Author Topic: Problems of SSTO and technologies to solve these  (Read 50489 times)

Offline hkultala

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Re: Problems of SSTO and technologies to solve these
« Reply #220 on: 02/10/2019 07:46 am »
Hrm, I wonder if you could get away with an electric pump for the TAN RP1 injection in the nozzle, and potentially avoid coking?

Electric pumps would be TERRIBLE for SSTO.

You would need to carry all the weight of the batteries all the way up to the orbit. Or, if you don't, your vehicle is not SSTO.

Or, in order to not have the heavy batteries, you would need to have generators in your main turbopump. I don't see the point in this.

« Last Edit: 02/10/2019 07:49 am by hkultala »

Offline john smith 19

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Re: Problems of SSTO and technologies to solve these
« Reply #221 on: 02/10/2019 10:14 am »

TAN is a great idea, but becomes unnecessary for SSTO if a (hinted at) 200:1 T/W dense fuel engine like Raptor is available.  Raptor would be only ~0.65% of launch mass which is only around 10% of the ~6.5% of GTOW that an SSTO powered by it can orbit.  Thought the boost it can give to vacuum isp (through use of higher expansion ratio TAN nozzle) is probably more significant.
That leaves the problem of landing.

A single engine SSTO design either needs to avoid relighting its engine for landing IE Gliding in, or have an engine capable of throttling  to something like 3% of its takeoff thrust.

The first is certainly possible the second is at least partly why SX use engine clusters.
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Offline Proponent

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Re: Problems of SSTO and technologies to solve these
« Reply #222 on: 02/10/2019 01:13 pm »
TAN is a great idea, but becomes unnecessary for SSTO if a (hinted at) 200:1 T/W dense fuel engine like Raptor is available.  Raptor would be only ~0.65% of launch mass which is only around 10% of the ~6.5% of GTOW that an SSTO powered by it can orbit.

Where does the ~6.5% figure come from?  Off hand, it seems to me that most SSTO proposals have considerably lower payloads fractions.

Offline RobLynn

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Re: Problems of SSTO and technologies to solve these
« Reply #223 on: 02/10/2019 07:03 pm »
TAN is a great idea, but becomes unnecessary for SSTO if a (hinted at) 200:1 T/W dense fuel engine like Raptor is available.  Raptor would be only ~0.65% of launch mass which is only around 10% of the ~6.5% of GTOW that an SSTO powered by it can orbit.

Where does the ~6.5% figure come from?  Off hand, it seems to me that most SSTO proposals have considerably lower payloads fractions.

The rocket equation.  9100m/s (typical dense fuel SSTO LEO deltaV). 340s Isp. 6.5%GTOW = total mass in LEO RLV+payload in orbit.  Number is not particularly important other than to make point of how small a factor engine mass is with high thrust to weight engine.
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Offline RobLynn

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Re: Problems of SSTO and technologies to solve these
« Reply #224 on: 02/10/2019 07:15 pm »

TAN is a great idea, but becomes unnecessary for SSTO if a (hinted at) 200:1 T/W dense fuel engine like Raptor is available.  Raptor would be only ~0.65% of launch mass which is only around 10% of the ~6.5% of GTOW that an SSTO powered by it can orbit.  Thought the boost it can give to vacuum isp (through use of higher expansion ratio TAN nozzle) is probably more significant.
That leaves the problem of landing.

A single engine SSTO design either needs to avoid relighting its engine for landing IE Gliding in, or have an engine capable of throttling  to something like 3% of its takeoff thrust.

The first is certainly possible the second is at least partly why SX use engine clusters.

If you are trying to do economic SSTO you need to find every scrap of performance you can.  I like the idea of a net catcher to eliminate mass of precise trajectory control and landing systems: 3 or more big multicopter drones with a net strung between them, dive to intercept (and reduce net impact velocity).
The glass is neither half full nor half empty, it's just twice as big as it needs to be.

Online edzieba

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Re: Problems of SSTO and technologies to solve these
« Reply #225 on: 02/11/2019 07:30 am »
No need for drone net shenanigans, mid-air recovery with fixed-wing aircraft is a well-practiced technique (using mk-1 eyeball guidance and timing) for collecting objects post re-entry, and has had a several demo missions successfully performed with helicopters used to snag lower-mass objects.

Mid-air recovery is attractive due not just to landing gear mass savings, but structural mass savings from not needing to handle landing impacts in the first place. To use Falcon 9 as an example, it would not just eliminated the legs and leg attachment points, but also the internal strengthening brace tubes inside the RP-1 tank, and some of the Octaweb mass. The attachment point for the recovery line can be the same point used for lifting for ground handling (minimal added structural mass) and the base needs minimal provision for being able to survive a ground impact (e.g. with helicopter capture, can be placed with precision into a receiving cradle). The obvious downside is loss of vehicle if anything interrupts groundside recovery operations (like weather) or if an abort places reentry outside of predefined recovery coverage areas.

Offline hkultala

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Re: Problems of SSTO and technologies to solve these
« Reply #226 on: 02/11/2019 08:54 am »

TAN is a great idea, but becomes unnecessary for SSTO if a (hinted at) 200:1 T/W dense fuel engine like Raptor is available.  Raptor would be only ~0.65% of launch mass which is only around 10% of the ~6.5% of GTOW that an SSTO powered by it can orbit.  Thought the boost it can give to vacuum isp (through use of higher expansion ratio TAN nozzle) is probably more significant.
That leaves the problem of landing.

