LV Design to launch full-size DreamChaser on Strato-Launch

[Todd Martin]

Regarding the likelihood of an LH2 tank rupturing at the attachment point to the Stratolaunch aircraft, certainly a whole new LH2 tank would need to be designed for this application.  I did a little research and found some of the puzzle pieces to see the size of the issue.

The contact area of a 5 meter fairing (197 inches) to a 26 foot wide wing (312 inches) is 61,464 square inches.  The LV weight of 500,000 lbs, so the pressure applied over this area from the weight would be 8.13 psi.  Not a ridiculous number.

The Shuttle ET was pressurized around 32 psi, a 5 meter tank should be able to go higher, perhaps as high as 50 psi.  The pressure inside the tank helps provide stiffness to the pressure body.

Buckling pressure can be predicted by measuring tank stiffness.  There is a linear relationship between measured stiffness and measured buckling pressure.  Tank stiffness increases with the square of the wall thickness.  So, if all else is equal and you need to double the tank stiffness, then the tank wall thickness would increase by the square root of 2.

Current tanks are primarily aluminum-copper or aluminum-lithium.  Since Stratolaunch has to develop a whole new tank anyway, they could choose to go with a composite tank.  Boeing has demonstrated some significant advances in composite tank technology for LH2:  http://www.parabolicarc.com/2014/04/05/composite-tank/ and they claim a 30% weight savings & a 25% cost savings to a traditional aluminum tank.

[RanulfC]

Is it even possible to carry and launch a cryogenic vehicle from this platform?

That's a good question.  From what I understand, there are 2 issues which must be considered.

Carry is the optimal word to me. In my mind's eye, this is what I see. You suspend that thing horizontally from the single suspension point. You then begin loading liquid prop. The fragile fuselage begins to sag at both ends from the weight. Well before the tanks are full, the thing just rips apart. Even if you manage to avoid that, you take off and the vibrational harmonics finish the job.

Boeing did some study of air-launching a Delta-IV by pretty much wrapping the "aircraft" airframe 2/3rds of the way around the LV so as not to have to significantly redesign the Delta-IV. (Keep in mind though the "real" reason for the exercise was to pitch replacing the Delta-IV with a cargo pod as a means to try and sell a new heavy lifter aircraft design )

In the real world though what you'd have to do is create a stong enough LV "frame" and integrated aircraft "strong-back" to carry the load which of course is why the folks doing the actual design have tossed out liquid propellant entirely in favor of solid since they are structurally far more robust than any liquid tankage can be.

The Shuttle ET, strong-points and SRB connections would be the place to start for such an arrangement I'd think.

Randy

[Todd Martin]

Boeing did some study of air-launching a Delta-IV by pretty much wrapping the "aircraft" airframe 2/3rds of the way around the LV so as not to have to significantly redesign the Delta-IV. (Keep in mind though the "real" reason for the exercise was to pitch replacing the Delta-IV with a cargo pod as a means to try and sell a new heavy lifter aircraft design )

In the real world though what you'd have to do is create a stong enough LV "frame" and integrated aircraft "strong-back" to carry the load which of course is why the folks doing the actual design have tossed out liquid propellant entirely in favor of solid since they are structurally far more robust than any liquid tankage can be.

The Shuttle ET, strong-points and SRB connections would be the place to start for such an arrangement I'd think.

Randy
[/quote]
Randy, that's a great point that it would be preferable to have the aircraft provide as much of the structural support needed as possible, like a strong-back.  A stratolaunch aircraft designed from the start to support a hydrolox stage may differ significantly from what they built.  The difference is a performance margin for the 1st stage.

As for using a drop-tank and DC as the second stage, it would remove the expense of 2 expendable RL10B 2nd stage engines, but I am unclear how it would be arranged.  You can't put the drop tank behind DC because the DC engine nozzles would impinge on the drop tank.  You can't put the drop tank in front of DC and be "pushed" due to plumbing issues.  That leaves a side-mount design, like the Shuttle.  The ET of the shuttle could be considered a drop tank and the MAKS concept shown above is a side-mount.  In this case, the 1st stage booster is limited to 5 meters due so that the aircraft landing gear can reach the ground.  A sidemount drop tank & DC would need to fit on top of a 5 meter booster without a fairing.  Could that be acceptable from an aerodynamic point of view?   

[jongoff]

Is it even possible to carry and launch a cryogenic vehicle from this platform?

That's a good question.  From what I understand, there are 2 issues which must be considered.

Carry is the optimal word to me. In my mind's eye, this is what I see. You suspend that thing horizontally from the single suspension point. You then begin loading liquid prop. The fragile fuselage begins to sag at both ends from the weight. Well before the tanks are full, the thing just rips apart. Even if you manage to avoid that, you take off and the vibrational harmonics finish the job.

