An SSTO as "God and Robert Heinlein intended".

[RGClark]

In post #136 I argued that small, low cost SSTO's are doable
now using lightweight design and high efficiency engines. However,
I was surprised to find after doing the calculation you don't even need
the high efficiency engines to get the SSTO. The low efficiency SpaceX
Merlin engines would be sufficient for example, IF you have altitude compensation.
The impetus for trying the calculation was from a report by SpaceX
that you could get the same performance from a planned heavy lift
first stage using a lower performance Merlin 2 engine compared to the
high performance RS-84 engine. The reason was the lower Isp of the
Merlin was made up for by its lower weight:

SpaceX Propulsion.
http://images.spaceref.com/news/2010/SpaceX_Propulsion.pdf


Now note that the biggest single contributor to the vacuum Isp of an
engine is not the chamber pressure, but the nozzle length. For
example, the Merlin Vacuum raises its vacuum Isp to 342 s from the 304
s Isp of the Merlin 1C by having a longer nozzle, even though the
chamber pressure remains the same, ca. 100 bar.
So I'll redo the calculation for the SSTO using the SpaceX Falcon 1
first stage but using Merlin engines this time. We'll assume that
using altitude compensation we are able to get an engine with the same
vacuum Isp as the Merlin Vacuum but able to launch from ground.
We'll use the soon to be introduced Merlin 1D:


SpaceX Plans To Be Top World Rocket Maker.
Aug 11, 2011
By Guy Norris
San Diego
http://www.aviationweek.com/aw/generic/story.jsp?id=news/awst/2011/08/08/AW_08_08_2011_p27-354586.xml&headline=SpaceX%20Plans%20To%20Be%20Top%20World%20Rocket%20Maker&channel=defense


Using the 160 to 1 thrust/weight ratio and 155,000 lbs. vacuum thrust
given, it has a mass of 970 lbs., 440 kg. However, this would make it
overpowered for the Falcon 1 first stage only. So we'll use two copies
of this stage powered by a single Merlin 1D.
The original Falcon 1 first stage with the Merlin 1C engine has a dry mass
of 1,360 kg. I estimated the mass of the Merlin 1C in the prior post to
be 650 kg. So without the engine, the stage weighs 710 kg. So two of
them will be 1,420 kg without engines, and adding on the Merlin 1D
engine gives this a mass of 1,860 kg.
The propellant mass of the two copies of the first stage is 43,080
kg. Then to calculate the payload that can be carried I'll again just
use the vacuum Isp and take the required delta-V as 9,150 m/s. We
conclude a payload of 1,140 kg can be lofted:

342*9.8ln(1 + 43,080/(1,860 + 1,140)) = 9,160.

Now we'll estimate how much the payload can be if we use a higher
energy density fuel such as methylacetylene and use lightweight
composites for the stage. I'll get a rough idea how high the Isp can
be for this case by assuming it is increased proportionally to the
same degree as for the high efficiency engine case. That is, using
methylacetylene in the high efficiency case resulted in increasing the
vacuum Isp to 384 s from the 360 s vacuum Isp for the kerosene.
Assuming the vacuum Isp will be increased to the same proportion here
gives us a vacuum Isp of 365 s for methylacetylene and the Merlin 1D
engine.
For the reduced stage weight using composites, assume again it will
be reduced by 40% aside from the engines. Then the stage weight with
the Merlin 1D engine will be .6*1,420 + 440 kg = 1,290 kg. Then will
be able to loft a payload of 2,320 kg:


365*9.8ln(1 + 43,080/(1,290 + 2,320)) = 9,160 m/s.


Also, quite likely SpaceX could make a half-size version of the
Merlin 1D engine. So you could use a single copy of the Falcon 1 first
stage. Then the payload would be approximately cut in half, 570 kg for
the kerosene/standard stage version and 1,160 kg for the
methylacetylene/composite stage version.

Note that low chamber pressure, low performance engines can also be
used to power the SSTO's is extremely important. Such engines have
less complicated combustion cycles and have to withstand much less
strenuous operating regimes. This makes them cheaper, simpler, easier
to maintain, and easier to make reusable. So the most costly component
of any rocket, the engines, become markedly cheaper for the proposed
SSTO.

