Stratolaunch - how much does it save?

So I read this very interesting article about Paul Allen’s Stratolaunch project:

I’m just wondering - how much launch energy does taking a huge rocket up to 35,000’ and Mach .8 (or whatever) actually impart to the vehicle? I mean, I understand that for every foot gained in altitude, that is one less that the booster will have to climb to to reach its ultimate goal. But how big of a fuel or weight savings are we talking about?

It also seems like a tough environment to monitor and launch a big rocket from. On a pad, you can have a lot of infrastructure around to assist in the launch logistics. Sure, you can probably get much of the same stuff in an airplane carrier vehicle, but it certainly won’t be as complete as a ground based facility. And what about aborts and flying around with a huge bomb (essentially) under the carrier aircraft?

Any-who…just seems like a pretty big project with questionable payoff…


I think Paul Allen is getting all Spruce Goosey. Someone must have run the numbers I guess, at least to pay for the fancy graphic, but intuitively it does seem funky not in a good way.


Yeah…I wanna see the thing fly and succeed. Just seems like a pretty far out project… Interesting that such a high tech shell is going to feature a lot of old 747 parts and cockpit. Seems to be the anti-Elon Musk approach…

I imagine aerodynamics plays a pretty significant part as well- drag due to air resistance will be a lot less at 35000, which means combed with the nonzero initial velocity will probably work wonders for offering a much higher specific impulse, and allow a smaller booster to put a heavier load into orbit, or a larger booster to lift a load even higher.

You should also take oxidizer into account. A jet can get oxidizer from the air meaning

A it does not need to be purchased and
B it does not need to be hauled in the vehicle.

Oxidizer can make up a large portion of the mass in a reaction. When looking at something like C8H18 (Octane) wich is a part of gasoline and could be a part of JET-A and JET-B ( All I know about them is that JET-A usually has 8 to 16 carbon atoms per molecule and JET-B 5 to to 15, so it could be a part of both) The reaction is as follows.

2 C8H18 + 25 O2 = 18 H2O + 16 CO2

The standard atomic weight for …

Carbon is 12 ( 12.01 actually but let’s get some nice easy to read numbers)
Hydrogen is 1 (1.008 but idem)
Oxygen is 16

For a reaction we need 2 Octane molecules and 25 dioxygen molecules.

16 × 12 = 192
36 × 1 = 36

36 + 192 = 228u worth of octane molecules.

We also had 25 dioxygen molecules.

16×2×25 = 800u

so we also have 800u worth of oxygen. This means that

of the mass in the reaction actually comes from the oxygen, wich the Jet scoops from the air for free while the rocket had to carry that onboard ( if it was a kerosene + LOX design). and propel the mass of the oxidizer as well as the other components.

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Big. In theory, at least.

First of all, the rocket isn’t re-usable, so every bit of cost you can remove from the structure is cost you save ON EVERY LAUNCH. And the cost savings can be big. Just using some nominal numbers on the back of this envelope here, and I guesstimate a delta-vee savings of 500 m/s (it takes a vehicle around 10,000 m/s to get to low Earth orbit, starting from the surface of the Earth) would result in about a 21% decrease in overall vehicle weight, for the same payload. If you squint hard enough, the cost of an aerospace vehicle is almost linearly related to weight, so that’s about a 21% cost savings. Not bad, especially when rockets that go to LEO are about $50 - $100 million smackaroons each.

This reusable launch platform isn’t going to be cheap, but you only have buy it ONCE (unless @BeachAV8R is flying, then you’d better get a backup) . Say this Roc thingy costs $100,000,000 to buy, and you can save $20,000,000 on each rocket you build to launch from this higher altitude. The thing pays for itself in a mere 5 launches!

Wow!, right? Sign me up for two!

But, unfortunately, this is where reality comes crashing in to ruin all your dreams. Again.

I have no idea what the actual math looks like for this project, but I’d expect a hefty development/build cost on this plane, and “return on investment” is probably going happen around more like 100 launches. Whatever the actual number is, the launching aircraft is going to have to be in service a long time before it breaks even. And during that service, it’s going to wrack up life-cycle cost.

Life-cycle costs are simply astounding. Think of all the gas you’ve put into your car; and oil changes, tires, windshield wipers, tolls, parking fees, repair costs, inspections, insurance, registration, etc. And your car isn’t a high-tech airplane that carries rockets to the stratosphere. Maintenance/upkeep, training, inspections, re-certifications, accidents, etc. will stack up steeply from Day 1 and subtract from the bottom line. These costs nearly always far exceed the initial purchase price (or “first cost”) and more than a few careless investors have never realized a return on investment due to this life-cycle cost creep.

