights carrying the same number of passengers as the Dragon capsule at a per unit cost for the vehicle comparable to that of business jets.
To carry large amounts of cargo, you combine these reusable SSTO's bimese or trimese fashion to get fully reusable multi-stage vehicles that allow you to carry large amounts of payload at costs 1 to 2 orders of magnitude lower than they are now.
In order to do that the realization that SSTO's are possible has to be made by using the most weight optimized stages and most sea level-to-vacuum efficient engines together.


Bob Clark

_________________
Single-stage-to-orbit was already shown possible 50 years ago with the Titan II first stage.
Contrary to popular belief, SSTO's in fact are actually easy. Just use the most efficient engines
and stages at the same time, and the result will automatically be SSTO.
Blog: http://exoscientist.blogspot.com


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Post Re: An SSTO as "God and Robert Heinlein intended".   Posted on: Sat Jun 18, 2011 2:55 pm
ckpooley wrote:
Rob's June 16 post:

"OK. But let me illustrate how much more payload you can carry with bimese staging ..."

The post misses the point I was trying to make. Any parallel staging requires more than one stage engine operate at launch, and exposes more than one stage to aerodynamic loads--neither being good for small designs, and not for Microlaunchers.


You may be right. It certainly is true that small launchers incur greater gravity loss than large ones. But I don't know how much greater. I'd like to see a simulation of the drag loss for the serial and parallel staging methods for small launchers.


Bob Clark

_________________
Single-stage-to-orbit was already shown possible 50 years ago with the Titan II first stage.
Contrary to popular belief, SSTO's in fact are actually easy. Just use the most efficient engines
and stages at the same time, and the result will automatically be SSTO.
Blog: http://exoscientist.blogspot.com


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