What is key though is to come up with ways to get the needed altitude
compensation without adding on too much to the engine weight. In a
following post I'll discuss some methods this might be accomplished.



Bob Clark

 

[Loru]

seriously - create a blog.

If SSTO is such a good idea from engineering/economic point of view why we haven't got one already?

 

[T.Neo]

How many times do people have to tell you that you can't just make half-sized Merlin engines, you can't just switch over to composite tanks, and you can't just modify things as you wish? It is not trivial. The engineering in this case is very intensive and interconnected and complex.

I don't even read the bulk of your posts anymore, RGClark. I'd appreciate your enthusiasm and effort more if you actually took more of a look at real engineering constraints relating to this sort of stuff.

If SSTO is such a good idea from engineering/economic point of view why we haven't got one already?

For multiple reasons, not limited to those already discussed here...

 

[Urwumpe]

RGClark: If real engineering would be really that easy, my coworkers and me would be out of work, since just a single beard with a whiteboard is enough. Luckily, the real world is so complex, that we can't just stop employing new specialists here, despite us only doing CAx services for many more engineers at the actual projects - good work is always rewarded with more work, and real engineering is a lot of work.

Your first order estimates here already fail to correlate with any experience values, so don't you think you should better calm down a bit, take a deep breath and then calculate really conservatively?

If you think that it is below your dignity to do proper CAE on your first-order estimates to prove that you are right despite the conservative first-order estimates being far away from your claims: There are always people like my company to do the dirty numerical work for you and let you concentrate on the decision-making. But be aware: Specialists cost money, because while we like our job, we like our job to be profitable.

 

[RGClark]

seriously - create a blog.
If SSTO is such a good idea from engineering/economic point of view why we haven't got one already?

Boeing believes it might be doable:

Boeing proposes SSTO system for AF RBS program.
The new issue of Aviation Week has a brief blurb about a Boeing
proposal for the Air Force's Reusable Booster System (RBS) program:
Boeing Offers AFRL Reusable Booster Proposal - AvWeek - June.13.11
(subscription required).
"Darryl Davis, who leads Boeing's Phantom Works, tells AvWeek that
they are proposing a 3-4 year technology readiness assessment that
would lead up to a demonstration of a X-37B type of system
but would be smaller. Wind tunnel tests have been completed. Davis
says the system would be a single stage capable of reaching low Earth
orbit and, with a booster, higher orbits. The system would return to
Earth as a glider.
Davis says "that advances in lightweight composites warrant another
look" at single-stage-to-orbit launchers."
http://www.hobbyspace.com/nucleus/index.php?itemid=30110

A key technology that has to be accomplished is an altitude compensation method that doesn't add too much to the engine weight. Note that Reaction Engines is investigating one possibility with the expansion-deflection nozzle for their Skylon vehicle. I'll discuss some other possibilities in a following post.



Bob Clark

---------- Post added at 03:03 AM ---------- Previous post was at 02:53 AM ----------

RGClark: If real engineering would be really that easy, my coworkers and me would be out of work, since just a single beard with a whiteboard is enough. Luckily, the real world is so complex, that we can't just stop employing new specialists here, despite us only doing CAx services for many more engineers at the actual projects - good work is always rewarded with more work, and real engineering is a lot of work.
Your first order estimates here already fail to correlate with any experience values, so don't you think you should better calm down a bit, take a deep breath and then calculate really conservatively?
If you think that it is below your dignity to do proper CAE on your first-order estimates to prove that you are right despite the conservative first-order estimates being far away from your claims: There are always people like my company to do the dirty numerical work for you and let you concentrate on the decision-making. But be aware: Specialists cost money, because while we like our job, we like our job to be profitable.

I don't agree with that. Conservative estimates say you can do it with currently existing stages and engines. Use of composites just gives you better payload fraction.


Bob Clark

 

[T.Neo]

Doable does not mean viable. Viable does not mean "magically cuts costs down to the region of $100/kg".

Just because Boeing is supposedly doing some far-out study project doesn't mean they are actively pursuing something that is viable. Or even doable.

I don't agree with that. Conservative estimates say you can do it with currently existing stages and engines. Use of composites just gives you better payload fraction.

