It's a bird, it's a plane .... it's Stratolaunch

from Michael Khan, 01. January 2012, 00:01

Since the start of the space age, technology development has been hampered by one major problem: The launch cost per unit of payload mass has always been prohibitively high. This very effectively dissuades users from developing large orbital applications. Because there are no large applications to be launched, there is no strong incentive for anyone to develop a large and significantly less costly launch vehicle. This leads to a deadlock.

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There have been some attempts to change this, so far to no avail. Enter Stratolaunch Systems, with a new and rather bold concept. We'll see whether their idea is any good, but it certainly is bold, no doubt about that.

There is a lot of money and experience behind Stratolaunch, so people take this venture seriously. The money is coming from Paul Allen, co-founder of Microsoft, and a man who probably knows how to make and increase a fortune. The experience is provided by Burt Rutan's Scaled Composites and Paypal's Elon Musk's SpaceX. Rutan won the X-Prize with White Knight/SpaceShipOne. Musk is in the process of entering the commercial launch services market in force; his company SpaceX already has brought a small and a mid-sized rocket and a manned spacecraft up to (or close to) operational status and may soon also offer a heavy lift launch vehicle.

Stratolaunch proposes to carry a fueled rocket to 9000 meters (30,000 ft) of altitude slung underneath a custom-built transport aircraft. At that altitude, the rocket will be deployed and its first stage engines ignited, so it can place the payload stored in its tip into a low Earth orbit. This is not a new idea as such - Orbital Sciences is already doing just that with their Pegasus rocket. The difference is in size: Stratolaunch wants to launch a rather large rocket, according to the information they provide, they foresee a launch mass, including payload and a full load of propellant, of 220 metric tons. That may sound a lot, but it isn't really. Other mid-sized launchers such as the Falcon 9 and the Soyuz are around 50% heavier than that. The Ariane 5 rocket weighs in at over 750 metric tons.

In the Stratolaunch project, the rocket mass is limited by the carrier aircraft. Though 220 metric tons is far from impressive for a rocket, it is a still a hefty external load for any aircraft to lug. In fact, there is no aircraft capable of doing this, so Stratolaunch will have to develop and build one that would weigh 540 metric tons at launch, have a wingspan of 117 meters (386 ft) and be propelled by 6 high-bypass turbofan engines of the kind used by the Boeing 747 (which has four engines). This new plane will have a double fuselage, similarly to Scaled Composites' White Knight Two. The external load will be attached to a deployment mechanism below the wing mid section. Stratolaunch cites a payload mass capability into low Earth orbit of 13,500 lbs (around 6200 kg), which would not suffice for SpaceX's manned Dragon space ship.

On Stratolaunch's youtube channel there is an animation of how they think a launch would work. Let's discuss this a bit more after you have checked it out.

No question about it: It is cool.

But of course all of this gives rise to a number of questions, namely:

   1.) Would it work?

   2.) What's with the wings?

   3.) What's the point?

I'm an aerospace engineer, so I prefer to do the sums and run some simulations before I judge an idea. So here goes: 

Would it work?

I don't have access to all the necessary data, so I have to make some guesstimates based on the information provided by Stratolaunch and the available technical data on the SpaceX Falcon rocket. Note that although SpaceX will provide the rocket to be used in the Stratolaunch system, that won't be a Falcon 9 but a distinctly smaller vehicle, so I'm going to have to do some extrapolation. 

Concerning the conditions at rocket deployment: The aircraft as shown in the animation will not be capable of a very high speed with its high aspect ratio, non-swept wings. The animation states a deployment altitude of 30,000 ft. That's 9 kilometres. I assume that the airspeed will be no higher than 800 km/h, around 430 knots. Probably it will be rather less than that, but let's give them the benefit of doubt.  

There's one thing I don't buy. The animation shows the aircraft gong into a steep ascent for deployment. Hardly. Can you imagine trying to recover that monster from a stall? Would the airframe withstand the resulting loads? I don't think so. I'll assume flight path angles of 0, 5 and 10 degrees (again giving them the benefiit of doubt).

The rocket is a two stage vehicle with, according to Stratolaunch's data, a wet mass of 220 metric tons. I don't have information on the staging, so by extrapolating from the Falcon 9 data I obtain 176 tons wet/13 tons dry for the first stage, 36 tons wet/2.7 tons dry for the second stage and on top of that, the payload, the fairing and the adapter.

They say that the first stage will have five engines and the second stage one engine. I assume that these will be SpaceX's Merlin 1C engines with a vacuum thrust level of 555 kN each. Now, with only five Merlins on the first stage, as was foreseen for the cancelled Falcon 5 rocket (which however would have had a much lower launch mass than 220 metric tons), the thrust appears to be on the low side and explains why they add wings and have to deploy at such a steep flight path angle.

