Author Topic: Falcon Tandem 1.5STO  (Read 14345 times)

Offline sevenperforce

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Falcon Tandem 1.5STO
« on: 04/07/2016 03:09 pm »
This is more speculative than anything else, and there's no evidence SpaceX is considering anything like this...

...but if they wanted to try pulling off 100% stage reuse for Falcon 9, I have an idea for how they could.

First of all, replace two of the Merlin 1D engines on a standard Falcon 9 first stage with Merlin 1D Vacuum engines (I think they would fit, though it might be tight):



Then strap on a Falcon Heavy style booster with crossfeed and put your payload on top:



I'm sure you can see where this is going. Launch with all nine engines on the side booster and the seven SL-optimized engines on the payload booster. Once you're past maxQ, ignite and throttle up the two vacuum engines while throttling down the seven SL engines on the core booster; allow the side booster to burn down to its landing reserves and then separate. With the higher specific impulse and thrust of the vacuum engines, plus additional fuel from crossfeed, the payload booster should have enough dV to enter orbit, deliver the payload, and then burn off enough velocity to re-enter on a RTLS trajectory after a single orbit.

The challenges would be making sure the SL engine exhaust doesn't damage the extended vacuum engine, figuring out center of thrust and center of mass due to the unbalanced launcher, and determining whether crossfeed would work. But if all that worked, then you'd have a parallel-staged fully-reusable launch system.

Offline the_other_Doug

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Re: Falcon Tandem 1.5STO
« Reply #1 on: 04/07/2016 03:21 pm »
It would be fully re-usable only if your main core stage that achieves orbit can survive an atmospheric entry from orbital speed.

I'm afraid that's a bit problematic, and in any event would require a lot more fuel reserve for the de-orbit burn to be added in addition to a longer entry burn (higher heating and for longer) and maintain enough fuel for a landing burn.
-Doug  (With my shield, not yet upon it)

Offline sevenperforce

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Re: Falcon Tandem 1.5STO
« Reply #2 on: 04/07/2016 03:39 pm »
I guess it depends on whether they can burn through re-entry to produce an exhaust shield.

Offline envy887

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Re: Falcon Tandem 1.5STO
« Reply #3 on: 04/07/2016 06:49 pm »
Is there an advantage to a single booster instead of two, assuming you're reusing everything?

You could put a couple Merlin vacs on a Falcon Heavy core stage, mate the payload right on top of the core stage, and accomplish the same thing with more margins and no oddly located centers of mass or thrust.

Offline OxCartMark

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Re: Falcon Tandem 1.5STO
« Reply #4 on: 04/07/2016 07:26 pm »
There's a whole lot of things to swing a comment at here but the largest it seems to me is that we more or less know the maximum re-entry speed is what SES-9 did, and that's right on the edge of what's possible.  So all of the dV to get from orbital speed down to SES-9's re-entry speed has to be done by burning fuel.  That would be a lot of fuel.  Looking at the webcast of the SES-9 launch I see MECO 1 at 8250 kph, this is more or less the speed you'd need to get down to before reentry.  You'd need to do that from (in the case of SES-9) a SECO of 26,980 kph.  That's a lot when you're accelerating a whole first stage and the fuel.

While the originally proposed concept has a lot of interesting tails to chase around and may make for a long thread I think the proposal above to put some Mvacs on a FH center core may be the one that we want to put some of our R&D time into.
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Offline sevenperforce

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Re: Falcon Tandem 1.5STO
« Reply #5 on: 04/07/2016 08:05 pm »
Is there an advantage to a single booster instead of two, assuming you're reusing everything?

You could put a couple Merlin vacs on a Falcon Heavy core stage, mate the payload right on top of the core stage, and accomplish the same thing with more margins and no oddly located centers of mass or thrust.
Yeah, that was my first thought as well. But if you can manage it with a single strap-on booster, you probably should, simply because it almost cuts the ground support requirements in half. Using a pair of strap-on boosters means twice as much side-booster refurbishment, 50% more fuel and fuel supply requirements, and so forth. Reuse isn't perfect; you're still cutting into the lifetime of each launch vehicle, and you're risking 50% more hardware each time you launch.

I'd need to do a bit of math to get a general notion of how much margin you'd get with one booster vs two.

There's a whole lot of things to swing a comment at here but the largest it seems to me is that we more or less know the maximum re-entry speed is what SES-9 did, and that's right on the edge of what's possible.  So all of the dV to get from orbital speed down to SES-9's re-entry speed has to be done by burning fuel.  That would be a lot of fuel.  Looking at the webcast of the SES-9 launch I see MECO 1 at 8250 kph, this is more or less the speed you'd need to get down to before reentry.  You'd need to do that from (in the case of SES-9) a SECO of 26,980 kph.  That's a lot when you're accelerating a whole first stage and the fuel.