Seriously? Better tell the Launcher One guys to call everything off. There's obviously no way to design a liquid fueled rocket that can be air-launched without snapping in two. </sarcasm>

~Jon

[NovaSilisko]

Boeing did some study of air-launching a Delta-IV by pretty much wrapping the "aircraft" airframe 2/3rds of the way around the LV so as not to have to significantly redesign the Delta-IV. (Keep in mind though the "real" reason for the exercise was to pitch replacing the Delta-IV with a cargo pod as a means to try and sell a new heavy lifter aircraft design )

I... would be very much interested in learning more about this study. I have never heard of this before.

[TomH]

Is it even possible to carry and launch a cryogenic vehicle from this platform?

That's a good question.  From what I understand, there are 2 issues which must be considered.

Carry is the optimal word to me. In my mind's eye, this is what I see. You suspend that thing horizontally from the single suspension point. You then begin loading liquid prop. The fragile fuselage begins to sag at both ends from the weight. Well before the tanks are full, the thing just rips apart. Even if you manage to avoid that, you take off and the vibrational harmonics finish the job.

Seriously? Better tell the Launcher One guys to call everything off. There's obviously no way to design a liquid fueled rocket that can be air-launched without snapping in two. </sarcasm>

~Jon

Thank you for such a pleasant reply Jon. Please look back in my post and see if you see where I wrote,  There's obviously no way to design a liquid fueled rocket that can be air-launched without snapping in two. Obviously a liquid fueled rocket can be designed for air launch, but not all rockets are the same. In this case, the OP hypothesizes about the use of Stratolaunch and a launcher large enough to place a full sized DC into LEO. To do that, you need a fusilage that is much stronger than a vertically launched rocket, particularly with regard to a rocket of this length given the leverage such a length will generate. So you end up adding mass to your LV. Building a strongback to support your LV adds mass and drag to your aircraft platform. All of this cuts into performance. I have not done any strength calculations, however anyone with basic understanding of physics should realize that horizontally air launching a liquid fueled rocket of this size from a single suspension point starts to become problematic.

Here's bidding you a good day.

[jongoff]

Thank you for such a pleasant reply Jon.

Apologies for the snark. It just seemed like such a silly issue.

Please look back in my post and see if you see where I wrote,  There's obviously no way to design a liquid fueled rocket that can be air-launched without snapping in two. Obviously a liquid fueled rocket can be designed for air launch, but not all rockets are the same. In this case, the OP hypothesizes about the use of Stratolaunch and a launcher large enough to place a full sized DC into LEO. To do that, you need a fusilage that is much stronger than a vertically launched rocket, particularly with regard to a rocket of this length given the leverage such a length will generate. So you end up adding mass to your LV. Building a strongback to support your LV adds mass and drag to your aircraft platform. All of this cuts into performance. I have not done any strength calculations, however anyone with basic understanding of physics should realize that horizontally air launching a liquid fueled rocket of this size from a single suspension point starts to become problematic.

Here's bidding you a good day.

The thing to remember is that pressurized structures tend to be extremely stiff in bending. But yeah, you'll need to make sure you design the launch supports and rocket internal structure to react the predicted loads (plus margin) reasonably well. If you do it poorly it won't work, but I don't think anyone who has ever looked at air-launched rockets has thrown in the towel for this reason. Rockets already experience rather high bending loads from aerodynamic forces during flight. I haven't run any specific numbers either, but I'd be really surprised if this were the show-stopper you think it is. But the loads probably will drive you toward a more limited set of design options than a vertically ground-launched design.

Apologies again for being rude before. I need to keep the flame without sharing it quite so often.

~Jon

[RanulfC]

Boeing did some study of air-launching a Delta-IV by pretty much wrapping the "aircraft" airframe 2/3rds of the way around the LV so as not to have to significantly redesign the Delta-IV. (Keep in mind though the "real" reason for the exercise was to pitch replacing the Delta-IV with a cargo pod as a means to try and sell a new heavy lifter aircraft design )

I... would be very much interested in learning more about this study. I have never heard of this before.

