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Author Topic: Stratolaunch  (Read 71762 times)

Offline martinbayer

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Re: Stratolaunch
« Reply #240 on: January 19, 2019, 10:49:52 pm »
To me, the saddest thing is that while I never expected the system to make any economic sense with expendable rockets of any size and payload capacity, the longer term vision of a fully reusable spaceplane, which would essentially be a current version of the Interim Hotol concept from almost three decades ago, but with less risky downward release, had the potential to be a true game changer.
« Last Edit: January 20, 2019, 12:02:22 am by martinbayer »
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Offline Orionblamblam

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Re: Stratolaunch
« Reply #241 on: January 20, 2019, 12:01:14 am »
To me, the saddest thing is that while I never expected the system to make any economic sense with expendable rockets of any size and payload capacity, the longer term vision of a fully reusable spaceplane, which would essentially be a current version of the Interim Hotol concept from almost three decades ago, but with less risky downward release, had the potential to be a true game changer.

How so? An air-launched SSTO is only slightly less technologically difficult than a ground-launched SSTO, but with added logistical difficulties. Since it looks like there was only ever going to be a single carrier aircraft, all the work spent on developing the SSTO could have flushed right down the tubes if the carrier aircraft ever broke down, crashed or kerploded.
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Offline martinbayer

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Re: Stratolaunch
« Reply #242 on: January 20, 2019, 12:17:08 am »
If you look at gross weight sensitivity curves, even a *slight* reduction of delta-v requirements for the upper stage of a TSTO (you know, any air launched vehicle really isn't a true SSTO, old chap - the carrier aircraft actually is the first stage, really) can reap significant rewards, and the drop down release eases the logistics challenges as opposed to an off-top separation, plus the aircraft launch gives you flexibility with respect to target orbit inclinations. And if the concept proved to be workable, let alone profitable, chances are the system would have been replicated in some form or another even in case of a disastrous failure, just like the DH Comet accidents didn't prevent the jet age.
« Last Edit: January 20, 2019, 12:27:53 am by martinbayer »
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Offline Orionblamblam

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Re: Stratolaunch
« Reply #243 on: January 20, 2019, 12:32:33 am »
and the drop down release eases the logistics challenges as opposed to an off-top separation,

But it's far from clear that it easier than vertical launch from a pad. Obviously the best way to prove all these ideas is to build them all, but it is folly to assume before something is even built that it will necessarily be not only successful but superior.
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Offline martinbayer

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Re: Stratolaunch
« Reply #244 on: January 20, 2019, 12:37:58 am »
and the drop down release eases the logistics challenges as opposed to an off-top separation,

But it's far from clear that it easier than vertical launch from a pad. Obviously the best way to prove all these ideas is to build them all, but it is folly to assume before something is even built that it will necessarily be not only successful but superior.

I completely agree - that's why I mourn for the lost opportunity of actually putting a fully reusable TSTO HTHL system to the test.
« Last Edit: January 20, 2019, 12:48:43 am by martinbayer »
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Offline Archibald

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Re: Stratolaunch
« Reply #245 on: January 20, 2019, 01:22:06 am »
somewhere I have a tech paper where japanese engineers mention the exact numbers for Air Launch. I mean, how much delta-v advantage it provides.
Values were 600 m/s, 900 m/s and 1200 m/s according to the aircraft speed but also angle of attack at release - 30 degree seems to be optimal.
I'll seek the paper later and quote it.

Well, so average 1 km/s. Good, earth orbit is 9 km/s.  then 1 out of 9 is more or less 10%.
Alas, it doesn't work that way...

The real issue is that the rocket equation is non linear, exponential, and with a logarithm stuck into it.
Hence 1 km/s out of 9 is not 10% but even less - 5% perhaps. 

Hence air launch cut barely 5% of the delta-v to orbit.

Well, 5% is not enough to justify all the hassles of carrying a big rocket under a big plane.

Virgin went around that limitation by using a plain old 747 from the Virgin Atlantic fleet. Clever, really, it probably didn't cost much to buy the aircraft. I was barely a transfer from one Branson company to another Branson company ! Still the maintenance cost of a 747 have to be paid. We will see if they manage to get any money out of Launcher one. Maybe Tristar = too small and Roc = too big, hence 747 = right size.

