Author Topic: Falcon Tandem 1.5STO  (Read 14339 times)

Offline ChrisWilson68

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Re: Falcon Tandem 1.5STO
« Reply #20 on: 04/10/2016 02:16 am »
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,

As others have pointed out on this thread, the Falcon 9 first stage is unlikely to survive re-entry at more than about Mach 6.  So Mach 6 is what you get from drag.  Any more than that and your stage burns up.

So, everything I said was true.

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.

That's completely irrelevant.  I was talking about delta-V.  Delta-V is delta-V.

When you talk about delta-V to orbit, it's the same number whether you're using 1 stage or 3.

Staging can help you get more delta-V, but it doesn't reduce the delta-V you need.

What we're talking about here is taking either Falcon Heavy minus its upper stage or Falcon Heavy minus both its upper stage and one of its two boosters and thinking that can be a fully-reusable system.  For that, the total delta-V you need is relevant.  You're taking a system with only one staging event with parallel staging trying to use it to get something approaching twice the delta-V to orbit.  It doesn't seem remotely likely the numbers will work out for that.
« Last Edit: 04/10/2016 02:16 am by ChrisWilson68 »

Offline Stan-1967

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Re: Falcon Tandem 1.5STO
« Reply #21 on: 04/10/2016 05:27 am »
Well I though I would ignore the problems of fitting M1-Vacs on the stage, &  run some math on this confuddled mess of a rocket.  I expect it won't be impressive.   The problem is that the events as described, specifically the air start of the M1Vac engines, don't fit any readily available computational models.  I will have model the air start at the beginning of vehicle staging.

So here is how I will model it:  ( don't like it, propose something better )

1.  First of all I will treat it as a two stage vehicle, but I will assign the propellant burned in both cores, prior to the M1-Vac engine start to stage 1. I am leaving out the mass of the capsule in the simulation, and simply looking at the payload to orbit that the side by side cores can offer with the proposed flight profile.
   
2.  The S1 thrust will be for 9+7=16 Merlins, running for 140 seconds ==> 12096 KN

3.  I will leave 40 tons of propellant in the booster core for re-use of that core.   That mass gets assigned to S1 as included in its dry weight. 

4.  For the sake of the model, I will not air start the M1-Vac until the other booster core is ready to jettison.  At the same time I will shut off the 7 sea level Merlins.   This was an undefined part of the proposed configuration, and puts some additional gravity losses into play.  Max Q is going to happen pretty quick in this rocket, probably around 75-85 seconds.   Staging is at 140 seconds. ( when the 9 engine booster has only 40t prop left for DPL) There is no throttle down to limit G's.  I calculated G's at 6.3 at staging, and 7.3 at S2 burnout.

So the mass/thrust/dry weight numbers break down like this:
Stage 1:  dry mass = 59 ton, propellant = 612 ton,  Thrust = 12096 KN, ISP = 304 s ( avg. value )
Stage 2:  dry mass = 19 ton, propellant = 118 ton,  Thrust = 1868 KN, ISP = 348 s


So even before running this through a mathematical simulator, look at how messed up this is!   The first stage already takes a big hit on the performance because of the fuel set aside for re-use, so it's mass fractions are very very poor.   The second stage has a whopping 19ton dry weight, and only 118 t of fuel left after the seven sea level Merlins burn through 267 tons in the 140 seconds to staging.  The T/W is lower than for a standard F9 V1.2 because we are leaving the launch pad with a 2 engine out scenario.

So I've loaded this into the simulator at:  http://www.silverbirdastronautics.com/LVperform.html
pressing the button now...

Launch Vehicle:     User-Defined Launch Vehicle
Launch Site:     Cape Canaveral / KSC
Destination Orbit:     185 x 185 km, 45 deg
Estimated Payload:     12611 kg  ==> Dragon V2 plus trunk weighs around 7200kg
95% Confidence Interval:     7286 - 19102 kg

I was wrong in my previous post.   This vehicle, if it could be built, would achieve orbit.  However....

Compare this to a standard F9 V1.2, which can place around 17t into the same orbit, and that calc. includes hauling up the 4000kg fairing & ocean recovery of S1.

This configuration adds 317t of mass in excess of a standard F9 V1.2, and yet it deliver 4.5 tons less to LEO!  Not efficient or impressive.

