Falcony Heavy with Raptor centre core and Merlin side boosters

[andyr]

Just wondered if this has been discussed anywhere ?

The advantages (forget the costs!) would be:

- Side boosters being Merlin can return to land.
- Central core is expendable
- F9 upper stage may move to Raptor anyway, so making the 1st stage Raptor also might not be too big a leap
- Central core has fewer Raptors (maybe 5?)
- Gains some Raptor flight time ahead of ITS

[macpacheco]

There's little point in considering a Falcon heavy type vehicle if putting raptor on the booster is a possibility.
With a 9 raptor booster + single raptor upper stage, assuming the stages are sized to efficiently use the higher thrust, we get a rocket that's more capable than FH with just 10 engines and no side boosters.
Each raptor has 2x the thrust plus there's the extra ISP.
Its notional performance to LEO would firmly invade SLS territory.
But far more important, it would have performance to launch 20 tons to GTO with full reuse, performance to launch a fully loaded red dragon to Mars or to the moon with at least booster reuse.
The key requirement to achieve such goals is to give up road transportability. It would have to be either constructed locally and shipped by barge. Something like a 5.2m diameter body to handle the lower density deep cryo metane.
With full reuse, road transportability becomes a much lower requirement.

If the goal is just to obtain more experience with Raptor in the easiest way, they just use it on the 2nd stage, where its been determined it will produce substantially higher performance even with the volumetric limitations of road transportability due to the higher ISP ruling in the 2nd stage.

In fact, I fully expect as soon as Raptor testing is concluded, SpaceX will either make a mini ITS (with multiple raptors in the upper stage, with a mix of vacuum and SL nozzles to enable retro landing), or a F9 on steroids with 6-9 raptors on the booster + a single raptor upper stage.

[andyr]

Hi - I was thinking the centre core would be of very similar size to the S1 current cores, with a reduced number of Raptors.

[Jim]

Hi - I was thinking the centre core would be of very similar size to the S1 current cores, with a reduced number of Raptors.

Not viable.  It would have less performance than the standard FH.  And mixing methane and Rp-1 doesn't make sense logistically.

[hkultala]

Just wondered if this has been discussed anywhere ?

The advantages (forget the costs!) would be:

- Side boosters being Merlin can return to land.

What is the point if you ado not care about cost?

- Central core is expendable

I don't see how this is "advantage".

- F9 upper stage may move to Raptor anyway, so making the 1st stage Raptor also might not be too big a leap

This is not advantage. You are just trying to claim  a "smaller disadvantage."
And there are NO released plans for raptor upper stage. It is simply not needed.

- Central core has fewer Raptors (maybe 5?)

5 raptors is still more expensive than 9 merlins. I don't see how than can have any "advantage".

- Gains some Raptor flight time ahead of ITS

... without recovering those engines and not getting to check them after flights. Just run them on test stand instead.

[gospacex]

There's little point in considering a Falcon heavy type vehicle if putting raptor on the booster is a possibility.
With a 9 raptor booster + single raptor upper stage, assuming the stages are sized to efficiently use the higher thrust, we get a rocket that's more capable than FH with just 10 engines and no side boosters.

I think thread originator meant F9-sized tankage - the same diameter as F9. Which means it can be built on the same factory with the same tooling as F9 cores are built, so it does make some economic sense.

9 Raptor booster can't be made with F9-sized tankage - 9 Raptors won't fit under it. One Raptor can.

[spacenut]

Someone calculated the nozzle size for a vacuum Raptor would be almost 5m wide.  That means it could not be used with the diameter of the existing upper stage of 3.7m.  Also only about 2-3 sea level Raptors could be used on the existing F9 core.  Too big.

It is not in the plans, but a mini-ITS say with a 6-7m core and 9 Raptors and one Vacuum Raptor upper stage would be a nice rocket.  It could match or exceed Falcon heavy with a single stick rocket.  But like I said, this in not in the plans. 

SpaceX is going straight for the ITS and the Big booster rocket. 

[Jim]

the same diameter as F9. Which means it can be built on the same factory with the same tooling as F9 cores are built, so it does make some economic sense.

No, it doesn't because it will have less performance and there is no plus side

[envy887]

Hi - I was thinking the centre core would be of very similar size to the S1 current cores, with a reduced number of Raptors.

Not viable.  It would have less performance than the standard FH.  And mixing methane and Rp-1 doesn't make sense logistically.

I'd like to see some math on the "lower performance" bit. The prop mass fraction is 21% better with kerolox, but Raptor's insane pressure and FFSC more than make up for it.

IF they hit Raptor's nominal specs, a 6100 kN, 2-Raptor booster with the exact same size and dry mass as the F9 booster, with the standard F9 upper stage, would put 3% more payload to LEO, by my math. And that's not accounting for savings on plumbing and thrust structure mass from having fewer engines, which you always say is significant.

With FH, making the center core methane is even better since as a "sustainer" type of stage it needs less thrust and higher ISP. A single-Raptor center stage (with same size/dry mass as FH core) would boost payload to LEO by around 10%.