What's with the wings?

Right, those wings. I'll tell you outright that I don't think they're a good idea. They need wings if they have a low thrust-to-mass-ratio on the first stage to prevent the rocket from diving down like a bomb. The wings, and the fact that the aircraft pulls up sharply at deployment are supposed to give the rocket some time to accelerate and gain altitude.

Orbital Science's Pegasus also has wings on its first stage. But the Pegasus is a three-stage-design, it has solid rocket motors with a high thrust, and it is smaller than the Stratolaunch rocket by a factor of 12. The Pegasus has a wing span of 6.7 meters. Just how big would the wings on the Stratolaunch rocket have to be? How much mass would that add and how much would those wings start messing with the aerodynamics of the carrier aircraft?

In my simulation runs, I tried a different approach: I did not simulate a winged rocket, but I included the first stage thrust level in in the list of optimization parameters, so the optimal thrust level would be worked out in the trajectory optimization. It turned out that the optimum value (that which maximized the achievable payload capoability) was in the direction of the thrust level obtained with nine Merlin 1C engines. So here's what I'd do: Lose the wings and just use the Falcon 9 first stage propulsion unit instead. Four more engines likely add far less weight than is saved by leaving out the wings, so there is a net gain. They can just take the propulsion unit off-the-shelf, it's been developed anyway. 

Some results of my simulation runs are here (click on the thumbnails to see larger versions):

Parametric Optimization Results: Altitude over Downrange Distance (left), Altitude over Velocity (right). Source: Michael Khan

Parametric Optimization Results: Flight Path Angle Profile (left) and G-Load Profile (right). Source: Michael Khan

In a nutshell: It would work, subject to the assumptions detailed above, i.,e., with no wings and increased first-stage thrust. For deployment from horizontal flight I compute a payload mass of 7100 kg. For deployment from ascent at 5 and 10 degrees flight path angle there are  associated payload gains of several hundreds of kg, respectively. The target orbit was a 200 km low Earth orbit at an inclination of 51.6 deg (the inclination of the ISS orbit plane). 7100 kg is not far from Stratolaunch's stated performance of 6200 kg. Close enough, considering the differences in  assumptions and the scarcity of hard data available to me.

So the rocket part works, at least on paper - I am not making any statement on whether the carrier aircraft can be built  and flown. 

Attentive readers will note that the g-loads are a bit high; they peak at 4.7 g before 1st stage burn-out and 6 g before 2nd stage burn-out. That's too high for manned operations but could be easily fixed by cutting two engines on the 1st stage towards the end of the thrust phase or throttling back the second stage engine shortly before burn-out. In the context of this analysis, this is just a minor side issue, not the major point.

What's the point?

Not everything that can be done is also sensible. Stratolaunch's proposal boils down to using a great deal of effort to place a payload of just over 6 tons in low Earth orbit. It's not likely that this capability can be stretched futher - how big can that carrier plane realistically be expected to get? 

The SpaceX Falcon 9 rocket - which is already up and running - has 50% higher launch mass and 50% more payload capability into the same target orbit. Therefore the Stratolaunch system offers no evident advantage over a conventional rocket that is launched vertically from a launch pad on the ground. Conversely, a conventional rocket and easily be scaled up to provide massively increased payload capabilities which the Stratolaunch system could not match.

If the Stratolaunch concept does not offer increased payload capacity, would it at least dramatically slash the launch costs? Unlikely. The rocket they would need is neither smaller nor less complex than conventional ones, so why would it be cheaper? On top of that, there is the development, unit and operations cost of the carrier aircraft. 

Yeah, it looks cool, granted. But this is just a dead end. It leads us nowhere and it will not revolutionize spaceflight.  I hope SpaceX will soon develop the Falcon 9 Heavy rather than wasting too many resources on the Stratolaunch venture.

Further Information

Article on Stratolaunch on, 13. December 2011, citing Paul Allen: "We are at the dawn of radical change in the space launch industry. Stratolaunch Systems is pioneering an innovative solution that will revolutionize space travel."

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  1. Torbjörn Larsson, OM The year of the Dragon [Jan 23, 2012 – Feb 9, 2013]
    01.01.2012 | 20:53

    What's the point? "Any orbit, any time." [2:25 of the movie.]

    I think they made very clear that their main business is to launch irrespective of fixed launch sites constraints of constrained orbital parameters and good weather conditions, cutting into their business.

    I don't think it is worthwhile either, in the larger view. Hopefully it makes space more affordable for smaller players.

