An SSTO as "God and Robert Heinlein intended".

[RGClark]

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?

Come on. EVERYBODY argues by authority. If you are having a discussion on relativity and you quote Einstein's opinion on an issue that's arguing by authority.


Bob Clark

 

[Urwumpe]

Come on. EVERYBODY argues by authority. If you are having a discussion on relativity and you quote Einstein's opinion on an issue that's arguing by authority.

No, that is absolutely horrible.

Einstein said that Black Holes are impossible and any such interpretation of his theories wrong. Today we know that Einstein was right in his theories but wrong about Black Holes. Einstein was wrong pretty often, and sometimes he admitted it.

There is no "because Einstein said so, it must be right". The same applies to any living person.

For me, arguing with Einsteins authority is futile. But using Einsteins arguments isn't. Einstein did never just say "I am Einstein, and this is fact." He explained his results.

Which is pretty much what we often demand from you. Not just claim something and then summon the divine authority of some engineer to back you up.

 

[RGClark]

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.

Yes they are and they're not even hard. In fact, they are no harder than the most efficient multi-stage launchers.
All you have to do is what you naturally do for the most efficient multi-stage launchers: use both the most efficient engines and the most efficient stages at the same time.
If you do that then what you will get will automatically be a SSTO stage. In fact the payload will be so high you can add reentry and landing systems and still carry significant payload. The full reusability will allow the costs to orbit to be cut by 1 to 2 orders of magnitude.
The only thing needed to accomplish this is to believe what the rocket equation is actually saying.

"Free your mind. The rest will follow."
- En Vogue

---------- Post added at 09:16 AM ---------- Previous post was at 09:03 AM ----------

No, that is absolutely horrible.
Einstein said that Black Holes are impossible and any such interpretation of his theories wrong. Today we know that Einstein was right in his theories but wrong about Black Holes. Einstein was wrong pretty often, and sometimes he admitted it.
There is no "because Einstein said so, it must be right". The same applies to any living person.
For me, arguing with Einsteins authority is futile. But using Einsteins arguments isn't. Einstein did never just say "I am Einstein, and this is fact." He explained his results.
Which is pretty much what we often demand from you. Not just claim something and then summon the divine authority of some engineer to back you up.

As I said, literally EVERYBODY argues by authority.
"Don't smoke cigarettes. Doctors say they're bad for you."
"Eat plenty of fruits and vegetables. Doctors say they are good for you."

Moreover, every academic paper in any field, science, the arts, the humanities, anything, that has ever been written for the last few hundred years argues by authority. The reason is they all use that format where you make an argument and you cite references at the end by experts in the field that support that position.
That's arguing by authority.


Bob Clark

 

[Ghostrider]

As I said, literally EVERYBODY argues by authority.
"Don't smoke cigarettes. Doctors say they're bad for you."
"Eat plenty of fruits and vegetables. Doctors say they are good for you."

That doesn't mean it's a valid argument. There's plenty of evidence that smoking is bad for you, visit pathology one of these days and take a look at a smoker's lung and you'll see. Doctors who are worth their salt will give you hard evidence to back their assertions.

As for scientific publications, they're not only peer reviewed, the data used to corroborate the findings must be made public as well. If tomorrow professor Hawking says he has invented a working transporter beam, wouldn't you want to see it working? Or just believe him because of who he is?

Argument by authority used to be called ipse dixit. Aristotle said that, so it's true. Doesn't work this way at all.

 

[RGClark]

That doesn't mean it's a valid argument. There's plenty of evidence that smoking is bad for you, visit pathology one of these days and take a look at a smoker's lung and you'll see. Doctors who are worth their salt will give you hard evidence to back their assertions.
As for scientific publications, they're not only peer reviewed, the data used to corroborate the findings must be made public as well. If tomorrow professor Hawking says he has invented a working transporter beam, wouldn't you want to see it working? Or just believe him because of who he is?
Argument by authority used to be called ipse dixit. Aristotle said that, so it's true. Doesn't work this way at all.

The point of the matter in this instance the expert does NOT just say it. He shows the equations that show it.
Note as well people who just make a blanket statement SSTO's are not possible are arguing by authority. They are relying on what people have said for decades from the earliest times of orbital rockets, not taking into account the advances in engines and materials since then.