Another challenge will be finding payloads - rockets will have to be specifically made to launch in this style (Orbital’s Pegasus is one example in a pretty sparse field of air-launched orbital delivery trucks) and, as we established above in my airy conjecture, this platform will need a plethora of launches to make its return on investment. Aerospace developers can’t exactly be excited about dumping their money into designing a vehicle for a very unique and relatively high risk launch platform - if the Roc fails, how else do they launch their rockets?

You don’t get into rocket science without being a rocket scientist, so I’m certain Señor Allen and his band of crafty aerospatial engineer Vuclans have thought through all of this (and a whole bunch more). I fully expect that they’ve worked out a solution that will turn a buck…but it’s not going to be easy money.

Good luck to them!.


Always nice to have a rocket scientist provide some great insights! :thumbsup:

One thing that seems kind of risky with the air launch thing is that I would assume the rocket must be dropped, then there has to be a heartbeat or two before the motor is lit. Aren’t launches frequently aborted in those last few seconds due to some valve not seating or a computer automatically shutting down the launch? I have visions of fully fueled rockets and their payload becoming Mk20000000 bombs.

I get that it isn’t exactly a new concept, and it has been done successfully in the past. Just seems like the odds are stacked against it. But I still think it is cool and hope it is ultimately a successful venture. Can’t wait for the HD videos! :wink:

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Time for a DCS DARPA mod…!

KSP has it figured out!

That’s so Chuck Yeager

Something that piques my curiosity is that overall system reliability needs to be impeccable, partially for that reason (no aborts once separated, obviously) and because I’d speculate most issues with the mothership would have their checklists begin with jettisoning the rocket and payload. Which relates to…

In a similar vein, the Shuttle system was meant and projected to create tremendous savings in space flight due to re-usability. Unfortunately, it turns out getting thrown into orbit and coming screaming back down again is a stressful process (who could’ve guessed) and as such the vehicle needed to be taken almost completely apart, inspected, and put back together again between each launch. Any cost savings were obliterated by maintenance, system upgrades and, sadly, replacements.

The scale and design of the Roc (if the conceptual drawings are accurate) make me a bit wary, especially the twin fuselages with a fairly wide separation connected only in one place. Structural strength and rigidity of a given material and design tends to not scale at the same rate as the phenomenon it needs to overcome. I’d be really interested to see how they plan on handling structural stress/vibrations and how their control system is going to work to prevent any odd twisting or bending of that ever-critical center joint.

I don’t necessarily mean catastrophic failures either- every bit of unnecessary stress on that airframe’s going to count against the vehicle’s lifespan. If that lifespan drops below the magic flight number X that Einstein talked about where the fuel savings starts paying for the cost of the program, they have a problem! But wait, it gets better! Don’t forget that time spent working this stuff out equals a higher program cost. Bonus points because they want to make it out of composite materials, which have part lifecycle and fatigue considerations just a bit nastier than metal or other typical aircraft materials.

That said, I’m a big fan of Burt Rutan and the wizards at Scaled Composites. If they’re building it, you better believe they’re confident they have a business case. Could you imagine that thing thundering down a runway?! :smiley:

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Re - your comments on the stresses. Yeah - I mean, you are pretty much instantly releasing all that weight…although I guess they can gradually reduce the “weight” (load factor on the wings) by flying a reduced G separation maneuver (like .25 G)

Like you said - I’m sure they have it all worked out and calculated. I don’t think this is all on a napkin in a Denny’s diner or something…LOL…

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I was wondering about that as well. Didn’t the Air Force/ NASA do the same thing with the X-series that were air launched, in terms of flying a low-g profile for their launches?

So, I was more thinking about the fact that the two fuselages are only connected at the wing, not at the tail. Just about every twin-fuselage aircraft I know of that made it off the drawing board is also connected at the tail, and the Roc has the distinction of being bigger and heavier than all of them! (Side note, check out that list, specifically the Conroy Virtus and the- did I read that right? TWIN FUSELAGE C-5 CONCEPT that Lockheed proposed could carry the shuttle).

This is interesting because if each fuselage’s tail isn’t working in concert, you could potential put some strange and nasty “towl-wringing” like moments on the center connection, or worse, you’d get some aeroelasticity or flutter that could cause an alternating-Indian-burn like phenomenon (to use a technical term), neither of which would be particularly good news for the fatigue and wear of the load-bearing structures in the wing.

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