Discussion SpaceX's Grasshopper RLV

[anemazoso]

Elon's talk at the National Press Club today:

http://www.c-spanvideo.org/program/SpaceFligh

 

[Arrowstar]

Elon's talk at the National Press Club today:

http://www.c-spanvideo.org/program/SpaceFligh

If Mr. Musk can pull off a fully reusable LV, then more power to him. I look forward to see his attempt. That all said... he really isn't a great public speaker, is he?

 

[Tychonaut]

The thing is, that they are not just trying to land the stage, they are trying to put it on a trajectory that takes it back to the launch site.

That's something in the video I found odd. Of course you can land the Dragon anywhere you want that your orbit will pass over, and if you have the cross-range capability (shades of STS) you can land the upper stage at the launch site after one orbit if you resolve the TPS and weight problems, etc. But how does the first stage get back to the launch site? It's trajectory is sub-orbital, so it's not going once around and landing. Unless they intend to land it someplace else and the KSC landing in the video is "for illustration purposes only."

 

[Ark]

I dunno where else they could land...are they thinking something like the old TAL abort options? Where are you going to land that doesn't carry the possibility of dropping a booster on a populated area?

 

[Tychonaut]

I dunno where else they could land...are they thinking something like the old TAL abort options? Where are you going to land that doesn't carry the possibility of dropping a booster on a populated area?

Transatlantic is just what I was thinking. Turning it around to land at KSC makes no sense, since they'd need enough fuel left to shed all the velocity the first stage has and then accelerate it back towards KSC. The video, however clearly it reflects their thinking, shows the stage flipping end-over-end and firing retrograde.
I have a hard time believing it could have enough speed and altitude to essentially make a full orbit and return to KSC the long way.
Transatlantic has its own problems, such as would they need multiple landing sites depending on their launch azimuth?
My impression from hearing Musk talk about retro-firing some of the first stage engines was they just wanted to slow it enough to come down in the ocean with minimal stress. I guess I got that wrong. :shrug:

Just tried launching an F9/Dragon from KSC to the ISS in Orbiter to see where the first stage separates. It was around 160 km downrange and 80 km up for my intended orbit. Not as far away as I thought, so perhaps without the mass of the second stage and Dragon to push it would be feasible to return to the launch site.
Certainly impressive if they can pull it off.

 

[N_Molson]

Maybe that dead mass is worth it.

Yeah, but if that means you could haul enough payload to orbit and save another launch... Also there is the multiple engines restarting that seems a little iffy.

For the propellant required, we could try with Velcro Rockets to get a rough idea (assuming that the stage, in retrograde orientation, produces no aerodynamical lift).

About a TAL, I think that building a new rocket could be cheaper than sending back the old one to USA and refurbishing it.

Also, as you pointed before, building more rockets lowers the costs.

 

[Ark]

Transatlantic is just what I was thinking. Turning it around to land at KSC makes no sense, since they'd need enough fuel left to shed all the velocity the first stage has and then accelerate it back towards KSC. The video, however clearly it reflects their thinking, shows the stage flipping end-over-end and firing retrograde.
I have a hard time believing it could have enough speed and altitude to essentially make a full orbit and return to KSC the long way.
Transatlantic has its own problems, such as would they need multiple landing sites depending on their launch azimuth?
My impression from hearing Musk talk about retro-firing some of the first stage engines was they just wanted to slow it enough to come down in the ocean with minimal stress. I guess I got that wrong. :shrug:

Just tried launching an F9/Dragon from KSC to the ISS in Orbiter to see where the first stage separates. It was around 160 km downrange and 80 km up for my intended orbit. Not as far away as I thought, so perhaps without the mass of the second stage and Dragon to push it would be feasible to return to the launch site.
Certainly impressive if they can pull it off.

It would make a lot more sense if they weren't launching from the Cape and could have a landing site a few hundred KMs downrange. The F9 first stage wouldn't be able to glide for crap, so any course deviation away from it's regular splashdown point is going to burn extra fuel.

I guess Musk & Co will just have to do the math and figure out whether the extra dead weight of the fuel and resulting payload penalty is worth the benefit of landing the stage intact for refurbishment.

 

[T.Neo]

But how does the first stage get back to the launch site? It's trajectory is sub-orbital, so it's not going once around and landing. Unless they intend to land it someplace else and the KSC landing in the video is "for illustration purposes only."

I believe their intention is for a boost-back to the launch site.

Also there is the multiple engines restarting that seems a little iffy.

Maybe, maybe not. MVac has restart capability, after all.

About a TAL, I think that building a new rocket could be cheaper than sending back the old one to USA and refurbishing it.

I severely doubt that. Shipping costs should be nothing compared to the cost of building an entire new vehicle.

Refurbishing on the other hand... well, the problem with reuse is that if the refurbishment costs are high, and the original unit costs are higher than for the expendable, then it doesn't really make sense.