Bob Clark

 

[T.Neo]

Yes they are and they're not even hard. In fact, they are no harder than the most efficient multi-stage launchers.
All you have to do is what you naturally do for the most efficient multi-stage launchers: use both the most efficient engines and the most efficient stages at the same time.
If you do that then what you will get will automatically be a SSTO stage. In fact the payload will be so high you can add reentry and landing systems and still carry significant payload. The full reusability will allow the costs to orbit to be cut by 1 to 2 orders of magnitude.
The only thing needed to accomplish this is to believe what the rocket equation is actually saying.

Let's take the S-II, strip the five J-2s out, stick three SSME on, and assume that the mass of the rest of the structure and the propellant loading stays the same.

John Schilling's Launch Vehicle Performance Calculator gives me a payload of ~15 tons.

The Atlas 531 has a similar payload, and it indeed does have a similar dry and wet mass. But here's the catch; the propulsion systems on the Atlas V are far less intensive than the SSME.

You don't want the most effective, efficient, high-tech engines. They have to withstand intensive engineering conditions and this makes them expensive.

Compare the 453 second SSME, costing $50-90 million, to the 409 second RS-68, costing $14-20 million. You do not want to aim for the most efficient engine possible if you can help it. You want to aim for the cheapest, most reliable engine that can get the job done.

I really do wonder what figures you calculated for reusability that compel you to say things like "reduce cost by 1 to 2 orders of magnitude". That simply won't happen, unless your vehicle works on pure magic...

And I really think, that all the recovery provisions for that rocket stage, would be at least on the order of 15 tons. Oops. There goes the payload... :shifty:

 

[RGClark]

...
I really do wonder what figures you calculated for reusability that compel you to say things like "reduce cost by 1 to 2 orders of magnitude". That simply won't happen, unless your vehicle works on pure magic...

It's coming from the idea that the the vehicle would be reusable in the range of, say, 100 times, instead of throwing it away after each use.
This also would require the maintenance costs to be low on each flight. Recall that one of the stated goals of the DC-X and the X-33 programs was to demonstrate this idea of low maintenance and quick turnaround time for space vehicles. The DC-X program did demonstrate this within it's operability envelope, which was low altitude, rocket flight. It would have been great to see for the X-33 if this is doable with vehicles that have to undergo reentry. Perhaps the X-37B will be able to demonstrate this.


Bob Clark

 

[T.Neo]

Refurbishment and maintainance have nonzero costs.

You might be able to optimise the costs pretty well (I believe there were some suggested improvements to the Shuttle's engines, for example, that would have made refurbishment easier), but they'll still be there, and I don't think it's a good idea to imagine them in an overly optimistic way.

Even Skylon only gets down to under $200/kg if it has a relatively high flight rate, which would not materialise without demand.

 

[Urwumpe]

It's coming from the idea that the the vehicle would be reusable in the range of, say, 100 times, instead of throwing it away after each use.

There is the free lunch.

This also would require the maintenance costs to be low on each flight.

And there is the price. There is no cheaper way to reduce maintenance costs than by throwing everything away. Producing a new vehicle is often cheaper than first evaluating the damage and then repair it.

Recall that one of the stated goals of the DC-X and the X-33 programs was to demonstrate this idea of low maintenance and quick turnaround time for space vehicles. The DC-X program did demonstrate this within it's operability envelope, which was low altitude, rocket flight. It would have been great to see for the X-33 if this is doable with vehicles that have to undergo reentry. Perhaps the X-37B will be able to demonstrate this.

AFAIR, it didn't achieve it except on the press releases. It did not do what Armadillo Aerospace also achieves on a fraction of the budget. Launch, land, lift-off again, land. And then get rolled into a hangar and get fixed.

The problem with this is, that it only gives the illusion of a quick overhaul. If you would measure the important metrics on 100 flights like that, you would quickly notice that the rocket again spends much more time on the ground than in the air, and the number of people involved repairing it in that time not being much lower than during the manufacture.

The only way to extend the time of the "little maintenance needed" mission phases is to extend the mean time between failures of the components. And this requires first of all a reduction in the peak performance. You can't go to the short-term limits, because you have now long-term limits. A seal deep inside the engine that has to be replaced after 25 minutes of operation is no longer an option.

