Stratolaunch: General Company and Development Updates and Discussions

[ncb1397]

Based on their payload mass targets, Hydrogen is the only option to get there.

Based on what exactly? All the Hydrogen SSTO's out there? Making a hydrogen SSTO is not appreciatively easier than making a kerosene or methane SSTO. All have their pros and cons.

No, based on physics.  The ability to succeed with SSTO means pulling out all the stops in every performance parameter you can within the design in both mass fraction as well as propulsion.  You have to do the crazy efficient propellant mass fraction no matter the propellant choice, but with hydrogen you can get an extra 30% in specific impulse over methane.  This fact, combined with their aggressive payload targets on both the cargo launchers as well as the SSTO spaceplane, leads me to assume hydrogen.

Don't stare yourself blind on specific impulse, an entire industry did that for decades without making any progress towards reusable vehicles, never mind SSTOs. And you are only going to LEO, where Isp matters the least. Keep in mind that a Atlas V (without SRBs) has a slightly worse payload fraction than an all-kerolox F9 (expendable) to LEO, despite having a hydrogen upper stage with an engine with much better Isp. There are more factors at play.

If comparing multistage to SSTO, it would be better to pick some other orbit than LEO because SSTO is going to have to provide ~9 km/s which is more akin to escape injections on multistage rockets. A TSTO stage to LEO only has to provide half that per stage. It would be better to use some high energy orbit like C3=10 km2/s2. In which case, the mass fractions are the following

Delta IV Heavy
mass:733,000 kg
payload: 9,285 kg
fraction: 1.26%

Falcon Heavy
mass: 1,420,000 kg
payload: 13,615 kg
fraction: .958%

Sigh. This is about is best for LEO delivery, since that is what “Black Ice” would go to. Or do you really think Stratolaunch is targeting C3=10 km2/s2 with this planned space plane? LEO is a relatively low energy target compared to that, so Hydrogen suffers.

Yeah, but LEO in one stage is high energy. It is basically as high energy as it gets.

[jbenton]

Air launch has a primary design limitation that favours dense propellants.

Could you remind me why that is? I know that Pegasus (to my knowledge the only successful orbital air-launch bird to date) uses all solids, but I wasn't sure why that is. I thought it was just easier to maintain or easier to transport without it exploding or something.

[nacnud]

I guess it's due to to denser rockets (of equivalent aspect ratio) having less air resistance while being carried under the launch aircraft.

[Archibald]

For those LH2-SSTO die hard fans: read the Mel Bulman paper I posted earlier. Or the one linked here. LH2 makes SSTO harder, not easier, for a simple reason. By some extraordinary coincidence, while 1 kg of LH2 has 2.5 more energy than 1 kg of kerosene, its density is also 2.5 lower.
Which mean that, liquid hydrogen tanks are giant PITA, as far as drag goes. They are voluminous, bulky, and very dragy. At the end of the day, all the issues with liquid hydrogen tanks exactly balance (or even negate) the huge specific impulse gain (460 vs 360 or less).
As Lar said, liquid hydrogen is, for SSTO, a red herring. For Centaur like upper stages it is not, of course.

II. Mission Analysis
A. Baseline Vehicle
We conducted a simple mission analysis to better understand the benefits of the TAN. We selected the single
stage to orbit mission as most in need of TAN benefits.
For our baseline, we selected a 2.5 Mlb gross liftoff weight (GLOW) vehicle powered by seven up-sized SSME
class hydrogen/oxygen engines operating at 3000 psia. Figure 6 shows the vehicle, which requires a little more than
100,000 cu ft of propellants to deliver 25,000 lb to orbit. Figure 7 shows the trajectory analysis for this vehicle. The
orbit we used was 200 nm at 28 deg, the most readily accessible orbit from Cape Canaveral station. The engine has
to throttle to 34 percent (3X or 1020 psia) to keep from exceeding the acceleration limits

With a payload fraction of 1 percent, it does not take much loss in engine performance or weight increase to
wipe out the payload. To see how sensitive the payload mass is to propulsion, we reduced the Pc to 1500 and
lowered the engine F/We 10 percent, and lost most of the payload (payload from 25 klbm to 1.7 klbm). It is clear
why no one would want to invest in such a risky endeavor.