So how does Sevenperforce propose that this vehicle with significantly less capability than a F9 V1.2 can return a 19 ton stage from orbital velocity when the V1.2 doesn't have the margins to return a 4.7 ton S2 from orbit?  At best it will have around 5 tons of excess payload for de-orbit, heat shields, landing etc.  5 tons of fuel is enough to fire one engine around 18 seconds.   Compare that to the DPL or RTLS profiles for returning the stage from far slower velocity.   

There is a good thread on what is the Falcon 9's secret to good performance.   It pretty much boils down to excellent mass fractions, high T/W, Optimized ISP for LEO, and sufficient ISP for GTO.   This proposal ignores basic fundamentals of the rocket equation and it's implications for design choice.  In spite of adding some 316 tons of mass over a standard F9 V1.2, the rocket gods punish bad mass fractions severely, and this configuration shows that.
« Last Edit: 04/10/2016 06:42 am by Stan-1967 »

Offline CorvusCorax

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Re: Falcon Tandem 1.5STO
« Reply #22 on: 04/21/2016 11:55 pm »
I think this all boils down to - how can a booster deorbit without burning up or using fuel for most of the dV (like from Mach 25 to Mach 6)

It has been speculated a few times that the reentry burn SpaceX does is not a delta-V manouver but an active plasma-heatshield manouver.

If that's the case, the booster would basically conduct atmospheric breaking, but detach the shock front from the rocket surface by a layer of exhaust (still hot but less hot than reentry shock front) gas by a few but crucial inches to prevent melting, while at the same time widening the shock front to increase friction and as such deceleration.

The physics behind that are anything but trivial.

We do know that Falcon 9 S1 currently cannot land beyond SES-9 speeds. But the reason for that is mainly that it will deplete its fuel supply to reach those speeds (with payload) and wouldn't have any left for reentry and landing burns. (as I understiid it, SES-9 even used a shortened reentry burn for that reason, despite the higher speed, thus SpaceX worry it might not survive reentry, but it did)

We know a single core F9 S1 could (barely) make SSTO.
If that's the case, then a tri-core FH with no 2nd stage and very light payload would also make it - especially if the side cores deplete early and RTLS, leaving the center core with spare of propellant for its orbital insertion. enough spare for the landing manouver
(it's basically a single core being given a push)

but of course even such a rocket would never have enough dV for a propulsive deceleration. It might however have enough fuel to make a 3 engine burn to create a plasma reentry shield during an aerobreaking manouver. (plus landing burn)

The main issue I see is, I know no way to calculate how much fuel is needed for this propulsively-shielded reentry manouver. It stands to reason it needs a lot less than a fully propulsive landing.

what could be calculated is how much spare fuel a FH center core would have if doing a CSTO (center stage to orbit, it's not really SSTO if you stage the side cores) if that's not enough for a regular landing burn plus around two minute of reentry-fire then you can forget the entire thing.


Offline alang

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Re: Falcon Tandem 1.5STO
« Reply #23 on: 04/24/2016 08:58 am »
Off topic but related. Has there been a discussion of the affect of the outer eight engine exhaust on the centre engine as kind of 'virtual' nozzle extension.

Offline Jim

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Re: Falcon Tandem 1.5STO
« Reply #24 on: 04/24/2016 12:55 pm »
Slapping hardware together and expecting somebody else to the the math is non starter.  If you design a vehicle and can't provide the numbers then you haven't designed a vehicle.  I suggest that this thread be locked

Offline intrepidpursuit

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Re: Falcon Tandem 1.5STO
« Reply #25 on: 04/24/2016 08:01 pm »
I think this all boils down to - how can a booster deorbit without burning up or using fuel for most of the dV (like from Mach 25 to Mach 6)

It has been speculated a few times that the reentry burn SpaceX does is not a delta-V manouver but an active plasma-heatshield manouver.

If that's the case, the booster would basically conduct atmospheric breaking, but detach the shock front from the rocket surface by a layer of exhaust (still hot but less hot than reentry shock front) gas by a few but crucial inches to prevent melting, while at the same time widening the shock front to increase friction and as such deceleration.

The physics behind that are anything but trivial.