And that is why I repeat myself again and again to explain you, that a SSTO is, while possible, just a PR stunt, something for the ivory tower, but no economic spacecraft. You can build one. Sure. Just go at the short-term limits of everything. It would not be reusable, but an SSTO that can carry really existing payloads. It would burn itself out during that flight and be on the brink of explosion seconds before MECO, but it would fly.

Even just reducing the short-term performance by 5% means even an expendable SSTO can't work. Let alone one that should be reused after flight and needs EDL systems.

But by just allowing two stages, you are already getting so much margin for your operations, that you could put more focus on economics and quality and less on performance (old classic engineering, for your SSTO, you would be at performance and quality or performance and economics)

A FFSC engine would likely have a 5% lower specific impulse as a comparable staged combustion engine. But its life time would be about 60-100 times longer, and the components in the engine not be forced to be designed for extreme conditions. There are still many problems in the technology (mixing gas generator exhaust and propellant flow, control of such a device) but it is really promising.

And thus I just can repeat again: you can dedicate your life building a SSTO. But somebody doing a simple TSTO would not just be done sooner, his second or third generation TSTO would already fly circles around your SSTO because of the faster development cycles.

And could maybe one day be a small first with a large second stage, that already is very close to a SSTO. Who knows what technology we will have in 20 years. But if you design an SSTO now, you can't include every new technology. you wouldn't get even to Phase B then, because of your long development cycles. A simpler TSTO could include new technology about every 5 years, similar to civilian aircraft. And still build on flight experience, that you wouldn't have.

I don't see much joy in SSTOs from the system engineering point of view. They are a fun challenge for bored engineers and aerospace students to make a Phase 0 study about them, but there is no practical way to design and build one without hell freezing over.

---------- Post added at 08:22 PM ---------- Previous post was at 08:18 PM ----------

Even Skylon only gets down to under $200/kg if it has a relatively high flight rate, which would not materialise without demand.

and remember: The Skylon doesn't even fly yet. While the calculations and planning has been found solid and the company managed to meet their engineering milestones, it does not mean the final flight costs will really be automatically at $200/kg.

Skylon is just a first generation vehicle for Reaction engines. I will not be surprised if things turn out different to the plans today. But if they manage to build their first vehicle and fly it, I see no reason to doubt that they could also build a second vehicle with much more competitive flight costs.

 

[T.Neo]

And could maybe one day be a small first with a large second stage, that already is very close to a SSTO.

This I find particularly interesting. If you take an SSTO, downgrade it considerably, and place it on top of a relatively 'small' first stage, you could end up with considerable performance.

A low seperation velocity for the first stage would make recovery easier as well.

and remember: The Skylon doesn't even fly yet. While the calculations and planning has been found solid and the company managed to meet their engineering milestones, it does not mean the final flight costs will really be automatically at $200/kg.

Very good point.

 

[Tacolev]

I'm still a fan of the "Plane so big it's still in ground effect at cruising altitude" first stage.

 

[RGClark]

I'm still a fan of the "Plane so big it's still in ground effect at cruising altitude" first stage.

How big would that have to be? Kilometers across?


Bob Clark

 

[T.Neo]

I'm still a fan of the "Plane so big it's still in ground effect at cruising altitude" first stage.

I didn't mean to imply a small stage as in physically small, but rather supplying a relatively small amount of dV and having a relatively low seperation velocity.

The seperation velocity of the EELV core stages is somewhere over 4000 m/s. The EELVs have very large first stages in comparison to their second stages.

Falcon 9 has a first stage seperation velocity of over 3000 m/s but under 4000 m/s, as far as I know.

Ares I, but comparison, had a very low first stage seperation velocity- 2000 m/s at the most, if I remember correctly.

So I'm talking about a stage that would have a relatively low seperation velocity, but would be enough to firstly take the second stage high up out of the atmosphere to enable it to use high expansion ratio engines, and secondly to add dV (of course) and let it fight gravity losses.

Of course, that means the second stage (the "SSTO" in this case) needs higher thrust. The EELV upper stages get away with low thrust because the first stages that deliver them to their "theatre of operation" take them to a pretty high velocity.

And another thing: P&W page on the Common Evolved Cryogenic Engine says it could perform 10 000 seconds in total of operation and 50 in-space starts. This could be enough for roughly 20 missions (including a two-burn profile). Could an engine really be built that could survive 20 consecutive flights without maintainance? What could such an engine do with maintainance ability on the ground?