« Last Edit: January 20, 2019, 01:24:05 am by Archibald »
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Offline Orionblamblam

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Re: Stratolaunch
« Reply #246 on: January 20, 2019, 02:23:19 am »

I completely agree - that's why I mourn for the lost opportunity of actually putting a fully reusable TSTO HTHL system to the test.

The aircraft still exists. The people who are responsible for it will probably be desperate to figure out how to make money with it. In the end that will probably mean sending it to the chop shop. But if a good case could be made that an air-launched SSTO would make sense, *somebody* might be willing to develop it, and hire the Stratolaunch to lurch it into the sky.

Remember that the original launcher for Strato was to be a modified Falcon 9. But the modifications needed to carry the thing horizontally were too much of a pain and SpaceX pulled out. This should be instructive for the dream of hauling an SSTO with the structure of an eggshell.
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Offline blackstar

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Re: Stratolaunch
« Reply #247 on: January 20, 2019, 06:54:56 am »
If you look at gross weight sensitivity curves, even a *slight* reduction of delta-v requirements for the upper stage of a TSTO (you know, any air launched vehicle really isn't a true SSTO, old chap - the carrier aircraft actually is the first stage, really) can reap significant rewards, and the drop down release eases the logistics challenges as opposed to an off-top separation, plus the aircraft launch gives you flexibility with respect to target orbit inclinations.

So come at this issue from another direction: if the advantages are so obvious, then why hasn't this been the preferred approach for launching rockets? What we've actually seen over the long history of spaceflight is that there have been very few proposals for air launch, and only one that's been moderately successful (Pegasus). That alone is a strong indicator that there's little advantage to air launch.

Beyond the performance issues, you also have to look at the logistics and infrastructure issues. People have claimed that one of the advantages of air launch is that the plane can fly anywhere to launch. But that's not really true. For starters, the satellite and rocket have to be serviced before launch, and that requires facilities. In the case of the satellite, it probably requires some kind of moderately clean room facility. The facilities may have to handle dangerous materials like hydrazine. So even though the plane can move, chances are that you still need a fixed facility that is near an airport, and that's going to put constraints on the "anywhere" aspect of launching. Plus add in the fact that maintenance and certification and insurance for the aircraft costs money. And then there's limitations on scalability--you cannot make the rocket any bigger without affecting the aircraft (much less of a problem for a fixed launch site).

When trying to compare standard launch vehicles and their fixed facilities vs. air launch, there are a lot more drawbacks to air launch. That explains why nobody really is doing it (except Pegasus).

Offline martinbayer

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Re: Stratolaunch
« Reply #248 on: January 20, 2019, 07:20:53 am »
I agree that for expendable launch vehicles air launch is probably more hassle than it's worth, but I suspect it still *might* show some advantages for a fully reusable system - after all, it worked well for a number of experimental rocket planes, including the X-15.
« Last Edit: January 20, 2019, 07:46:29 am by martinbayer »
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Offline sferrin

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Re: Stratolaunch
« Reply #249 on: January 20, 2019, 08:32:32 am »
after all, it worked well for a number of experimental rocket planes, including the X-15.

Which of those went into orbit?
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Offline martinbayer

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Re: Stratolaunch
« Reply #250 on: January 20, 2019, 08:40:13 am »
after all, it worked well for a number of experimental rocket planes, including the X-15.

Which of those went into orbit?

None, but for a fully reusable orbiter air launch just might make all the difference between being able to achieve LEO with a meaningful payload or not without the use of any expendable components.
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Offline Archibald

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Re: Stratolaunch
« Reply #251 on: January 20, 2019, 09:58:34 am »
There it is
https://engineering.purdue.edu/AAECourses/aae450/2008/spring/report_archive/reportfinaluploads/pdfs/Report_Section_7.pdf

"A study by Klijn et al. concluded that at an altitude of 15,250 m, a rocket launch with the carrier vehicle having a zero launch velocity at an angle of attack of 0 to the horizontal experienced a Δv benefit of approximately 600m/s while a launch at a velocity of 340m/s at the same altitude and angle of attack resulted in a Δv benefit of approximately 900m/s. The zero launch velocity situations can be used to represent the launch from a balloon as it has no horizontal velocity.  