You have the RS-68, for example, and the SSME. The SSME costs $50 at the lowest, the RS-68 costs something like $14 million. The problem is, the SSME costs about that to reuse, which makes the whole thing pointless.

Hopefully SpaceX will do better than NASA did with the shuttle.

Also, as you pointed before, building more rockets lowers the costs.

Yes and no. It's important to realise where the savings are and what kind of savings they are. Think of it in this analogy:

- A car from a factory producing 100 cars per year would be relatively expensive.

- A car from a factory producing 10 000 cars per year would be much cheaper.

- It's still far more economic to reuse either car and service it than buy a new one for each trip.

Also, a lot of cost reduction from higher flight rate has to do with amortising development costs and overheads relating to facilities (like launch facilities), rather than engineering and construction (those cost savings would kick in at higher launch rates, rely on improving production to keep up with demand, and is, I think, a more speculative assertion).

 

[Moach]

It would make a lot more sense if they weren't launching from the Cape and could have a landing site a few hundred KMs downrange. The F9 first stage wouldn't be able to glide for crap, so any course deviation away from it's regular splashdown point is going to burn extra fuel.

I guess Musk & Co will just have to do the math and figure out whether the extra dead weight of the fuel and resulting payload penalty is worth the benefit of landing the stage intact for refurbishment.

well, judging for the animation, if the thing is able to land THAT precisely, then it could very well do so onto a barge somewhere, no? :hmm:


and for the capsule, if it were up to me, i'd have it pop a drogue chute and let it slow down before leading it on a [hybrid parachute/powered approach and landing

much less fuel to worry about


parachutes are a lot cheaper than rockets, is that not? - makes sense to leave those unrecovered if you can land the expensive bits safely :sweet:

 

[Andy44]

Launch from Texas. Land stage 1 in Florida. Bribe the Florida politicians to grease the legal skids. Early shuttle studies considered the Texas coast as a new launch site.

 

[T.Neo]

That would indeed be interesting... but beyond speculation on the internet, I don't believe there is any indication that SpaceX is looking toward constructing a Texas launch site.

Things would likely get complicated when trying to fly to different inclinations... you might overfly stuff you don't want to (even when launching due east- your upper stage and payload are going to sail over Florida).

parachutes are a lot cheaper than rockets, is that not? - makes sense to leave those unrecovered if you can land the expensive bits safely

You have to then pack the parachutes for every flight and you won't have that much of a pinpoint landing.

I'd imagine the choice of rockets is due to supposed ease of use.

 

[RGClark]

Elon Musk believes as I do that making the launchers fully reusable can cut the costs to orbit by two orders of magnitude:

Elon Musk Outlines Grand Plans In Speech (Video).
29 september 2011
"Musk said that reusability is the key to the dramatic cost savings that will enable advancements in human exploration of space. He said, the importance of RLVs (reusable launch vehicles) is probably lost on the press (in this room) and the public at large."
...
"Musk stated that SpaceX has a design for rocket reusability that works on paper. He said they will check their calculations against “reality” and they are hoping for a match. “If it does work, it’ll be pretty huge” Musk said. It will lead to a 100 fold reduction in launch costs."
http://moonandback.com/2011/09/29/elon-musk-outlines-grand-plans-in-speech-video/


Bob Clark

 

[N_Molson]

:shrug:

 

[Urwumpe]

:shrug:

That is not really correct. That is a small letter Delta[math]\delta[/math] instead of a big Delta [math]\Delta[/math], which is a completely different meaning in mathematics.

A big Delta is a macroscopic difference between between values (initial and final velocity), a small Delta is a infinitesimal small change of a function, for example the change in velocity in an extremely short period of time. But this makes only sense in context of a differential function, for example to say that when you approach a certain mass ratio, then the change in velocity that you get for every second will be less than [math]\delta V[\math]. In better words: if you have a [math]\epsilon[/math] given for the inputs of a function, you can use the [math](\epsilon, \delta)[/math]-pair of that function to give a maximal deviation that the real function results will have of you propagate the inputs [math]\epsilon[/math] into the future.

 

[N_Molson]

I should have guessed... :rofl:

Is this one more orthodox ?

Where mi = initial mass ; mf = final mass

and Ve = Velocity exhaust, or ISP*9.81

and ln = "logarithme naturel"

(All that to say that adding dead mass like landing gear or parachute systems on rockets seem, at the first glance, to go against physics. Because the higher is mf, the lower is the result, given that the other variables are unchanged).

 

[Urwumpe]

Well, it isn't about orthodoxy...

[math]\delta V = I_o \ln{\frac{M_1}{M_0}}[/math] is simply useless without also giving a [math]\epsilon[/math] for which this limit applies.

I think they wanted to write [math]\Delta V[/math] but ruined it.

 

[RGClark]

I should have guessed... :rofl:

Is this one more orthodox ?

