F9 Third Stage

[arachnitect]

If it's ever been talked about, I've missed it. Something like Blok D or Transtage. Why not?

Right now, F9 has no way of turning its good LEO performance into higher energy. More Stages is how everyone else solves this problem.

Alternative is a whole new second stage (probably with a higher energy propellant and engine cycle) or using FH for payloads that really shouldn't need it.

Would be simple compared to many of spacex' other development projects.

[gospacex]

I believe three stage version is called Falcon Heavy.

[guckyfan]

Right now, F9 has no way of turning its good LEO performance into higher energy. More Stages is how everyone else solves this problem.

Alternative is a whole new second stage (probably with a higher energy propellant and engine cycle) or using FH for payloads that really shouldn't need it.

Would be simple compared to many of spacex' other development projects.

SpaceX is certainly not "everyone else".

A new second stage with higher energy propellant? An expensive detour, no way they would do that. Though a while back I thought they might do a methane upper stage just for training before they build the new large launch vehicle. We know now this is not happening.

Simple would be a third stage with hypergolics. But Falcon Heavy with reusable boosters is easier once a Heavy has been built.

[IRobot]

Right now, F9 has no way of turning its good LEO performance into higher energy. More Stages is how everyone else solves this problem.

Musk had said before that keeping stages to minimum is a goal, to reduce costs and improve reliability.

They don't need more performance if they are cheap and reliable.

[M129K]

Simple would be a third stage with hypergolics. But Falcon Heavy with reusable boosters is easier once a Heavy has been built.

A Falcon 9 with a third stage would be able to bridge the massive gap between F9 and FH, and a hypergolic third stage wouldn't have to be that expensive compared to a higher energy upper stage, yet could increase F9 GTO performance to about 7 tons.

[Tomness]

Super draco derived third stage would be awesome, but i doubt they want or need it.

[inventodoc]

Super draco derived third stage would be awesome, but i doubt they want or need it.

The Superdraco would be really cool and I can see why that would seem appealing.  I thought that a cluster could even power a whole rocket stage (or enable new classes of VSTOL aircraft) but the problem with Superdraco is the crummy Isp - I think it's only around 235 which is fairly poor efficiency.   Merlin gets in the low 300's and the Raptor is aiming for 365ish.   I think that hydrolox rockets get around 430.

One of the first posters mentioned 'Falcon heavy'.  I guess they are right that Falcon Heavy is truly a three stage rocket.   The side boosters are stage one (with cross propellant flow to the center),  the still mostly full center booster is part of stage 1 and all of stage two, and the upper stage is stage 3, so there you have it.

[A_M_Swallow]

Try a reusable space tug.  A Raptor engine with appropriate sized fuel tanks can fly several tons of cargo to lunar orbit and the Lagrange points.  It can pick up 13 tonnes of cargo from a Falcon 9 in LEO and lots of fuel from the Falcon Heavy.

[rst]

A Falcon 9 with a third stage would be able to bridge the massive gap between F9 and FH, and a hypergolic third stage wouldn't have to be that expensive compared to a higher energy upper stage, yet could increase F9 GTO performance to about 7 tons.

They could also bridge that gap with fully-reusable, RTLS Falcon Heavy (eating the heavy performance penalty for RTLS on the core stage).  More generally, the reason that you don't see people launching small payloads on big rockets is that you're throwing away a lot more rocket than you have to.  The long-term goal for SpaceX is to get the rocket back; if they can do that, and keep refurbishment costs to a minimum, that changes the economics.

[sdsds]

I'm guessing SpaceX won't add a third stage to F9 because those few missions that could use it could instead incorporate a solid motor into the payload.

[guckyfan]

I'm guessing SpaceX won't add a third stage to F9 because those few missions that could use it could instead incorporate a solid motor into the payload.

That would require development on many different satellites by many suppliers. A SuperDraco third stage would be a solution for all purposes. The ISP of SuperDraco gets a lot better with a vacuum extension of the bell. It would beat the Ariane 6 before Ariane construction has even begun.

That's not saying I believe it will happen, I don't.

[darkenfast]

Why Super Dracos?  Why not use a Kestral?   Two spherical tanks, helium tanks and a Kestral, contained inside the fairing.  There's not a lot you can do, as this is all eating into payload and space, but is there any niche that a Kestral 3rd stag would fill?  Perhaps certain space probes?  I don't know, I'm just wondering if there would be a business case for this.