[HMXHMX]

For those LH2-SSTO die hard fans: read the Mel Bulman paper I posted earlier. Or the one linked here. LH2 makes SSTO harder, not easier, for a simple reason. By some extraordinary coincidence, while 1 kg of LH2 has 2.5 more energy than 1 kg of kerosene, its density is also 2.5 lower.
Which mean that, liquid hydrogen tanks are giant PITA, as far as drag goes. They are voluminous, bulky, and very dragy. At the end of the day, all the issues with liquid hydrogen tanks exactly balance (or even negate) the huge specific impulse gain (460 vs 360 or less).
As Lar said, liquid hydrogen is, for SSTO, a red herring. For Centaur like upper stages it is not, of course.

II. Mission Analysis
A. Baseline Vehicle
We conducted a simple mission analysis to better understand the benefits of the TAN. We selected the single
stage to orbit mission as most in need of TAN benefits.
For our baseline, we selected a 2.5 Mlb gross liftoff weight (GLOW) vehicle powered by seven up-sized SSME
class hydrogen/oxygen engines operating at 3000 psia. Figure 6 shows the vehicle, which requires a little more than
100,000 cu ft of propellants to deliver 25,000 lb to orbit. Figure 7 shows the trajectory analysis for this vehicle. The
orbit we used was 200 nm at 28 deg, the most readily accessible orbit from Cape Canaveral station. The engine has
to throttle to 34 percent (3X or 1020 psia) to keep from exceeding the acceleration limits

With a payload fraction of 1 percent, it does not take much loss in engine performance or weight increase to
wipe out the payload. To see how sensitive the payload mass is to propulsion, we reduced the Pc to 1500 and
lowered the engine F/We 10 percent, and lost most of the payload (payload from 25 klbm to 1.7 klbm). It is clear
why no one would want to invest in such a risky endeavor.

Regrettably, I don't have the time to go into detail on the LH2 vs. denser propellants discussion.  But I will note (based on work I did for an unnamed client about four years ago) that while I am definitely a fan of dense propellants for ground launch SSTO or nSSTO configurations, the matter is not so simply resolved for a GLOW-limited air-launch.  Obviously, in a ground launch concept GLOW is a weak forcing function of cost, but when you are looking at an a/c that has a fixed payload limit, LH2 looks better, generally.  I don't think the Black Ice concept is automatically a bad design if it choses to employ LH2.

I'll also note that propulsion is far less important for air-launch than mass fraction or flight path angle.  Unfortunately, for a winged spaceplane, the spaceplane itself must do the turn at some delta-v cost.  Burt originally wanted use hybrid rockets to do the gamma turn using the Roc carrier itself, but that ends up being the wrong choice, since it cuts the payload mass that the carrier can release.  Better to put that mass to work as higher energy propellant in the spaceplane and release straight and level at optimum launch altitude (which is not "as high as you can fly" by the way).

[Archibald]

Burt originally wanted use hybrid rockets to do the gamma turn using the Roc carrier itself,

Wait... are you telling me he wanted to boost the entire Roc with rockets - at least to get the nose at a high alpha angle, which is better for air launch ?

Then again, there is this http://www.thespacereview.com/article/1608/1

If a 747 could do it... Roc has 747 DNA, so why not ?

But what an awesome sight it would have been, that giganormous Roc aircraft climbing, nose(s) pointed upwards, through the sky on rocket power      - Like a freakkin' Me-163 Komet, except a little bigger...

Alternatively, they could pour liquid hydrogen into the Roc turbofans for a massive boost of power, but that's easier said than done !!!

[HMXHMX]

Burt originally wanted use hybrid rockets to do the gamma turn using the Roc carrier itself,

Wait... are you telling me he wanted to boost the entire Roc with rockets - at least to get the nose at a high alpha angle, which is better for air launch ?

Then again, there is this http://www.thespacereview.com/article/1608/1

If a 747 could do it... Roc has 747 DNA, so why not ?

But what an awesome sight it would have been, that giganormous Roc aircraft climbing, nose(s) pointed upwards, through the sky on rocket power      - Like a freakkin' Me-163 Komet, except a little bigger...

Alternatively, they could pour liquid hydrogen into the Roc turbofans for a massive boost of power, but that's easier said than done !!!

Yes, that was the "original" plan, circa 2007, using six of the SS2 hybrid motors.  I don't recall the final angle but it wasn't small.  Something like 35-40° sticks in my mind.

I think Len Cormier proposed an LH2-augmented turbofan approach in the late 1970s, and of course the MIPCC method (water, LAIR or LOX injection into the compressor inlet) was the "secret sauce" of the DARPA RASCAL project, about 2003-2004.