Always use conservative estimates. Always. It doesn't matter whether you're working with aluminium or steel or graphic epoxy.

Composites don't magically solve problems. They can have cost issues and thermal problems as well, for example.

 

[RGClark]

Doable does not mean viable. Viable does not mean "magically cuts costs down to the region of $100/kg".
Just because Boeing is supposedly doing some far-out study project doesn't mean they are actively pursuing something that is viable. Or even doable.

I don't agree it is far out. Boeing is well aware of the weight savings that can be accomplished using composites because of their experience with the Dreamliner 787 and the X-37B.
It's not a coincidence that Boeing is the one investigating this.


Bob Clark

 

[Urwumpe]

I don't agree it is far out. Boeing is well aware of the weight savings that can be accomplished using composites because of their experience with the Dreamliner 787 and the X-37B.
It's not a coincidence that Boeing is the one investigating this.

Don't let me get started on this one. Well, too late.

The Dreamliner is actually heavier when empty than the old Boeing 767 it is meant to replace. It just carries a tiny bit more fuel per dry mass, but when looking at the engineering inside an aircraft, this isn't because there is more volume for fuel in the Dreamliner, but because the Dreamliner has much more effective wings as the obsolete 767.

Effective weight-savings by Boeing when using composite structures? extremely little. The A330 of Airbus, that has only a conservative use of composite structures, has almost the same mass properties as the 787, but a slightly shorter range (but Airbus has much longer experience in that field than Boeing, especially since Europe did already do research for civilian use when Boeing left composite structures to military projects with high maintenance requirements)

Again: Composite materials are alone no guarantee for lower mass. They can be lighter, no question, especially in places with low dynamic loads, but overall, their impact is very low.

Engineering is a science, and every science has its paradoxes, because not everything follows a simple linear model. You might think that a part made of aluminum is lighter than a part made of steel for the same requirements, but that is often wrong. For example pistons in car engines are MUCH lighter when made of steel.

 

[RGClark]

Don't let me get started on this one. Well, too late.
The Dreamliner is actually heavier when empty than the old Boeing 767 it is meant to replace. It just carries a tiny bit more fuel per dry mass, but when looking at the engineering inside an aircraft, this isn't because there is more volume for fuel in the Dreamliner, but because the Dreamliner has much more effective wings as the obsolete 767.

You'll have to give to give me a reference on that one. What I saw was that use of composites did result in weight savings in the Dreamliner that will result in reduced fuel usage compared to a comparably sized aircraft.
Certainly, Rutan has shown weight savings in his aircraft and now with his spacecraft using composites.

Bob Clark

 

[Urwumpe]

You'll have to give to give me a reference on that one. What I saw was that use of composites did result in weight savings in the Dreamliner that will result in reduced fuel usage compared to a comparably sized aircraft.

[ame="http://en.wikipedia.org/wiki/Boeing_787_Dreamliner"]Boeing 787 Dreamliner - Wikipedia, the free encyclopedia[/ame]
[ame="http://en.wikipedia.org/wiki/Boeing_767"]Boeing 767 - Wikipedia, the free encyclopedia[/ame]
[ame="http://en.wikipedia.org/wiki/Airbus_A330"]Airbus A330 - Wikipedia, the free encyclopedia[/ame]

The question on the 787 is: Could you have build such wings without composites? I think no. But the composite hull for PR reasons should have been scrapped. No weight advantage there and lots of expensive maintenance in service.

Certainly, Rutan has shown weight savings in his aircraft and now with his spacecraft using composites.

Rutan could also make a lead aircraft fly. This isn't really about the material. Also most sail planes are also made of composites today, because of weight constraints, but still, you have to differ there a lot HOW the material is used.

A thrust structure made of composites for example (which is one of the heaviest structures in a rocket), would fail after a few seconds, regardless how strong you make it. Composites don't like strong vibrations. Same problem if you have a moving part that needs a certain hardness. Temperature resistance can be a problem. you can use composites for some decoupled structures that don't experience the full vibrations. But alone aerodynamic vibrations can mean a lot of trouble for simple composites structures.

If you properly combine composite structures with metal structures, you save weight. But there is not much room for composites in rockets. Some interstages work great, but tank structures of composites alone are pretty impossible, cryogen temperatures neutralize all advantages of composites and make them much weaker as comparable metal tanks.