[CuddlyRocket]

A Falcon 9 with a third stage would be able to bridge the massive gap between F9 and FH, and a hypergolic third stage wouldn't have to be that expensive compared to a higher energy upper stage, yet could increase F9 GTO performance to about 7 tons.

They could also bridge that gap with fully-reusable, RTLS Falcon Heavy (eating the heavy performance penalty for RTLS on the core stage).  More generally, the reason that you don't see people launching small payloads on big rockets is that you're throwing away a lot more rocket than you have to.  The long-term goal for SpaceX is to get the rocket back; if they can do that, and keep refurbishment costs to a minimum, that changes the economics.

This is probably the lower cost alternative and SpaceX optimises for cost, not performance.

[arachnitect]

Simple would be a third stage with hypergolics. But Falcon Heavy with reusable boosters is easier once a Heavy has been built.

A Falcon 9 with a third stage would be able to bridge the massive gap between F9 and FH, and a hypergolic third stage wouldn't have to be that expensive compared to a higher energy upper stage, yet could increase F9 GTO performance to about 7 tons.

How do you figure 7t to GTO? I'm playing around with a spreadsheet and don't really see that much performance increase for heavier payloads. I'm more interested in what kind of dV you can get for 1800kg payload.

If I'm doing this right (not likely I admit), a surprisingly good option is a Falcon 1 second stage (Kestrel). Also really good: Castor 30B.

Storable propellant stages get screwed twice. First with low Isp, then with bad mass fraction.


As for cost of RTLS FH boosters vs. a 3rd stage, even if you get the boosters back, it's not free to fly them. I have no idea how to even begin estimating the costs though so we'll just have to wait and see I guess.

[macpacheco]

If it's ever been talked about, I've missed it. Something like Blok D or Transtage. Why not?

Right now, F9 has no way of turning its good LEO performance into higher energy. More Stages is how everyone else solves this problem.

Alternative is a whole new second stage (probably with a higher energy propellant and engine cycle) or using FH for payloads that really shouldn't need it.

Would be simple compared to many of spacex' other development projects.

Short term solution is Falcon Heavy, long term solution is scaled up Falcon 9 using Raptors. The later rocket would have twice the payload of Falcon Heavy to most missions, making it possible to accomplish every Falcon Heavy mission in expendable mode with full reuse.
Using a true third stage make little sense. The FH solution is very smart, and scaling up the F9 with Raptor should provide a very affordable launch system (cheap fuel, full reuse).
Remember, SpaceX is about cost effectiveness, not making the technically perfect rocket that is way too expensive. Without economics the Mars Colony won't be viable. Every launch system should be designed for maximum flexibility with minimum cost.
The main issue of F9 and FH efficiency wise is it's low ISP in Vacuum mode compared with LH rockets. Using methane should bridge that gap very well, maintaining a single engine for first and second stage.

[Zed_Noir]

SpaceX might developed a Super-Draco based upper stage if it could also be used as Dragon service module component. Solid motors and nonstorable liquid motors are not applicable for SM usage.

[Elmar Moelzer]

Do we have any official numbers for the Isp for the SuperDraco. According to Musk, a lot of development effort went into that engine. So I would assume that it has better than average performance numbers.

[guckyfan]

Do we have any official numbers for the Isp for the SuperDraco. According to Musk, a lot of development effort went into that engine. So I would assume that it has better than average performance numbers.

It is very limited due to its engine bell size. It should be easy though to add a vacuum nozzle and significantly enhance ISP, considered its high pressure. Easy as rocket science goes.

[Elmar Moelzer]

It is very limited due to its engine bell size. It should be easy though to add a vacuum nozzle and significantly enhance ISP, considered its high pressure. Easy as rocket science goes.

Hmm, wonder what the limits given an optimal engine bell size would be.

[arachnitect]

It is very limited due to its engine bell size. It should be easy though to add a vacuum nozzle and significantly enhance ISP, considered its high pressure. Easy as rocket science goes.

Hmm, wonder what the limits given an optimal engine bell size would be.

Chamber pressure doesn't make much difference once you're in vacuum, expansion ratio is more important.

AJ-10 gets ~320s. I would consider anything above 330s absurdly good. Pump fed has gotten to 340s.

Mass fraction for pressure fed engine is an issue, high chamber pressure becomes a liability.