[Lar]

As Lar said, liquid hydrogen is, for SSTO, a red herring. For Centaur like upper stages it is not, of course.
 

I think that was Lars-J, not me. I don't have a dog in this hunt (although I admit to being easily swayed)

[Archibald]

Yes, that was the "original" plan, circa 2007, using six of the SS2 hybrid motors.  I don't recall the final angle but it wasn't small.  Something like 35-40° sticks in my mind.

My mind is blown.

...by the way, you really, really need to write your biography someday.  oh please...

I think that was Lars-J, not me. I don't have a dog in this hunt (although I admit to being easily swayed)

d'oh !!!

[jbenton]

All this talk about hydrolox put a question into my mind:

What if the rocket explodes while still attached to the plane? I know the probability of that is low, but I was wondering if they have ejector seats in the Roc's crew cabin or some other such device.

Is it optionally manned?

[jbenton]

Also, have they given up on the triple Pegasus configuration, or is that option still open? (at least as an interim measure or something)

[Zed_Noir]

Does anyone think the Roc could do a zoom climb launch  after a long shallow dive? Presuming the Roc is carrying a single core Kraken.

[Hobbes-22]

So anyone think they will try to go SSTO right from the get go? 2 stage really doesn't make much sense as a development path to  the space plane.

The image of the tri-core pretty clearly looks like a multi-stage rocket,  the lines on the single core are hard to make out.

Try zooming in a bit. Both the single-core and the 3-core look like 2-stage vehicles to me.

[jbenton]

Burt originally wanted use hybrid rockets to do the gamma turn using the Roc carrier itself,

Wait... are you telling me he wanted to boost the entire Roc with rockets - at least to get the nose at a high alpha angle, which is better for air launch ?

Then again, there is this http://www.thespacereview.com/article/1608/1

If a 747 could do it... Roc has 747 DNA, so why not ?

But what an awesome sight it would have been, that giganormous Roc aircraft climbing, nose(s) pointed upwards, through the sky on rocket power      - Like a freakkin' Me-163 Komet, except a little bigger...

Alternatively, they could pour liquid hydrogen into the Roc turbofans for a massive boost of power, but that's easier said than done !!!

Maybe Virgin Orbit should consider this to improve LauncherOne performance. The article seems to suggest that the 747 was practically made for such a mission

[TrevorMonty]

All this talk about hydrolox put a question into my mind:

What if the rocket explodes while still attached to the plane? I know the probability of that is low, but I was wondering if they have ejector seats in the Roc's crew cabin or some other such device.

Is it optionally manned?

That brings up question of fuelling rocket while attached to plane. When do crew board? Before or after fuelling.

Once under way should be safe, assuming they don't have COPV come loose. Engine will not start until after separation when plane is clear.

[groundbound]

Also, have they given up on the triple Pegasus configuration, or is that option still open? (at least as an interim measure or something)

Whether or not they think they are doing it, that will never work out for economic reasons.

Take a rocket that flies very seldom because of a lack of customers, and is increasingly seen as overpriced. Then propose that you launch three at once? The technical feasibility is almost irrelevant.

[ThePhugoid]

Based on their payload mass targets, Hydrogen is the only option to get there.

Based on what exactly? All the Hydrogen SSTO's out there? Making a hydrogen SSTO is not appreciatively easier than making a kerosene or methane SSTO. All have their pros and cons.

No, based on physics.  The ability to succeed with SSTO means pulling out all the stops in every performance parameter you can within the design in both mass fraction as well as propulsion.  You have to do the crazy efficient propellant mass fraction no matter the propellant choice, but with hydrogen you can get an extra 30% in specific impulse over methane.  This fact, combined with their aggressive payload targets on both the cargo launchers as well as the SSTO spaceplane, leads me to assume hydrogen.

Don't stare yourself blind on specific impulse, an entire industry did that for decades without making any progress towards reusable vehicles, never mind SSTOs. And you are only going to LEO, where Isp matters the least. Keep in mind that a Atlas V (without SRBs) has a slightly worse payload fraction than an all-kerolox F9 (expendable) to LEO, despite having a hydrogen upper stage with an engine with much better Isp. There are more factors at play.

