Re: Elon Musk's SpaceX to build 'Grasshopper' hover-rocket
Robert Clark
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
Robert Clark
> A couple of suggestions for the reusable version of the Falcon 9.
> 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
> EThttp://www.space.com/11948-robot-spa...hing-2020.html
>
Europe Aims to Launch Robotic Mini-Shuttle By 2020.
Bob Clark
Robert Clark
> Whatever happened to the "Glide Back" booster proposals that
> would allow the booster to fly downrange, as well as vertically,
> and still be able to return to the launch site?
Elon doesn't like wings because of the extra weight. He mentions this
again during the National Press Club speech:
NATIONAL PRESS CLUB LUNCHEON WITH ELON MUSK.
http://www.spacex.com/npc-luncheon-elon-musk.php
BTW, I think the payload lost in making the vehicle reusable is being
overstated. Elon himself during the speech spoke ruefully of cutting
into the 2%-3% payload fraction of launch vehicles. But actually a
small percentage of the vehicle's dry weight, which is the important
parameter not the gross weight, would need to go the reentry/landing
systems.
The reason is this is for a multi-stage launcher, and a key fact is
for the larger first stage any extra kilo added to the first stage dry
weight subtracts only ca. 1/10th of a kilo from the payload.
And also for multi-stage launchers, the upper stage dry weight is
usually rather small, in fact frequently smaller than the payload.
We can estimate the added weight for the Falcon 9. This page
estimates the weights for this launcher:
Space Launch Report: SpaceX Falcon Data Sheet.
http://www.spacelaunchreport.com/falcon9.html#components
The dry weight for the first stage is given as 19 mT, and 3 mT for
the upper stage. These weights might even be overestimated. Some
references for instance give the dry weight for the first stage as in
the range of 15 mT.
Now estimate the mass of reentry/landing systems. First, Robert
Zubrin gives an estimate of about 15% of the landed weight for reentry
thermal protection:
Reusable launch system.
http://en.wikipedia.org/wiki/Reusable_launch_system#Reentry_heat_shields
Secondly, an estimate of 10% is often cited for the wings for glided
landing or for the fuel for powered landing:
Reusable launch system.
http://en.wikipedia.org/wiki/Reusable_launch_system#Horizontal_takeoff
Finally the estimated weight for the landing gear is about 3%:
Landing gear weight.
http://yarchive.net/space/launchers/landing_gear_weight.html
This totals to 28%. However, it is important to keep in mind that
with modern materials this can probably be reduced to half this.
So 14% of 19 mT on the first stage is 2,660 kg. But remember for a
first stage this will only subtract about 1/10th this from the
payload. So 270 kg lost.
For the second stage 14% of 3 mT is 420 kg. So the total is in the
range of 700 kg lost from the Falcon 9 payload capacity to LEO of
10,000 kg.
But by doing this you are making the vehicle reusable and cutting
costs by a factor of 100.
Bob Clark
Robert Clark
> 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.
>
the "parashield" idea of the Dr. David Akin or the inflatable heat
shield NASA is investigating. These might make the reusable Falcon 9
easier and quicker to implement since the usual cylindrical shaped
stages could be used:
Phoenix: A Low-Cost Commercial Approach to the Crew Exploration
Vehicle.
http://www.nianet.org/rascal/forum2006/presentations/1010_umd_paper.pdf
"Figure 5.9-1: Phoenix ParaShield in stowed and deployed
configurations."
http://oi51.tinypic.com/14e9vd4.jpg
Another advantage of the parashield is that it can also serve as a
parachute once the vehicle has passed through reentry.
And for NASA's inflatable heat shield:
NASA Launches New Technology: An Inflatable Heat Shield.
UPDATE: 08.17.09
http://www.nasa.gov/topics/aeronautics/features/irve.html
See the video on this page describing the inflatable heat shield.
Bob Clark
Alan Erskine
very careful with centre of gravity.
Robert Clark
> 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:
>
> 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
> 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.
>
be of the Japanese HYFLEX hypersonic test vehicle:
Hypersonic Flight Experiment "HYFLEX".
http://www.jaxa.jp/projects/rockets/hyflex/index_e.html
HYFLEX.
http://www.rocket.jaxa.jp/fstrc/0c02.html
This was successfully tested all the way back in 1996 at a Mach 15
reentry speed.
It's roughly cylindrical shape would mean you would lose a relatively
small degree on the mass efficiency of cylindrically shaped tanks.
However, rather than redesigning the tanks you might want to just use
a composite aeroshell on the usual Falcon 9 stages. A conical
aeroshell for example was used on the DC-X.
This would make the reusable Falcon 9 more quickly and easily to be
implemented. The mass of the aeroshell though would contribute to the
mass lost from payload.
As with the above cases, if used with altitude compensating nozzles
on the Merlins or with existing high efficiency engines with just
their standard nozzles, these HYFLEX-shaped stages could also be
SSTO's.
Bob Clark