Where mi = initial mass ; mf = final mass

and Ve = Velocity exhaust, or ISP*9.81

(All that to say that adding dead mass like landing gear or parachute systems on rockets seem, at the first glance, to go against physics. Because the higher is mf, the lower is the result, given that the other variables are unchanged).

Dead weight means something that has no value.
It's not dead weight if it can cut your costs to orbit by a factor of 100.

Bob Clark

 

[Urwumpe]

Dead weight means something that has no value.
It's not dead weight if it can cut your costs to orbit by a factor of 100.

Bob Clark

Let's be realistic. Even if it means just 5% cost-savings it is already a decisive victory.

But still, you have to call it dead weight in some phases, because during these phases the mass is just inert ballast and should be as small as possible. In later phases, this may apply to other masses. It is maybe not nice to call the parachute dead weight during launch. But a realistic estimate of how useful the parachute is for reaching orbit.

 

[N_Molson]

In the same manner, a propellant reserve dedicated to powered descent is dead mass on the way to orbit (assuming that the goal is to deliver a payload in orbit...)

 

[RGClark]

A couple of suggestions for the reusable version of the Falcon 9. First, model it on the DC-X. In the SpaceX video of the proposed reusable launcher the first and second stages have the same straight sides of the expendable versions. But having sloping sides helps to protect the sides of the vehicle during reentry as well as increasing aerodynamic stability during reentry.
Note that as long as the cross-section remains circular for a conical shaped stage you should still get the high tankage ratio that obtains for cylindrical tanks:

Space Access Update #91 2/7/00.
The Last Five Years: NASA Gets Handed The Ball, And Drops It.
"...part of L-M X-33's weight growth was the "multi-
lobed" propellant tanks growing considerably heavier than promised.
Neither Rockwell nor McDonnell-Douglas bid these; both used proven
circular-section tanks. X-33's graphite-epoxy "multi-lobed" liquid
hydrogen tanks have ended up over twice as heavy relative to the
weight of propellant carried as the Shuttle's 70's vintage aluminum
circular-section tanks - yet an X-33 tank still split open in test
last fall. Going over to aluminum will make the problem worse; X-
33's aluminum multi-lobed liquid oxygen tank is nearly four times as
heavy relative to the weight of propellant carried as Shuttle's
aluminum circular-section equivalent."
http://www.space-access.org/updates/sau91.html

The McDonnell-Douglas version mentioned there was the scaled up DC-X.
There are a couple of ways this DC-X styled Falcon 9 could be implemented. As this is to be a multi-stage launcher, you could have each stage have the same sloping sides as the DC-C. Then each stage would have the shape of a truncated cone, a frustum, and when stacked one on top another the vehicle would have the shape of a single cone.
However, I prefer another method. It is known that you can increase your payload using parallel staging with cross-feed fueling. Indeed SpaceX intends to increase the payload of its Falcon Heavy launcher using this method. Then another method for this reusable Falcon 9 would have each stage in the shape of a full cone, but the second stage instead of being placed on top of the first stage would be placed along side of it in parallel fashion.
In addition to increasing the payload this would have an another key advantage. The high mass ratio of the Falcon 9 first stage, above 20 to 1, means that if it had high efficiency engines such as the NK-33 or RD-180 instead of the rather low efficiency Merlin 1C it would have SSTO capability. However, because of the high investment of SpaceX in the Merlin engines they no doubt are committed to its use.
But a key fact is that IF you have altitude compensation then even a low efficiency, i.e., low chamber pressure, engine can achieve high vacuum Isp while still providing good performance at sea level. Methods of altitude compensation such as the aerospike have been studied since the 60's. Then SpaceX could provide their DC-X styled Falcon 9 stages with altitude compensation to give their stages SSTO capability while still using the Merlin engines.
Then these SSTO stages could serve as low cost launchers for smaller payloads, including being used for private, manned orbital vehicles.

The second model for the reusable Falcon 9 stages would be on the ESA's proposed Intermediate eXperimental Vehicle (IXV):

Article:
Europe Aims to Launch Robotic Mini-Shuttle By 2020.
Rob Coppinger, SPACE.com ContributorDate: 13 June 2011 Time: 02:58 PM ET
http://www.space.com/11948-robot-space-plane-europe-ixv-launching-2020.html

This does not use the powered landing of the DC-X but rather uses a glided landing via its lifting body shape. SpaceX does not like the use of wings for landing because of the extra weight. But this design would not have wings. It would have larger thermal protection weight because the horizontal underside would have to be covered, whereas in the DC-X mode only the base has to be covered. However, it would make up for this in not requiring fuel for the powered landing.
In this case because the stages would have to maintain the aerodynamic shape, they could not be stacked as for serial staging. Parallel staging would have to be used. Once again this means the separate stages could be used as SSTO's.



Bob Clark