    But I wouldn't presume on Rutan's and Musk's abilities to come up with working designs, I am sure they have calculated on that.

    Nor their own business sense, I don't think Musk would squander resources from his own business.

    "Stratolaunch cites a payload mass capability into low Earth orbit of 13,500 lbs (around 6200 kg), which would not suffice for SpaceX's manned Dragon space ship."

    Considering that SpaceX gave the Dragon dry mass as ~ 4 200 kg @ 2008, I don't see how that figures.

    Assume dry mass remains close to that, add some ~ 500 kg fuel for steering and landing thrusters, ~ 200 kg for chutes, and you will still have room for 7 astronauts @ 80 kg a person and leave ~ 700 kg for seats and air. Tight but doable, say for space hotel trips.

    They did show a Dragon capsule, and the cargo version would be mostly a waste at some 6 000 kg payload capability.

  2. Michael Khan Answer to Torbjörn Larsson OM
    02.01.2012 | 00:13

    I am aware that Stratolaunch's selling point is "any orbit, any time". However, firstly, "any orbit" should not be taken literally.

    There are limits on the range of orbits into which a 220 ton two-stage rocket can deliver a wothwhile payload. "Any orbit" means "a low Earth orbit at any inclination". But one can just as well launch into almost any inclination, except very low ones) from the Kwajalein atoll, where SpaceX has a launch pad.

    If very low inclinations are desired, one could think of using a converted oil rig at a launch base, which is what Boeing Sea Launch is doing.

    However, for low Earth orbits - which is what a vehicle of that size can achieve with any sensible payload - low inclinations are of little to no interest, so Kwajalein would be perfectly sufficient.

    So what remains as a selling point is really only "any time".

    Conventional rockets have the added advantage that one can launch bigger versions of them. With a Stratolaunch that option is out, because there is a strong upper limit to the wet mass through the limited lifting capability of the carrier plane.

    The target orbit for the cited payload figure is not stated, so I assume it to be a maximum value for the lowest possible orbit, i.e., one with an altitude of around 200 km orbit, more or less the bare minimum required to remain in orbit for a few days. The payload capability is reduced with the height of the target orbit - going to ~400 km where the future applications are going to be, including those for tourism, exacts a performance penalty, probably by several hundreds of kg.

    One can still launch into a 200 km orbit and use the onboard propulsion system to reach a higher orbit and come back from there (Soyuz, ATV and the Space Shuttle all do it this way), but then 500 kg of propellant would be barely enough only for the initial orbit raising and final reentry initiation. Significantly more is needed for target approach, rendezvous and docking, and of course attitude mnanoeuvres. That is why the Dragon's tank capacity is significantly larger, around 1200 kg, if memory serves.

    There again: Why not just use a Dragon 9 from KSC or Kwajalein. That could do it with margins to spare.

    I certainly do not wish to be misconstrued as presuming on Rutan's and Musk's (or rather SpaceX's engineers') abilities. As I said, I am confident that the rocket part would work. I'm not saying that the airplane part would not work, I just take the information they cite as is.

    Concerning the rocket, all I can say is that my calculations indicate that with a bit more thrust on the first stage, a horizontal launch without a winged stage is possible and even appears advantageous in terms of performance. That's more than just an opinion, I did a significant amount of simulation work before writing this post.

    I doubt that the animation with which Stratolaunch went public represents the final layout. Much of what is shown is just PR and CGI. We'll see what the final design looks like when they are ready for roll-out an few years down the road.

    I certainly do not doubt Rutan's and Musk's business acumen. I assume the money comes from Allen. Rutan will get something out of this either way. Even if Stratolaunch fails to perform up to expectations, he's still have an outsize freight plane which can be used profitably for more mundane applications than space launches. As for Musk, he is in the confortable position of not being a risk-sharing partner in this venture, so for SpaceX, Stratolaunch will just be another customer.

  3. Space Lover Stratolaunch Profitability
    14.06.2012 | 07:15

    The only one at risk, is Paul Allen, as it is his money. Rutin will get a carrier plane big enough to launch rockets into orbit. And they could possibly use other rockets. Maybe Orbital, with a bigger Pegasus, would be interested.

    SpaceX has no risk. Also, the calculations were done with Merlin 1C engines. SpaceX has Merlin 1D engines with Thrust to Weight ratio of 160 to 1. The best of any liquid rocket. And their are rumors they are developing a more powerful 2nd stage rocket.

    They could go after tourism and Bigelow space stations. The flexibility in launching may allow them to pick up a niche market. SpaceX may want the flexibility to help avoid launch delays or reshuffle their schedules.

    It seems it could work, but as a niche.

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