The only majorly used reusable rocket engine we've had so far has been the SSME, and that has been a pretty high performance engine. I wonder what the refurbishment requirements would be like if someone tried to make a reusable engine with performance in the range of the J-2 or Vulcain... :hmm:

 

[RGClark]

From what I've read the DC-X did achieve its performance goals:

LESSONS LEARNED
DC-X OPERATIONS.
A.J. Polizzi
p. 8
http://www.ispcs.com/files/ww/files/presentations/nino.pdf

DC-X experimental lander set up Boeing for future NASA work.
by Ed Memi
"The DC-X was designed for reliability,
maintainability, supportability and operability.
Given the uncertainties of the design, the
plan was to produce a deliberately simple test
vehicle and to “fly a little, break a little” to
gain experience with a fully reusable quickturnaround
spacecraft. Demonstration objectives
included a 7-day turnaround between
flights with a 3-day goal and use of 50 or
fewer on-vehicle maintenance personnel.
The program achieved a 26-hour turnaround
with 10 maintenance personnel."
http://www.boeing.com/news/frontiers/archive/2008/aug/i_history.pdf

The new FFSC (full-flow staged-combustion) engine being investigated
though is interesting in being able to cut down the maintenance costs
while extending the engine life to the range of 200 missions:

New liquid rocket engine cycle fares well in test.
http://www.sae.org/aeromag/techfocus/10-2006/2-26-9-27.pdf

Rethinking engines.
March 8, 2007
Stephen J. Mraz
Rocket engines and the internal-combustion cycle get a remake, thanks to engineering ingenuity.
http://machinedesign.com/article/rethinking-engines-0308


Bob Clark

---------- Post added at 01:42 AM ---------- Previous post was at 01:20 AM ----------

Just saw this:

The SpaceX
Falcon Heavy Booster: Why Is It Important?
by John K. Strickland, Jr.
September, 2011
"What amazes people is that SpaceX has broken the long-sought 1,000
dollars a pound to orbit price barrier with a rocket which is still
expendable. 'How can he (SpaceX CEO Elon Musk) possibly do this?' they
ask. The Chinese have said flatly that there is no way they can
compete with such a low price. It is important to remember that this
was not done in a single step. The Falcon 9 already has a large price
advantage over other boosters, even though it does not have the
payload capacity of some of the largest ones. The 'Heavy' will even
this score and then some. At last count, SpaceX had a launch manifest
of over 40 payloads, far exceeding any current government contracts,
with more being added every month. These are divided between the
Falcon 9 and the Falcon Heavy."
http://www.nss.org/articles/falconheavy.html

I think the most important accomplishment of SpaceX might turn out
to be that they showed in stark terms that privately financed spacecraft,
both launchers and crew capsules, can be accomplished at 1/10th the
developmental cost of government financed ones. Imagine a manned,
reusable orbital launcher, for example, instead of costing, say, $3 billion,
only costing $300 million to develop.
As I argue, the key variable that made this reduction possible is that
the launcher was privately financed. That is, it was the launch
company's own money that was financing its development. In that case
it makes sense the company would be more fiscally responsible in
developing it.
Then the first step in reducing the price to orbit is making the
vehicles be privately financed. But if the launch companies are going
to spend their own money, they have to be convinced they can make a
profit on them. This will come if there is a significant market.
My view is that there would be a significant market for small,
privately owned, SSTO's. When you consider that with orbital refueling
such craft can also make lunar missions, the market becomes even more
apparent.
An additional finance stream of such vehicles that would make them
marketable I argue could be salvage of satellites in LEO or GEO:

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/

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


Remember the TV show Salvage 1?

Bob Clark

 

[Urwumpe]

Does not sound like it did meet its goals, when it took over twice as long as planned as goal. And the Armadillo vehicles had much shorter overhauls between flights... but that is because of their simplicity. NASA can't be simple.

Also, ONE 26 hours overhaul isn't that special - if it would have done so for a whole week or month, it would have been a good start. For commercial orbital services, you need to plan with one day in flight and maximal 12 hours on the ground - otherwise you have too much time on the ground for one vehicle to really cut costs. It isn't really about being able to launch every second day. But about doing many flights on demand when the customer needs it, with a minimal number of vehicles. Normal rockets already approach 14 days from order to launch.

