Discussion SpaceX's Grasshopper RLV - Page 3 - Orbiter-Forum

T.Neo · 09-30-2011, 07:17 PM

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).

Quote:

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 · 10-01-2011, 02:55 PM

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/el...-speech-video/

Bob Clark

N_Molson · 10-01-2011, 03:13 PM

**Urwumpe** · 10-01-2011, 03:48 PM

Quote:

Originally Posted by N_Molson

{image}

That is not really correct. That is a small letter Delta $\delta$ instead of a big Delta $\Delta$ , 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 $\epsilon$ given for the inputs of a function, you can use the $(\epsilon, \delta)$ -pair of that function to give a maximal deviation that the real function results will have of you propagate the inputs $\epsilon$ into the future.

N_Molson · 10-01-2011, 03:53 PM

I should have guessed...

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** · 10-01-2011, 04:05 PM

Well, it isn't about orthodoxy...

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

I think they wanted to write $\Delta V$ but ruined it.

RGClark · 10-01-2011, 04:08 PM

Quote:

Originally Posted by N_Molson

I should have guessed...

Is this one more orthodox ?

{image}

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** · 10-01-2011, 04:12 PM

Quote:

Originally Posted by RGClark

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 · 10-01-2011, 04:15 PM

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 · 10-02-2011, 02:40 AM

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-spa...hing-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

T.Neo · 10-02-2011, 12:12 PM

Quote:

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

Yeah, wouldn't reusability result in awesome cost savings?

(Ok, the thing had expendable components- but it was the reusable bit that made it more expensive than competing launch vehicles.)

I hope that SpaceX can succeed where STS failed. We shouldn't write off reusability just yet, there are a lot of solutions to this particular problem and the failed STS solution certainly isn't the only one.

Also: the interstage and fairing are not (as far as I know) reused...

RGClark · 10-02-2011, 01:40 PM

Quote:

Originally Posted by 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...)

This is not a big issue of what is considered "dead weight". You can call it dead weight if it does not help you get to orbit. But should you call it dead weight if it is required for you to accomplish your mission?
Were the heat shields on the Apollo capsules dead weight? Were the parachutes?

Bob Clark

N_Molson · 10-02-2011, 02:19 PM

Quote:

Were the heat shields on the Apollo capsules dead weight? Were the parachutes?

Yes, from a physical point of view, they were : regardless of any context, the less empty mass, the more Delta-V. Pure theoric point of view.

From the engineering point of view, they were "required dead mass". Those devices were designed as a compromise between efficiency and functionality, and every effort was made to make them as light as possible. From there is the idea of "mass budget" : what can be spared on a component can be spent on another.

Now if you add economics and reusability constraints, it will clash with efficiency. There, studies and experiments have to be made to see which mass budget can be allowed to reusability, knowing that it will anyway compromise efficiency. And then calculate on a serie of launches if it is economically viable or not.

T.Neo · 10-02-2011, 04:00 PM

Efficiency isn't all about mass budgets and dV capability and stuff like that. If a more massive vehicle can achieve lower costs, higher reliability, or other features that improve its usefulness, then it can be truely better.

Landing gear on aircraft are also not needed at cruising altitude, over the ocean. There they are unecessary, inefficient mass. But they are required for the aircraft to be reused- to operate repeatedly, to do its job.

There may come a time, when lack of recovery equipment on a rocket stage, will be viewed in a similar manner to removing the landing gear of an airliner...

anemazoso · 10-02-2011, 04:06 PM

I love this idea and feel a little silly that one of us in the Orbiter community haven't published and add-on with this type of system because it really is a "head slapper" to me, so simple.

That being said I think Elon (and I am a self admitted fanboy) bit off more than he can chew this time. This system would be in the category "to-good-to-be-true." I think it can be done but not sure if Elon can do it with the budget he can spend on it.

Then again I said before on this forum that the Falcon Heavy would never really fly because "who wants to trust 27 engines" and I am probably closer to being wrong on that one at this point in time.

We shall see. Can't wait for Grasshopper to fly!

09-30-2011, 07:17 PM	#31
T.Neo SA 2010 Soccermaniac	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). Quote: 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. Last edited by T.Neo; 09-30-2011 at 07:19 PM.