Enough with the flippant responses.  I was giving my opinion based on a lot of background and experience working this type of problem, which is a very hard problem, hence the reason people have to go the specific impulse route.  It isn't because of emotion or preference or what have you, I like methane as much as the next guy but a winged-body spaceplane that is also air-launched has a lot of factors that go against any route besides maximizing Isp.  I'll explain:

Some round numbers first
GLOW-constrained SSTO booster of 200,000 kg
Made up payload of 1,000 kg
dV of 9,000 m/sec

Right off the bat and assuming two candidate Isp values, you can evaluate the methane vs. LH2 trade
I'll assume 370 sec for methane assuming some badass staged combustion perpetual motion machine
I'll assume 450 sec for LH2

Just right there, starting at 370 seconds Isp for methane, amazing in its own right, the rocket equation alone says this vehicle has to have a 92% propellant mass fraction to make payload.  That leaves just under 20,000 kg for all the inert systems on a winged body spaceplane, a configuration for which many inert systems do not give one crap what propellant choice you made.  Systems like a gigantic wing, the TPS that covers said gigantic wing, landing gear, control surfaces, mechanisms inside those surfaces, ECLSS for the crew, abort systems, yada yada yada.  These systems also don't care how dense your propellant choice was.  They still have to exist and at similar scale given the mission ConOps.  And as was properly stated by a later post, you can't solve any of that by growing the entire vehicle as you would a ground-launched one, where the problem gets easier as you get bigger.

On top of the above, that's only the NTE operating design requirement.  A real program would start by requiring the engineers to design in margin, which in a simplistic problem like this we would just toss in as extra dV.  So by saying your design must meet say, 9,500 m/sec on day of PDR passage so you can hopefully shake a stick at the operating 9,000 m/s, you've now got a design that must meet 95-96% propellant mass fraction at PDR so that mass growth by the day you're integrated hasn't broken the whole mission. They again won't have that handy method of saving performance by adding propellant, because this isn't a ground launched vehicle.

Rewinding and leaving all the same while assuming you get the Isp of hydrogen, you automatically are allowed to start with an NTE propellant mass fraction of 87.5%.  Still, definitely super hard with a winged body spaceplane with people in it, but at least you haven't murdered your chance of succeeding before you've started. You live to die another day.

Until someone invents an unobtainium set of materials that you can build the most lightweight winged bodies as this with and reach impossible mass fractions, us engineers are going to have to continue staring ourselves blind on specific impulse.  There are no other simple options.

[TrevorMonty]

With LH fuselage will be long due to larger tanks but weight for most of airframe will be same as methane ie cockpit, wings, landing gear. So while LH tank may need to be for example 30% bigger doesn't result in 30% increase vehicle weight.

[Lar]

With LH fuselage will be long due to larger tanks but weight for most of airframe will be same as methane ie cockpit, wings, landing gear. So while LH tank may need to be for example 30% bigger doesn't result in 30% increase vehicle weight.

I don't quite see how you can make a larger fuselage without increasing the fuselage weight (and thus the weight of the wings, gear, etc... )... Yes, the cockpit can weigh the same, that's true. But 30% bigger tankage doesn't mean 0% increase in weight. Maybe 25% instead of 30% since the cockpit is the same but that's still an increase.

(numbers are made up, someone more rigorous than I can do real calculations)

[envy887]

...many inert systems do not give one crap what propellant choice you made...

This crucial point isn't entirely true. A hydrolox tank will be twice as large as a methalox tank (assuming fixed GLOW), meaning it will weigh more (pressure vessel mass is linear in volume), have much heavier engines (fixed GLOW means fixed thrust, but LH2 engines have much lower TWR), and require more TPS area, and LH2 needs extra insulation to prevent freezing air out on everything.

The heavier tanks and engines and TPS and insulation means the wings have to be larger to land at the same speed, and the landing gear have to be heavier to support the greater dry mass. The larger heavier empty vehicle needs larger control surfaces for aerodynamic flight, a larger RCS system for control in vacuum, and so on.

The increased tank and engine mass spirals out into other vehicle components, and your LH2 vehicle ends up 4 points worse in mass fraction, mostly eating the 5 points lower mass fraction you gained by switching for the higher ISP. This is readily apparent in upper stages, but those usually get away with very light low thrust engines since they don't start subsonic and deep in the atmosphere.

And on top of that you now have to deal with aerial fueling/topping of a really finicky deep cryogen instead of two soft cryogens. Hydrogen is a giant PITA, and it's hardly clear that it's the best choice for this application. It may win out in the trade space, but that's not at all obvious.