 

[T.Neo]

Wasn't the DC-X a McDonnel Douglas project first? Wikipedia seems to make out here that was transferred to NASA when it became the DC-XA.

Also, an interesting excerpt from the same article:

In a post-accident report, NASA's Brand Commission blamed the accident on a burnt-out field crew who had been operating under on-again/off-again funding and constant threats of outright cancellation. The crew, many of them originally from the SDIO program, were also highly critical of NASA's "chilling" effect on the program, and the masses of paperwork NASA demanded as part of the testing regimen.

NASA, picture of efficiency...

 

[RGClark]

The SpaceX
Falcon Heavy Booster: Why Is It Important?
by John K. Strickland, Jr.
September, 2011
"What amazes people is that SpaceX has broken the long-sought 1,000
dollars a pound to orbit price barrier with a rocket which is still
expendable. 'How can he (SpaceX CEO Elon Musk) possibly do this?' they
ask. The Chinese have said flatly that there is no way they can
compete with such a low price. It is important to remember that this
was not done in a single step. The Falcon 9 already has a large price
advantage over other boosters, even though it does not have the
payload capacity of some of the largest ones. The 'Heavy' will even
this score and then some. At last count, SpaceX had a launch manifest
of over 40 payloads, far exceeding any current government contracts,
with more being added every month. These are divided between the
Falcon 9 and the Falcon Heavy."
http://www.nss.org/articles/falconheavy.html

I think the most important accomplishment of SpaceX might turn out to be that they showed in stark terms that privately financed spacecraft, both launchers and crew capsules, can be accomplished at 1/10th the
developmental cost of government financed ones. Imagine a manned, reusable orbital launcher, for example, instead of costing, say, $3 billion, only costing $300 million to develop.
As I argue, the key variable that made this reduction possible is that the launcher was privately financed. That is, it was the launch company's own money that was financing its development. In that case it makes sense the company would be more fiscally responsible in developing it.
Then the first step in reducing the price to orbit is making the vehicles be privately financed. But if the launch companies are going to spend their own money, they have to be convinced they can make a profit on them. This will come if there is a significant market.
My view is that there would be a significant market for small, privately owned, SSTO's. When you consider that with orbital refueling such craft can also make lunar missions, the market becomes even more apparent.
...