We do know that Falcon 9 S1 currently cannot land beyond SES-9 speeds. But the reason for that is mainly that it will deplete its fuel supply to reach those speeds (with payload) and wouldn't have any left for reentry and landing burns. (as I understiid it, SES-9 even used a shortened reentry burn for that reason, despite the higher speed, thus SpaceX worry it might not survive reentry, but it did)

We know a single core F9 S1 could (barely) make SSTO.
If that's the case, then a tri-core FH with no 2nd stage and very light payload would also make it - especially if the side cores deplete early and RTLS, leaving the center core with spare of propellant for its orbital insertion. enough spare for the landing manouver
(it's basically a single core being given a push)

but of course even such a rocket would never have enough dV for a propulsive deceleration. It might however have enough fuel to make a 3 engine burn to create a plasma reentry shield during an aerobreaking manouver. (plus landing burn)

The main issue I see is, I know no way to calculate how much fuel is needed for this propulsively-shielded reentry manouver. It stands to reason it needs a lot less than a fully propulsive landing.

what could be calculated is how much spare fuel a FH center core would have if doing a CSTO (center stage to orbit, it's not really SSTO if you stage the side cores) if that's not enough for a regular landing burn plus around two minute of reentry-fire then you can forget the entire thing.

This entire argument still doesn't explain why you don't reenter a second stage rather than a first stage. Getting down from orbit is hard. Doing it with a massive first stage that has already been burning for 9.5 minutes would be so much harder then with a much smaller second stage and would make for a smaller payload.

I agree with Jim as well, if you are going to stick to your guns so tightly then at least calculate how much payload this would get to orbit and how much fuel would be left. My speculation is that both numbers will be close to 0.

Offline francesco nicoli

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Re: Falcon Tandem 1.5STO
« Reply #26 on: 04/24/2016 08:13 pm »
The second stage does not fall immediately to Earth, right?
If so, and if you could A) add a refuelling capability to a falcon upper stage, and B) maintain a fuel deposit il low LEO where a Falcon first stage could go by itself, then potentially you could refuel the second stage in that low orbit so that it gets the fuel it needs to slow down back to Earth. A F9 S1 should be able to bring to the depo enough fuel to service several missions that way. The main problem, I guess, is keeping that very low fuel deposit up there...

(my 2 cents from a non-technical point of view. But being an economist, I hate waste of productive capacity and fixed investment...)

Offline envy887

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Re: Falcon Tandem 1.5STO
« Reply #27 on: 04/24/2016 10:13 pm »
The numbers have been run. This monstrosity could put about 12t in orbit. A Falcon Heavy could throw the core booster plus about 19t into orbit while returning the side boosters to ASDS.

That's not enough for deorbit prop or even an orbital return heatshield, so at that point you've just wasted a $40 million booster while putting less mass to orbit than a reusable F9 with a cheap expendable upper stage.

Why would you ever use a expensive booster to do what cheap upper stage is designed and optimized for? That's completely backwards. SpaceX knows what they are doing.


Offline sevenperforce

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Re: Falcon Tandem 1.5STO
« Reply #28 on: 04/25/2016 01:56 pm »
Indeed, this idea was a non-starter from the moment OxCartMark pointed out that the MVac nozzle can't handle atmospheric stresses.

Otherwise it might have been useful to re-run Stan-1967's numbers with a triamese approach rather than a biamese approach, just to see whether the payload reaches F9 levels with sufficient remaining fuel for re-entry. Might also have been useful to see whether a virtual-heatshield re-entry burn as suggested by CorvusCorax would work.

But without MVac capability this concept is dead in the water.

Offline envy887

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Re: Falcon Tandem 1.5STO
« Reply #29 on: 04/25/2016 03:41 pm »
Indeed, this idea was a non-starter from the moment OxCartMark pointed out that the MVac nozzle can't handle atmospheric stresses.

Otherwise it might have been useful to re-run Stan-1967's numbers with a triamese approach rather than a biamese approach, just to see whether the payload reaches F9 levels with sufficient remaining fuel for re-entry. Might also have been useful to see whether a virtual-heatshield re-entry burn as suggested by CorvusCorax would work.

But without MVac capability this concept is dead in the water.

I did that for the triple core, assuming you light the center stage after the sides burnout. With three M1D's on the center core you get ~9t to 185km LEO. With a single M1D Vac you get ~19t to LEO, thanks to higher ISP. Neither of those is enough for any meaningful payload if you need landing fuel and a heatshield. There's definitely not enough overhead for a 2-minute reentry burn.

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