I don't think the payload gain is in any way useful enough to be worth bothering the the logistical headaches, but it is at least not a net loss.

[tyrred]

Proving that the NSF Shepard is a healthy skeptic?

[Moderas]

@envy887 How would they land a 2-raptor (or 1 for that matter) booster? Why would SpaceX spend money designing a new vehicle that can not be reused, because it can not land, for a 10% performance gain over their current rocket which already has excess performance?

[Stan-1967]

Just wondered if this has been discussed anywhere ?

The advantages (forget the costs!) would be:

- Side boosters being Merlin can return to land.
- Central core is expendable
- F9 upper stage may move to Raptor anyway, so making the 1st stage Raptor also might not be too big a leap
- Central core has fewer Raptors (maybe 5?)
- Gains some Raptor flight time ahead of ITS

It has been discussed elsewhere. 
http://forum.nasaspaceflight.com/index.php?topic=41361.0

It is an interesting thought, the performance hit isn't that great if used as the propulsion for a non-reusable center sustainer core, but the reasons that it will never happen are very well founded.   

[envy887]

@envy887 How would they land a 2-raptor (or 1 for that matter) booster? Why would SpaceX spend money designing a new vehicle that can not be reused, because it can not land, for a 10% performance gain over their current rocket which already has excess performance?

How? Probably with 4 ten tonne gaseous methalox thrusters.

Why? It would have to be a while combination of factors pushing them in that direction, none of which are really likely for reasons already mentioned here.

[macpacheco]

- Gains some Raptor flight time ahead of ITS

... without recovering those engines and not getting to check them after flights. Just run them on test stand instead.

Let me add that IF (and thats a mighty big IF) SpaceX wanted Raptor flight time, that would be much better achieved like this:
Build F9 Raptor 2nd stage.
Make a variant with a sea level nozzle.
Launch and land that stage 10x at Spaceport America or any other rocket test range that would accept SpaceX. Given the development contract with USAF, I'd say SpaceX would be welcomed with open arms anywhere that has enough spare time for SpaceX.
Perhaps launch from Boca Chica and land at Boca Chica LZ.

Rocket engineering isn't done in the air, flying.
Rocket engines aren't flown before they have been tested extensively on the stand and have performed flawlessly.
Some might reply, oh but test stands don't replicate all flight conditions, true, but the likely hood of bona fide engine problems in flight is quite small.
There could be problems in the rocket-engine interface or in the interaction of a boatload of engines producing heat together. But such problems are likely specific to the design of a specific stage type rather than engine design/fabrication problem.

In conclusion, its quite unlikely that SpaceX needs Raptor flight time before ITS. The key is testing the whole rocket stage in its final configuration.
Even then, that testing is done with a full/partial mission duration test fire somewhere.

I will agree that it will be quite likely that once SpaceX builds either a full raptor rocket or a raptor 2nd stage to F9/FH that if that rocket configuration is fully reusable, it will undergo some flight testing program, perhaps 3 test launches, cause they can afford to do it with a fully reusable rocket that can be relaunched dozens of times.

[gospacex]

@envy887 How would they land a 2-raptor (or 1 for that matter) booster? Why would SpaceX spend money designing a new vehicle that can not be reused, because it can not land, for a 10% performance gain over their current rocket which already has excess performance?

They wouldn't land it.
This theoretical LV is intended to serve the heaviest payloads, which otherwise can't be launched by FH at all, thus they go to some competitor instead of SpaceX.
If the center core is single-raptor, expending it may be acceptable.

[DanielW]

I am pretty sure that falcon heavy will be SpaceX's last foray into kerbal style rocketry. By their own admission multi-core was much harder than anticipated.

[andyr]

Thanks for the (considered) replies.

My post was only for a discussion. My thoughts were that to have the central core Methalox (both 1st and 2nd stage), using current tankage size, but reusing Merlin powered side boosters, there could be some gains. Both in terms of payload and data.

I was expecting that this would have better performance TBH.

[Prettz]

More performance to do what?

[Brovane]

Just wondered if this has been discussed anywhere ?

The advantages (forget the costs!) would be:

- Side boosters being Merlin can return to land.
- Central core is expendable
- F9 upper stage may move to Raptor anyway, so making the 1st stage Raptor also might not be too big a leap
- Central core has fewer Raptors (maybe 5?)
- Gains some Raptor flight time ahead of ITS

Some simulations of performance with an FH and a Raptor S2 was run in L2.
http://forum.nasaspaceflight.com/index.php?topic=37599.300

[Steve G]

The only upgraded performance EM would want out of a Falcon Heavy would be to increase the Mars payload for those regular missions he mentioned. This would be best addressed by upgrading the upper stage and keeping everything else the same.  This is assuming he'd need more payload and if all he's launching is a Red Dragon that wouldn't be necessary. I don't see any changes to the Falcons once V5 is done.

The only way we would see a smaller iteration of the ITS is if they discover the challenges of building a full size ITS are too formidable in terms of construction or cost and compromise on a smaller design.