Space vehicle launches could be routine if they could take off horizontally from airliner runways as a single stage, like aircraft. It was thought that wings would just be dead weight on ascent but in fact following a lifting trajectory can cut in the range of 40% off the propellant requirements from a SSTO if it is at high lift/drag ratio. So wings can "carry their own weight", so to speak even on ascent.
Here's a heuristic argument that an SSTO making a lifting trajectory at high L/D ratio can save on propellant requirements. I'll regard the straight-line path as my X-axis and the perpendicular to this as the Y-axis. Note this means my axes look like they are at an angle to the usual horizontal and vertical axes, but it makes the calculation easier. Call the thrust T, and the mass, M. Then the force component along the straight-line path, our X-axis, is Fx = T - gMsin(θ) - D and the force
component along the Y-axis is Fy = L - gMcos(θ).
We'll set L = gMcos(θ), since the vehicle is traveling along the straight-line, our X-axis, so the force component in the Y-direction is zero. Then the force along the straight-line is Fx = T -gMsin(θ) - gMcos(θ)/(L/D). As with the calculation for the usual rocket equation, divide this by M to get the acceleration along this line, and integrate to get the velocity. The result is V(t) = Ve*ln(M0/Mf) - g*tsin(θ) - g*tcos(θ)/(L/D), with M0 the initial mass, and Mf, the mass at time t, a la the rocket equation. If you make the angle θ (theta) be shallow, the g*tsin(θ) term will be smaller than the usual gravity drag loss of g*t and the (L/D) divisor will make the cosine term smaller as well.
Now note that the equation includes *both* the gravity and air drag. Secondly, note that though using aerodynamic lift generates additional, large, induced drag, this is covered by the fact that the L/D ratio includes this induced drag, since it involves the *total* drag.
Some preliminary calculations I did suggest you could save in the range of 40% off your propellant requirements by reducing the gravity loss in this fashion if indeed your L/D ratio is 7+, at the speed range up to the high supersonic to low hypersonic.
A reduction in the propellant requirements this high means you could carry significant payload to orbit even with standard wing weight, estimated as 10% of the vehicle weight, which would be the gross takeoff weight for a horizontally launched vehicle.
However, we can cut this weight even further. First, even for standard wings you could cut this wing weight by half with modern materials. But a key fact is that you don't even need wings for a horizontal liftoff. Lifting bodies can perform horizontal takeoffs without additional wings. Lifting bodies have long been investigated for use as reentry vehicles, but it seems to have been overlooked to use them also for horizontal takeoff.
The problem though would be to get mass efficient propellant tanks for the vehicle. The propellant tanks are frequently the heaviest component of the dry weight of the vehicle, even more than the engines.
But the lifting body would have non-cylindrical shape. What killed the X-33/VentureStar program was the inability to get light-weight tanks for the X-33's conformally shaped tanks. Even using composite materials the non-circular cross sections would have resulted in tanks twice as heavy as aluminum tanks of usual cylindrical shape [1].
It turns out the determining factor for the heavy weight of rocket propellant tanks is they have to be pressurized. This is because of the requirements for proper operation of the turbopumps on rocket engines [2]. Pressurized tanks have to have a certain thickness to safely hold the contents.
However, quite key is the fact that this is a requirement of turbopumps but not other types of pumps [3]. XCOR because it's using wing tanks for its Lynx suborbital vehicle plans to use reciprocating (piston) pumps [4]. These require reduced pressurization in the tanks, if any. And the tanks in the wings on aircraft commonly are also not pressurized. XCOR had to develop these since they intend to carry the propellant in wing tanks which being non-circular would have been very heavy if they had to be pressurized.
Another possibility would be to use inflatable wings. These can also save weight over standard wings [5].


Bob Clark



1.)Space Access Update #91 2/7/00.
The Last Five Years: NASA Gets Handed The Ball, And Drops It.
"...part of L-M X-33's weight growth was the "multi-
lobed" propellant tanks growing considerably heavier than promised.
Neither Rockwell nor McDonnell-Douglas bid these; both used proven
circular-section tanks. X-33's graphite-epoxy "multi-lobed" liquid
hydrogen tanks have ended up over twice as heavy relative to the
weight of propellant carried as the Shuttle's 70's vintage aluminum
circular-section tanks - yet an X-33 tank still split open in test
last fall. Going over to aluminum will make the problem worse; X-
33's aluminum multi-lobed liquid oxygen tank is nearly four times as
heavy relative to the weight of propellant carried as Shuttle's
aluminum circular-section equivalent."
http://www.space-access.org/updates/sau91.html

2.)Fuel tank scaling laws (Henry Spencer).
http://yarchive.net/space/launchers/fuel_tank_scaling_laws.html

3.)[ame="http://en.wikipedia.org/wiki/NPSH"]NPSH - Wikipedia, the free encyclopedia[/ame]

4.)XCOR Aerospace and United Launch Alliance.
Announce Successful Hydrogen Piston Pump Tests.
http://www.xcor.com/press-releases/...nounce_successful_hydrogen_pumping_tests.html

5.)An inflatable wing using the principle of Tensairity.
http://www.empa.ch/plugin/template/empa/*/107170

 

[Urwumpe]

I would like to counter this lengthy argument with just one (composite) word :

Induced Drag

(Disclaimer: As much as I am a big big fan of space planes - you can only really discuss them by also naming the bad things)

 

[T.Neo]

The propellant tanks are frequently the heaviest component of the dry
weight of the vehicle, even more than the engines.

Wasn't this argued not to be the case? I tried to give examples in this post showing that engines could make up a fourth to a third the total stage dry mass, and I didn't include the thrust structure. And the figures here for the Atlas D show a booster mass of over 50% of the vehicle's dry mass! That isn't even including the sustainer engine and its thrust structure.


Also, does anyone have a source describing the mass breakdown percentages of rocket stages? I've been trying to find this information for months! :shrug:

 

[Urwumpe]

I only have this one for winged TSTOs.

http://smartech.gatech.edu/jspui/bitstream/1853/8398/1/aiaa_2001-4542.pdf