Furthermore, by increasing the angle of attack of the carrier vehicle to 30 and launching at 340m/s, they obtained a Δv gain of approximately 1,100m/s.

Increasing the launch velocity to 681m/s (= Mach 2) and 1,021m/s (Mach 3) produced a Δv gain of 1,600m/s and 2,000m/s respectively."

Sure, 2 km/s helps... but only a little. And separation issues as mach 3 can be interesting, just ask the crew of Lockheed M-21, the SR-71 that carried a D-21 drone on its back.

A SSTO needs a propellant mass fraction of at least 0.94 that is, the complete vehicle with crew, payload, tank, represents 6% , and 94% is pure propellants.

No 0.94 and then no 9 km/s to Earth orbit;

Air launch substracts 2 km/s out of 9 km/s, fine, but even 7 km/s is still a very high hurdle. 7 km/s can be done with a PMF  of  0.90 - still 10% for the vehicle itself, and 90% of the mass allocated to the propellants.
At the end of the day, either 90% or 94% of the mass, being the raw propellants - doesn't change much, building such a thing is still a huge conundrum...

I wouldn't buy a car which would weight 1000 kg with the gasoline tank full, and only 50 kg with the tank empty.
Which mean that 950 kg of the car would have to be gasoline itself. Pure gasoline. no kidding.
The tank around the gasoline, the car around the tank, and the car payload could not mass more than 50 kg. Bad luck, I alone weight a little more than that. And then the car would not work.
Other example
A 450 mt 747-8 is 1/3rd structure (150 mt) 1/3rd payload (150 mt) and 1/3rd kerosene (150 mt). PMF is thus 33% or 0.33. A SSTO would need 95% or 0.95.  Air launch just can't change that, or not enough to make a useful difference. That's the issue...
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Offline martinbayer

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Re: Stratolaunch
« Reply #252 on: January 20, 2019, 11:37:21 am »
I do think that air launch could potentially make just enough difference (90% PMF vs. 94% - the STS ET had a PMF of 96%) to make a single stage orbiter a la Interim Hotol feasible.
« Last Edit: January 20, 2019, 11:51:05 am by martinbayer »
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Offline steelpillow

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Re: Stratolaunch
« Reply #253 on: January 20, 2019, 12:18:27 pm »
Following up on Archibald's comment. The big problem with a horizontal-launch space stage is the mechanical stresses in the body/fuselage. In a vertical-launch rocket, the optimum aerodynamic cylinder is also the optimal structure, with the fuel pressure able to provide sufficient structural rigidity. But in a horizontal position the cylinder tends to sag in the middle and the fuel pressure is nowhere near enough to stabilise it. A surprising amount of of structural reinforcement needs to be added, and that means weight.

The problem is even worse with Pegasus because the cylinder is chopped into discontinuous stages, with the structural forces having to be transferred across multiple joints.

A subsonic hike to a few miles up is no compensation at all. Whether SSTO or air-dropped, launch to orbit is on the edge of impossible. Stratolaunch could probably send up a sub-orbital second stage that could in turn launch an orbiter, but it's an awful lot cheaper to do it the SpaceX way. My guess is that this was why Elon Musk parted company - he woke up and smelled the coffee before Paul Allen did.
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Offline Archibald

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Re: Stratolaunch
« Reply #254 on: January 20, 2019, 11:56:35 pm »
I do think that air launch could potentially make just enough difference (90% PMF vs. 94% - the STS ET had a PMF of 96%) to make a single stage orbiter a la Interim Hotol feasible.

I respect that opinion, the jury is really split over what PMF can be done with present technology. 0.85, 0.88, 0.90 ? no idea. For example, Skylon C1 manual give a PMF of 0.87 (I have to check that).
Did Elon Musk ever made public Falcon 9 stages weight budgets ?
What is sure is every single % of PMF gained, is a hard fought battle. Even 0.85 and 0.90 makes a huge difference, either in delta-v or propellant.  There is also the case of weight creep during prototype construction, even the 787 or A350 ended overweight by 10 or 15%, despite Airbus and Boeing strict controls and best efforts.
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