It wasn't a generally available "concept" that I've found. I found it initially in a book/DVD that my wife bought me that discussed "future" launch vehicles that was published around 2010 IIRC. Illustration mentioned/showed the AC and LV in flight and launching and then a smaller illustration of the LV replaced with a cargo/passenger pod. At the time I found a couple of mentions on-line about it being pitched to the AF as a "responsive space" LV concept and a couple of dozen more that mentioned it as being pitched to the AF as a heavy lift cargo aircraft concept. Then nothing. Short search today has turned up nothing but I expected that. Once I get home I'll see if I can get some links to at least the illustrations

Randy, that's a great point that it would be preferable to have the aircraft provide as much of the structural support needed as possible, like a strong-back.  A stratolaunch aircraft designed from the start to support a hydrolox stage may differ significantly from what they built.  The difference is a performance margin for the 1st stage.

Well in all honesty the carrier aircraft DOES in fact provide some of that as the "stong-back" or interface provides an extensive support structure to transfer carry-loads between AC and LV. TomH's point as I understood it (and as he confirms later) is that given a need for a certain size of tankage to achieve the goal, and especially using LH2, the overall size and length of the tankage is going to require quite a bit of structure ON THE LV to achieve which is very likely going to have a large adverse effect on the payload capability of the LV. And it becomes a spiral as you need to add more propellant and therefore bigger tanks and therefore more mass for support...

LauncherOne is small and starte OUT more compact and structurally "solid" and of course the current StratoLaunch vehicle is "very" solid

Anyone who's done any air-travel will recall "bumping" down the taxi or runway at some point. You may not feel it in the bigget jets but those "bump" transits are still there and they would be transfered to the LV  and most LVs are simply not designed to handle those kind of pressure loads coming from point-sources like those of a carry-and-release mechanism. So your LV ends up being built more like an aircraft and less like a rocket to handle those loads. Jon is right though that pressure stabilization can do a lot, look at the AirLaunch LV which was pretty much "dumped" off the carrier and ramp But during most of the flight and transport the MAJORITY of the load bearing was the carrier platform NOT the LV which is not how the StratoLaunch aircraft works where the majority of the loading is on the LV all the time.

It CAN be done, I mean look at the bending loads the Shuttle ET was subjected to, but its not going to be a "simple" set of tankage and vehicle to accomplish

As for using a drop-tank and DC as the second stage, it would remove the expense of 2 expendable RL10B 2nd stage engines, but I am unclear how it would be arranged.  You can't put the drop tank behind DC because the DC engine nozzles would impinge on the drop tank.  You can't put the drop tank in front of DC and be "pushed" due to plumbing issues.  That leaves a side-mount design, like the Shuttle.  The ET of the shuttle could be considered a drop tank and the MAKS concept shown above is a side-mount.  In this case, the 1st stage booster is limited to 5 meters due so that the aircraft landing gear can reach the ground.  A sidemount drop tank & DC would need to fit on top of a 5 meter booster without a fairing.  Could that be acceptable from an aerodynamic point of view?   

Well I wasn't actually suggesting using a drop-tank/ET configuration as much as I was stating that a good starting point for the TANKAGE design would be the Shuttle-ET as it had to handle ALL its forces in an in-direct manner
But aerodynamically it probably COULD be done... Sort of...
http://www.astronautix.com/lvs/shuls200.htm
http://www.astronautix.com/lvs/staipper.htm
http://www.pmview.com/spaceodysseytwo/spacelvs/sld019.htm

Which would require a HUGE redesign of the DC I'd think to accomplish, but could be done. Yes you're "pushing" the tanks but the plumbing really is pretty straight-forward if you do it that way and one thing I NEED to point out to keep in mind here:

Your "configuration" HAS to be "aerodynamic" because a requirement in the design (StratoLaunch) is that the LV has to be capable of doing its OWN "pull-up" from level to near vertical flight.
(Actually unlike most "top-launch" plans where the carrier AC does a pull-up and "push-over" for release, bottom launch tends to have a begining "negative" AoA due to the release of the LV and it dropping away from the carrier AC. As far as I know only the AirLaunch "Lanyard-and-trapieze" system which orientated the LV before full release and the Crossbow concept where the LV engine was ignited prior to release to allow the carrier AC to bear the full aerodynamic loading didn't have this "problem" )

A very robust airframe structure is required to perform this and the LV has to have enough aerodynamic stability and structural margin to "glide" then take full engine thrust followed by a pull-up manuever to the proper AoA. Typical LV design tends to VERY much "skimp" on structure that is not required and typically no LV is going to experiance ANY loading of a similar nature so the "margin" simply isn't there. "Hanging" from an interface-structure or stong-back is going to also move most of your "loading" for transit loads to being spread among a number of "hold-and-release" mechanical supports (more is better) but those are still going to be "point" load sources that the LV structure itself is going to have to spread and distribute throughout the vehicle.

I highly suspect (HMX "might" chime in here ) that was the reason AirLaunch advocated a pressure-fed (and therefore stabilized) LV for their concepts despite the lower performance versus a turbopump design. The pressurized design probably required much less mass for structure than the non-pressure-fed design.