10-01-2011, 03:48 PM	#34
Urwumpe Donator	Quote: Originally Posted by N_Molson {image} That is not really correct. That is a small letter Delta $\delta$ instead of a big Delta $\Delta$ , 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 $\epsilon$ given for the inputs of a function, you can use the $(\epsilon, \delta)$ -pair of that function to give a maximal deviation that the real function results will have of you propagate the inputs $\epsilon$ into the future. Last edited by Urwumpe; 10-01-2011 at 03:53 PM.

10-01-2011, 03:53 PM	#35
N_Molson Addon Developer	I should have guessed... Is this one more orthodox ? Where mi = initial mass ; mf = final mass and Ve = Velocity exhaust, or ISP9.81 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). Last edited by N_Molson; 10-01-2011 at 04:10 PM.*

10-02-2011, 02:40 AM	#40
RGClark Mathematician	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-spa...hing-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 Last edited by RGClark; 10-02-2011 at 01:43 PM. Reason: clarity

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10-01-2011, 02:55 PM	#32
RGClark Mathematician	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/el...-speech-video/ Bob Clark

10-01-2011, 04:05 PM	#36
Urwumpe Donator	Well, it isn't about orthodoxy... $\delta V = I_o \ln{\frac{M_1}{M_0}}$ is simply useless without also giving a $\epsilon$ for which this limit applies. I think they wanted to write $\Delta V$ but ruined it.

10-01-2011, 04:08 PM	#37
RGClark Mathematician	Quote: Originally Posted by N_Molson I should have guessed... Is this one more orthodox ? {image} 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

10-01-2011, 04:12 PM	#38
Urwumpe Donator	Quote: Originally Posted by RGClark 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.

10-01-2011, 04:15 PM	#39
N_Molson Addon Developer	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...)

10-02-2011, 12:12 PM	#41
T.Neo SA 2010 Soccermaniac	Quote: Elon Musk believes as I do that making the launchers fully reusable can cut the costs to orbit by two orders of magnitude: Yeah, wouldn't reusability result in awesome cost savings? (Ok, the thing had expendable components- but it was the reusable bit that made it more expensive than competing launch vehicles.) I hope that SpaceX can succeed where STS failed. We shouldn't write off reusability just yet, there are a lot of solutions to this particular problem and the failed STS solution certainly isn't the only one. Also: the interstage and fairing are not (as far as I know) reused...

10-02-2011, 01:40 PM	#42
RGClark Mathematician	Quote: Originally Posted by 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...) This is not a big issue of what is considered "dead weight". You can call it dead weight if it does not help you get to orbit. But should you call it dead weight if it is required for you to accomplish your mission? Were the heat shields on the Apollo capsules dead weight? Were the parachutes? Bob Clark

10-02-2011, 02:19 PM	#43
N_Molson Addon Developer	Quote: Were the heat shields on the Apollo capsules dead weight? Were the parachutes? Yes, from a physical point of view, they were : regardless of any context, the less empty mass, the more Delta-V. Pure theoric point of view. From the engineering point of view, they were "required dead mass". Those devices were designed as a compromise between efficiency and functionality, and every effort was made to make them as light as possible. From there is the idea of "mass budget" : what can be spared on a component can be spent on another. Now if you add economics and reusability constraints, it will clash with efficiency. There, studies and experiments have to be made to see which mass budget can be allowed to reusability, knowing that it will anyway compromise efficiency. And then calculate on a serie of launches if it is economically viable or not.

10-02-2011, 04:00 PM	#44
T.Neo SA 2010 Soccermaniac	Efficiency isn't all about mass budgets and dV capability and stuff like that. If a more massive vehicle can achieve lower costs, higher reliability, or other features that improve its usefulness, then it can be truely better. Landing gear on aircraft are also not needed at cruising altitude, over the ocean. There they are unecessary, inefficient mass. But they are required for the aircraft to be reused- to operate repeatedly, to do its job. There may come a time, when lack of recovery equipment on a rocket stage, will be viewed in a similar manner to removing the landing gear of an airliner...

10-02-2011, 04:06 PM	#45
anemazoso Addon Developer	I love this idea and feel a little silly that one of us in the Orbiter community haven't published and add-on with this type of system because it really is a "head slapper" to me, so simple. That being said I think Elon (and I am a self admitted fanboy) bit off more than he can chew this time. This system would be in the category "to-good-to-be-true." I think it can be done but not sure if Elon can do it with the budget he can spend on it. Then again I said before on this forum that the Falcon Heavy would never really fly because "who wants to trust 27 engines" and I am probably closer to being wrong on that one at this point in time. We shall see. Can't wait for Grasshopper to fly!