While the originally proposed concept has a lot of interesting tails to chase around and may make for a long thread I think the proposal above to put some Mvacs on a FH center core may be the one that we want to put some of our R&D time into.
Yeah, putting Mvacs on a standard FHE core, particularly if they get crossfeed up and running, could be a serious boost to BLEO payload. You cut launch thrust by something like 14% but you have the capacity to seriously multiply your dV once drag drops to manageable levels. I really like that idea.

I don't think you need to get down to the SES-9 MECO speed before re-entry. The problem with re-entry for SES-9 was not re-entry; it came through re-entry beautifully and headed straight for OCISLY (well, straight for the poor barge's corner). The problem was not having enough remaining fuel for a one-engine suicide burn, and instead requiring a three-engine suicide burn to compensate for drag. Plus, if your booster core is in orbit, you can RTLS without a boostback burn, as you'll be swinging around in a single orbit before re-entry. The plan would be to use a gentle burn to get onto your aerobraking trajectory, then swing around into retrograde with RCS and burn through the re-entry so that your exhaust serves as a makeshift "ablative" heat shield while also slowing you down.

Offline robert_d

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Re: Falcon Tandem 1.5STO
« Reply #6 on: 04/07/2016 08:07 pm »
I guess it depends on whether they can burn through re-entry to produce an exhaust shield.
Sorry. But unless you have the math that says otherwise, this is not likely to work. You would be taking the weight of the legs and grid fins almost all the way to orbit. Cross feed plumbing will weigh more too. As others have said, slowing down again would take a lot of fuel. Thus, if it comes from the booster, that booster could not provide the delta-V itself and still  have enough propellant for return.

Offline OxCartMark

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Re: Falcon Tandem 1.5STO
« Reply #7 on: 04/07/2016 08:13 pm »
The assumption that you can go one orbit and land in the same place is a bit flawed unless that place is two places or unless that one orbit has more orbits.
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Offline Robotbeat

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Re: Falcon Tandem 1.5STO
« Reply #8 on: 04/07/2016 08:17 pm »
The assumption that you can go one orbit and land in the same place is a bit flawed unless that place is two places or unless that one orbit has more orbits.
...unless that orbit (and launch/landing site) is equatorial. ;)
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Offline sevenperforce

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Re: Falcon Tandem 1.5STO
« Reply #9 on: 04/07/2016 08:35 pm »
I guess it depends on whether they can burn through re-entry to produce an exhaust shield.
Sorry. But unless you have the math that says otherwise, this is not likely to work. You would be taking the weight of the legs and grid fins almost all the way to orbit. Cross feed plumbing will weigh more too. As others have said, slowing down again would take a lot of fuel. Thus, if it comes from the booster, that booster could not provide the delta-V itself and still  have enough propellant for return.
Yeah, it's a lot more dry mass. No doubt about that.

I guess the worst-case scenario dV yield can be modeled in X discrete stages assuming no crossfeed:

1. Full thrust from launch to maxQ, 78% fuel consumption on the payload booster.
2. Payload booster MECO; full thrust to separation on side booster.
3. Dual MVac ignition; full thrust and then throttle down to orbital insertion.
4. Payload separation at apogee (we'll assume a Dragon V2 so it can do the circularization itself).
5. Momentary burn at apogee to define once-around re-entry trajectory.
6. Slowdown burn before and through re-entry using MVacs.
7. MVac cut off; coast to landing site.
8. Single-engine suicide burn; legs down.

A bit of math for me (or anyone else who wants to take a stab at it) there, but not insurmountable.

The assumption that you can go one orbit and land in the same place is a bit flawed unless that place is two places or unless that one orbit has more orbits.
An apogee burn to define re-entry, combined with the re-entry burn and the grid fin control, should permit enough crossrange for RTLS on any launches reasonably close to the equatorial plane.

Offline starsilk

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Re: Falcon Tandem 1.5STO
« Reply #10 on: 04/07/2016 11:09 pm »
mvac bells are paper thin. exposure to atmosphere at any sort of velocity (up or down) will tear them to bits.

Offline OxCartMark

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Re: Falcon Tandem 1.5STO
« Reply #11 on: 04/07/2016 11:29 pm »
An apogee burn to define re-entry, combined with the re-entry burn and the grid fin control, should permit enough crossrange for RTLS on any launches reasonably close to the equatorial plane.
For the LEO circular orbits burning at apogee doesn't get you much more than you'd get at perigee.  The grid fins are only good for about 10km of cross range.
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Offline sevenperforce

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Re: Falcon Tandem 1.5STO
« Reply #12 on: 04/08/2016 02:30 pm »
mvac bells are paper thin. exposure to atmosphere at any sort of velocity (up or down) will tear them to bits.
If true, that's disappointing. Source?

Offline OxCartMark

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Re: Falcon Tandem 1.5STO
« Reply #13 on: 04/08/2016 02:43 pm »
Actulus Ferociter!