 

[Tacolev]

As a bit of supreme irony, refusal to admit that an aluminum tank would be every bit as light and plenty stronger than a composite tank is what killed the last serious SSTO project--the X-33/Venturestar. A composite tank can't be any shape you like, and making it fit an aerodynamic profile results in heavy joining sections that make a metal tank lighter and stronger nine times out of ten.

 

[RGClark]

Rutan could also make a lead aircraft fly. This isn't really about the material. Also most sail planes are also made of composites today, because of weight constraints, but still, you have to differ there a lot HOW the material is used.

That's not the point. He focused on composites because of the weight savings. Note that for SpaceShipOne and SpaceShipTwo even the propellant tanks are composites.
Here's a video of the assembly of SpaceShipTwo indicating the lightness of the structure by the handling of the technicians:

Bob Clark

---------- Post added at 01:44 AM ---------- Previous post was at 12:59 AM ----------

...
If you properly combine composite structures with metal structures, you save weight. But there is not much room for composites in rockets. Some interstages work great, but tank structures of composites alone are pretty impossible, cryogen temperatures neutralize all advantages of composites and make them much weaker as comparable metal tanks.

No. It is quite key to get the weight savings for composites that the tanks also be composites. This is important because the tanks typically take up the largest portion of the dry weight of the rocket, even more so than the engines.

Andrews Space Delivers Composite Liquid Oxygen Tank to the
Air Force Research Lab.
http://www.andrews-space.com/news.php?subsection=MzI0

SUCCESSFUL QUALIFICATION OF FULL-SCALE,
ALL-COMPOSITE CRYOGENIC LOX TANK OPENS THE WAY
TO LOW-COST, RESPONSIVE LAUNCH-ON-DEMAND.
http://www.smad.com/42_inch_tank_pressrelease.pdf


Bob Clark

 

[Urwumpe]

The actual tank structure mass has always been very low on rockets, compared to the thrust structure holding the engines. About 70% of a rockets dry mass is engines and thrust structure.

Also, as tiny question to your ability to differ between press release and reality: The press release is from 2006, today is 2011. Five glorious or not so glorious years have passed. Notice something?

http://www.scorpius.com/about.htm

Not only is the company that is customer of Microcosm actually a spin-off of Microcosm, since 2006, nothing there happened at all.

Also: They talk always about overwrapped tanks in context of cryogenic fuel tanks. Those aren't really a new thing, even the Shuttle already contained overwrapped tanks. Overwrapped tanks are already standard technology, having a full-composite wrap structure isn't: Those aren't really popular, because of their rapid aging in a dynamic environment.

(overwrapped tanks are thin metal shells supported by a composite structure - no direct contact of the fuel with the composite, but making large tanks with that technology is extremely hard to impossible)

What is really interesting:

http://www.scorpius.com/Documents/First gauntlet thrown 2 _2_ _1_.pdf

Helium (gas) tanks without inner liner. This means a very high standard of quality assurance, since it is generally pretty hard to make gas-tight composites. But no cryotank. Also nothing of a large scale. And also nothing that is designed to deal with dynamic loads.

 

[RGClark]

The actual tank structure mass has always been very low on rockets, compared to the thrust structure holding the engines. About 70% of a rockets dry mass is engines and thrust structure.

This is not correct. The propellant tanks are always a significant mass for any rocket. This is especially true for hydrogen fueled vehicles since the tanks have to be so large to hold the low density LH2.
I am aware that on the Saturn V the thrust structure had to be particularly heavy. But this is probably the case because the Saturn V was such a large vehicle, with the large masses of the upper stages and payload that had to be lofted.
This report by an aerospace professor uses some mass estimating relationships that rocket engineers use to estimate masses in designing their vehicles:

Mass Estimating Relations
• Review of iterative design approach
• Mass Estimating Relations (MERs)
• Sample vehicle design analysis
http://spacecraft.ssl.umd.edu/academics/483F09/483F09L13.mass_est/483F09L13.MER.pdf

This shows that for a SSTO the thrust structure is a relatively small proportion of the dry mass compared to the mass of the engines and propellant tanks.
It also confirms that an expendable SSTO is feasible using hydrogen fuel.
However, these same mass estimation formulas can also be used to show that a dense propellant SSTO can be made of smaller dry mass to carry the same payload.