Randy

[RanulfC]

Please look back in my post and see if you see where I wrote,  There's obviously no way to design a liquid fueled rocket that can be air-launched without snapping in two. Obviously a liquid fueled rocket can be designed for air launch, but not all rockets are the same. In this case, the OP hypothesizes about the use of Stratolaunch and a launcher large enough to place a full sized DC into LEO. To do that, you need a fusilage that is much stronger than a vertically launched rocket, particularly with regard to a rocket of this length given the leverage such a length will generate. So you end up adding mass to your LV. Building a strongback to support your LV adds mass and drag to your aircraft platform. All of this cuts into performance. I have not done any strength calculations, however anyone with basic understanding of physics should realize that horizontally air launching a liquid fueled rocket of this size from a single suspension point starts to become problematic.

Uhm point Tom? No one has proposed or suggested that a "rocket of this size" be suspended from a single point and no concept I've ever seen has suggested such either which is why (I'm sure) Jon snarked. Your argument misses that a larger strong-back/carry assembly is the BASELINE for the StratoLaunch design. Yes this effects carrier AC performance but that's already taken into account. (What I didn't see taken into account in the original design was how much stress this was going to put on the third, liquid, stage as all its loading was going to be carried through the second stage mounting AND internal structure which was going to be pretty large even just aerodynamically due to the size difference in the stages) I suspect that a larger one would be needed for a full LH2 design but what you lose in performance for your carrier AC is probably offset by the increased overall performance of the LV itself.

I believe I understand what you're trying to point out (see above) but its not quite as bad as you believe it would be due to the nature of carried payloads. As I noted on the other hand though, by their nature 'carried payloads' that are not fully supported by the carrier aircraft (top or internal carry as opposed to bottom carry) have to provide a good margin of self support and load bearing but its not hopeless from the start or their wouldn't be any suggestions of this type in the first place

Randy

[Patchouli]

Well I wasn't actually suggesting using a drop-tank/ET configuration as much as I was stating that a good starting point for the TANKAGE design would be the Shuttle-ET as it had to handle ALL its forces in an in-direct manner
But aerodynamically it probably COULD be done... Sort of...
http://www.astronautix.com/lvs/shuls200.htm
http://www.astronautix.com/lvs/staipper.htm
http://www.pmview.com/spaceodysseytwo/spacelvs/sld019.htm

Going with hypergolic ,kerosene,or methane engines in the vehicle would reduce the physical size of the tanks.

[Todd Martin]

Let's apply the Rocket Equation to my proposal to see how it much margin there is left over to play with (LV figures are at the beginning of the thread).

Mass Ratio
  MR = (Mpt/Me)+1
  where,
     Mpt = Mass of the Propellant
     Me = Mass of rocket empty of propellant

The mass ratio of the 1st stage is,
  MR1 = (169883kg/22900kg+2780kg+20410kg+11300kg)+1
  MR1 = 3.96015

The mass ratio of the 2nd stage is,
  MR2 = (20410kg/(2780kg+11300kg)+1
  MR2 = 2.44957

As stated earlier, the aircraft is traveling at 250 m/s

Ve = Exhaust Velocity = ISP * 9.806 m/s

The Exhaust Velocity of the 1st stage is:
  Ve1 = 419 * 9.8066 m/s
  Ve1 = 4108.9654 m/s

The Exhaust Velocity of the 2nd stage is:
  Ve2 = 470 * 9.8066 m/s
  Ve2 = 4609.102 m/s
 
Delta-V = Ve1  * ln(MR1) + Ve2 * ln(MR2) + DeltaVstrato
Delta-V = 4108.9654 m/s * ln(3.96015) + 4609.102 m/s * ln(2.44957) + 250m/s
Delta-V = 10034.45 m/s

There are drag & gravity losses to reach Low Earth Orbit (LEO).  Normally, this is 1500 to 2000 m/s.  In this case we are beginning launch at 30,000 feet so these losses would be reduced by an estimated 600 m/s.  An average total drag & gravity loss for Stratolaunch would be (1500 m/s + 2000 m/s) / 2 - 600 m/s = 1150 m/s 

Net Delta-V to LEO = 10034.45 m/s - 1150 m/s = 8884.45 m/s

The International Space Station (ISS) is orbiting at 7660 m/s.

Delta-V margin = 8884.45 m/s - 7660 m/s = 1224.45 m/s

[Patchouli]

DC also has a fair amount of delta-V it's self not sure how much but I think it's much more then Soyuz and Dragon but less then Orion so it's effectively a third stage.