Offline starsilk

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Offline sevenperforce

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Re: Falcon Tandem 1.5STO
« Reply #15 on: 04/08/2016 04:30 pm »
Source?
  Source is SpaceX.

Quote
twice the thickness of a soda can at the end
 
http://www.spacex.com/press/2012/12/19/update-cots-demo-1-launch-activities
Wow.

So re-entry is probably a nix. I still think it might be possible to significantly increase payload on FH launches with expendable cores; that's awfully thin, but the niobium alloy can't be THAT weak.

Offline Lars-J

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Re: Falcon Tandem 1.5STO
« Reply #16 on: 04/08/2016 10:21 pm »
This is more speculative than anything else, and there's no evidence SpaceX is considering anything like this...

...but if they wanted to try pulling off 100% stage reuse for Falcon 9, I have an idea for how they could.

First of all, replace two of the Merlin 1D engines on a standard Falcon 9 first stage with Merlin 1D Vacuum engines (I think they would fit, though it might be tight):


No, you would be wrong.  :)  Here is an image to give you an idea of the size difference between M1D and M1D-Vac - and this is to scale. You just cannot fit more than ONE M1D-Vac in a 12ft diameter, unless you REALLY truncate the nozzle, and then you lose most of the benefit.

And I don't think you realize how tightly packed the M1Ds are on the first stage - see picture 2:
« Last Edit: 04/08/2016 10:24 pm by Lars-J »

Offline ChrisWilson68

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Re: Falcon Tandem 1.5STO
« Reply #17 on: 04/09/2016 12:43 am »
I'd need to do a bit of math to get a general notion of how much margin you'd get with one booster vs two.

I haven't done the math either, but my guess is that the margins would be negative in both cases, even with zero payload.

Remember, the delta-V to get back to a landing from orbit is equal to the delta-V to get to orbit.  So orbit and back requires twice the delta-V to get to orbit.  Some of that you get from drag, but not too much or your vehicle burns up.

How about someone does the math before we discuss this further?  A little math could save us all a lot of wasted time.

I don't think you need to get down to the SES-9 MECO speed before re-entry. The problem with re-entry for SES-9 was not re-entry; it came through re-entry beautifully and headed straight for OCISLY (well, straight for the poor barge's corner).

Yes, it's true that on SES-9 it survived re-entry fine.  But the reason people think SES-9 MECO speed is the max for safe re-entry is that SpaceX said something to the effect that they thought it likely the first stage wouldn't even survive re-entry for SES-9.  That indicates that SpaceX thinks the max safe re-entry speed is in the neighborhood of the SES-9 MECO speed.

Offline matthewkantar

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Re: Falcon Tandem 1.5STO
« Reply #18 on: 04/09/2016 08:08 pm »
Remember, the delta-V to get back to a landing from orbit is equal to the delta-V to get to orbit.  So orbit and back requires twice the delta-V to get to orbit.  Some of that you get from drag, but not too much or your vehicle burns up.

Untrue. Drag offers lots of delta V, and don't forget that going up you are pushing the second stage and most of the propellant in the first stage. Coming down, the stage only need stop enough propellant to stop the itself.

Matthew

Offline Stan-1967

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Re: Falcon Tandem 1.5STO
« Reply #19 on: 04/09/2016 09:17 pm »
This is more speculative than anything else, and there's no evidence SpaceX is considering anything like this...

...but if they wanted to try pulling off 100% stage reuse for Falcon 9, I have an idea for how they could.

First of all, replace two of the Merlin 1D engines on a standard Falcon 9 first stage with Merlin 1D Vacuum engines (I think they would fit, though it might be tight):



Then strap on a Falcon Heavy style booster with crossfeed and put your payload on top:



I'm sure you can see where this is going. Launch with all nine engines on the side booster and the seven SL-optimized engines on the payload booster. Once you're past maxQ, ignite and throttle up the two vacuum engines while throttling down the seven SL engines on the core booster; allow the side booster to burn down to its landing reserves and then separate. With the higher specific impulse and thrust of the vacuum engines, plus additional fuel from crossfeed, the payload booster should have enough dV to enter orbit, deliver the payload, and then burn off enough velocity to re-enter on a RTLS trajectory after a single orbit.

The challenges would be making sure the SL engine exhaust doesn't damage the extended vacuum engine, figuring out center of thrust and center of mass due to the unbalanced launcher, and determining whether crossfeed would work. But if all that worked, then you'd have a parallel-staged fully-reusable launch system.

This vehicle will not work.  You are basically putting two cores that are barely SSTO as stand alone's, with near zero payload, side by side, and then throttling down half of and it, which will incur more gravity losses.

You will not need to worry about de-orbit burns.  It will not achieve orbit.   

On a lighter note, it could be the first fictional paper rocket to be self banning from the "BEST LOOKING ROCKET" thread.

Try putting a interstage adapter on the two S1 cores, and put a redesigned S2 with 1 or 2 Merlin vac. engines. That would at least get something to orbit.


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