Bob Clark

 

[Urwumpe]

But there is absolutely NO advantage in expendable SSTOs.

Also, the estimates in the presentation are not really realistic, did you already apply the estimates to existing rockets? If they would be really first order or second order estimates, they should be slightly higher (conservative estimate) than the real masses.

The professor did not really go into details how he arrived at the formulas, also in the few cases you manage to see real measurement samples in a plot, you notice a really small number of samples.

This is good enough for teaching students the work flow, but I would recommend correcting the estimates - especially the tank and tank insulation masses are way off if you use real numbers, simply because you don't know how high the pressure in the tank is and how high the structural loads are that pass through the tank structure.

 

[RGClark]

The increasing problem of space junk is getting greater discussion recently:

Space junk at 'tipping point', now getting worse on its own.
More collisions generate more debris, so more collisions.
By Gavin Clarke
Posted in Space, 2nd September 2011 11:18 GMT
http://www.theregister.co.uk/2011/09/02/space_junk_danger/

One company is planning on reusable in-space vehicles to refuel and service satellites. The Air Force favors this since this may also be used to tow inactive satellites out the way of operable satellites, thus reducing the problem of space debris:

Article:
World's First Space Gas Station for Satellites to Launch in 2015.
by Clara Moskowitz, SPACE.com Senior Writer
Date: 15 March 2011 Time: 06:03 PM ET
"Until now, satellites orbiting around Earth have been limited by how much fuel they carry onboard. Once those tanks run dry, the satellites die, sometimes languishing in space as uncontrollable debris that then poses the risk of colliding with other spacecraft.
"The new plan offers the potential not just to extend the lives of working satellites, but to help combat the growing space junk problem. The satellite, called the Space Infrastructure Servicing (SIS) vehicle, is designed not just to transfer more fuel into existing satellites, but to inspect, tow, reposition and make minor repairs to them.
"In addition to its tank of fuel, the refueling satellite will carry a robotic arm that can be used to grab onto satellites and tug at stuck solar array panels, for example, or attempt other minor fixes to broken parts.
'This is a first-time-ever, huge, huge, huge event,' said Andrew Palowitch, director of the Space Protection Program, a joint project of U.S. Air Force Space Command and the National Reconnaissance Office, speaking at a National Research Council workshop on orbital debrislast week.
"Palowitch stressed that the ability to tow or refuel dead satellites in order to steer them out of the way would have a big impact on the growing problem of dangerous space debris clogging the crowded corridors of Earth orbit. [Worst Space Debris Events of All Time]
'In the context of debris removal, this is the absolute best and absolute most fantastic new venture for the entire space community,' he said.
"The refueling satellite will be able to move dead spacecraft to what's called the 'graveyard orbit,' where they are high enough that they should not pose a risk to working satellites, or maneuver them low enough that they break apart in Earth's atmosphere."
http://www.space.com/11135-satellite-refueling-mission-space-debris.html

Then the Air Force recognizes the usefulness of reusable vehicles, when they are in-space. However, the importance of reusable SSTO's is that they could also return these satellites to Earth for repair or salvage.
Remember the old science fiction series Salvage 1:

Salvage 1.
[ame="http://en.wikipedia.org/wiki/Salvage_1"]Salvage 1 - Wikipedia, the free encyclopedia[/ame]

The theme of the show was a small "home made" spaceship was used to return space junk to Earth. I used to think the show was quite implausible because the spaceship went all the way to the Moon and everybody "knows" it takes huge Saturn V sized rockets to do that.
However, as I discussed in post #136 small, low cost SSTO's are indeed possible. And it is a known fact that if you have refueling in LEO then an SSTO can go all the way to the Moon, land, take off, and return to Earth on that one single refueling. So in fact the idea of salvaging spacecraft or satellites from the Moon and/or from GEO is feasible with SSTO's and on-orbit propellant depots.
Then this provides another financial benefit for SSTO's for private developers and for the Air Force. Imagine being able to retrieve satellites, the largest of which can cost upwards of a billion dollars, for reuse or possibly for sale. This does though raise the question of what would be the salvage laws for space.


Bob Clark

 

[T.Neo]

I can't speak for what amount of vehicle mass the thrust structure makes up (I haven't a clue what percentage it is), but it would make sense for it to be quite heavy.

Engines, however, are easier to compare as their masses as individual items are usually available. Let's compare a few stages, using data on Encyclopedia Astronautica:

S-IC:
Unfuelled mass: 135 218 kg

F-1 engine mass: 8 391 kg

Engines as percentage of total dry mass: ~31%

Delta CBC:
Unfuelled mass: 26 760 kg

RS-68 engine mass: 6 597 kg

Engines as percentage of total dry mass: ~25%

Atlas CCB:
Unfuelled mass: 22 461 kg

RD-180 engine mass: 5 480 kg

Engines as percentage of total dry mass: ~24%

As you reduce tank mass (for example by moving to a pressure supported structure), engine mass becomes more and more of the mass of the vehicle. And while you can only reduce tank mass so much, you cannot really reduce engine mass, because lighter engines will be under more intensive engineering demands and will therefore be:

- Less reliable.

- More expensive.

- Increasingly difficult to reuse.

In short, you just have to live with engine mass.

Because engines are so heavy and high levels of thrust are not needed late in ascent, this makes jettisoning engines and uneeded support structure attractive.

If Astronautix is to be believed, the Atlas D 'booster stage' accounted for over 50% of the dry mass of the Atlas D vehicle!

But there is absolutely NO advantage in expendable SSTOs.

Agreed. NONE whatsoever, the extra cost and reliability issues of the seperation system are by far a better option than having to deal with the physical disadvantages of an SSTO.

 

[RGClark]

But there is absolutely NO advantage in expendable SSTOs.
Also, the estimates in the presentation are not really realistic, did you already apply the estimates to existing rockets? If they would be really first order or second order estimates, they should be slightly higher (conservative estimate) than the real masses.
The professor did not really go into details how he arrived at the formulas, also in the few cases you manage to see real measurement samples in a plot, you notice a really small number of samples.
This is good enough for teaching students the work flow, but I would recommend correcting the estimates - especially the tank and tank insulation masses are way off if you use real numbers, simply because you don't know how high the pressure in the tank is and how high the structural loads are that pass through the tank structure.

No offense, but he's a professor and the chairman of the dept. of aerospace engineering at the University of Maryland. I trust you will not be offended if I take his opinion more seriously on the issue than others. The formulas he was using are the same ones used by NASA in estimating masses of launch vehicles. At the end of that report by Dr. Akin he gives the references to the NASA technical reports where these formulas are described.
In discussing this issue of SSTO's I've found that the opposition is frequently of the nature of just assuming that they can't be done. Calculations that show otherwise are disregarded.


Bob Clark

 

[T.Neo]

In discussing this issue of SSTO's I've found that the opposition is frequently of the nature of just assuming that they can't be done. Calculations that show otherwise are disregarded.

Personally my opposition of SSTOs has nothing to do with insisting that it can't be done, it's about SSTOs not being practical. Don't get me wrong, I see the benefits of an SSTO system... but those benefits are not worth it if they come at a cost that is simply too high.

The fact that an S-IVB or S-II with SSMEs strapped onto it would be able to make orbit is a novelty and does not mean much for real-world LV design.

Trying to find optimal solutions isn't about taking your favourite idea and driving it to death. If an idea isn't that good of an idea, you move on and try to find a new idea which works better.

 

[Urwumpe]

No offense, but he's a professor and the chairman of the dept. of aerospace engineering at the University of Maryland. I trust you will not be offended if I take his opinion more seriously on the issue than others.

If you like to be argued by authority, fine. But if you are unable to talk to me like to a sentient being and don't have better arguments than "it is so because this professor said so", you might already have noticed that I am not impressed at all.

My personal experience with professors tells me that preparing lectures is not always very high on their schedule, and even the better ones often like to use one set of lecture compatible models done in a week over researching better models in a year.

Also, if I would be professor (which I am really not, but how could you tell if I don't tell you so), would my argument then become automatically better than the argument of somebody else who is not?