A single engine SSTO design either needs to avoid relighting its engine for landing IE Gliding in, or have an engine capable of throttling  to something like 3% of its takeoff thrust.

The first is certainly possible the second is at least partly why SX use engine clusters.

If you are trying to do economic SSTO you need to find every scrap of performance you can.

Exactly the opposite.

In order to do anything cheaply you have to leave out all the expensive experimental stuff.

Quote
  I like the idea of a net catcher to eliminate mass of precise trajectory control and landing systems: 3 or more big multicopter drones with a net strung between them, dive to intercept (and reduce net impact velocity).


Have you got any idea how big and expensive those "drones" need to be?

But landing is not part of SSTO. SSTO means Single Stage to orbit.

you are already talking about.. SSTOL? Single Stage To Orbit and Landing, except then you are not single stage, you are two stage to landing,  TSTOL ? (two stage to orbit and landing?)

And then it makes much more sense to stage on ascent than stage on descent.

Staging on ascent gives much bigger benefits than staging on descent, and it's much cheaper, easier, better for performance etc.


Offline Proponent

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Re: Problems of SSTO and technologies to solve these
« Reply #227 on: 02/11/2019 01:43 pm »
The rocket equation.  9100m/s (typical dense fuel SSTO LEO deltaV). 340s Isp. 6.5%GTOW = total mass in LEO RLV+payload in orbit.  Number is not particularly important other than to make point of how small a factor engine mass is with high thrust to weight engine.

Oh, right.  I had mistakenly thought you were suggesting that an SSTO's payload might be ~6.5% of its GTOW.

I would expect, though, that an SSTO's payload as a fraction of GTOW will be quite a bit less than 6.5%.  Even the fully-expendable two-stage Atlas 401, for example, has a payload fraction of only about 3.8%.  For a reusable SSTO, 0.65% might be the difference between a positive payload and a negative one.

Offline jongoff

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Re: Problems of SSTO and technologies to solve these
« Reply #228 on: 02/11/2019 06:15 pm »
With TAN coming off patent soonish, perhaps it's time to revisit nozzle tricks. Specifically a vacuum optimized nozzle with TAN injection of a high density fuel to keep the plume stable and achieve high thrust when it counts, AKA a tripropellant engine but a bipropellant chamber. Say a LOX+methane/RP1 combo.

Hrm, I wonder if you could get away with an electric pump for the TAN RP1 injection in the nozzle, and potentially avoid coking?

Or my favorite, doing a LOX/LH2 core, with a very oxidizer-rich LOX/LH2 TAN injection... Shift the overall stage mass fraction to closer to a typical LOX/Methane stage, push the engine T/W ratio up into a healthier range, with LOX/HC Isp performance at low altitude and LOX/LH2 Isp performance for the majority of the burn...

~Jon

Offline RobLynn

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Re: Problems of SSTO and technologies to solve these
« Reply #229 on: 02/11/2019 07:00 pm »
The rocket equation.  9100m/s (typical dense fuel SSTO LEO deltaV). 340s Isp. 6.5%GTOW = total mass in LEO RLV+payload in orbit.  Number is not particularly important other than to make point of how small a factor engine mass is with high thrust to weight engine.

Oh, right.  I had mistakenly thought you were suggesting that an SSTO's payload might be ~6.5% of its GTOW.

I would expect, though, that an SSTO's payload as a fraction of GTOW will be quite a bit less than 6.5%.  Even the fully-expendable two-stage Atlas 401, for example, has a payload fraction of only about 3.8%.  For a reusable SSTO, 0.65% might be the difference between a positive payload and a negative one.

Yeah, and other huge impact of rocket mass is the vehicle centre of mass - and requirement for heavy wings or other planform area to balance all the tail mounted engines.  Maybe it is worth chasing engine T/W uber alles?  A TAN version of Raptor might get up to 3-400:1 T/W with Vac Isp of ~380s.
The glass is neither half full nor half empty, it's just twice as big as it needs to be.

Offline john smith 19

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Re: Problems of SSTO and technologies to solve these
« Reply #230 on: 02/11/2019 08:05 pm »
Yeah, and other huge impact of rocket mass is the vehicle centre of mass - and requirement for heavy wings or other planform area to balance all the tail mounted engines.  Maybe it is worth chasing engine T/W uber alles?  A TAN version of Raptor might get up to 3-400:1 T/W with Vac Isp of ~380s.
With the proviso this is always in the context of a VTOL SSTO system.

HTOL SSTO systems can have options.
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Offline RobLynn

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Re: Problems of SSTO and technologies to solve these
« Reply #231 on: 02/13/2019 12:29 am »
Have you got any idea how big and expensive those "drones" need to be?

Yes.  Assuming a relatively big 50 tonne returning RLV they each need to lift about 20 tonnes, If you want off-the-shelf then 3x Mi-26s to do the job would cost about $20k an hour to run each, software development and testing would cost a lot more but there are off-the-shelf helicopter autopilots available to help nowadays.  A pretty small cost in RLV development terms.

Or if you want to build something cheaper specifically for the job then at about 200kW/ton for helicopter disk loadings would need about 6000kW - most cheaply done with a collection of about 20 V8 crate engines (<$10k each) each geared to an appropriate lift rotor and mounted on a lightweight hexagonal cell structure space frame.  (like 18 rotor evolo electric helicopter). Cost a few million to build.  A very cheap highly scalable approach.
(I should mention, that crate engines are typically capable of running at max power for 500hours, done as part of the standard high speed endurance testing procedure for new engines)

It would probably only work with a wingless and squat kankoh maru type RLV design with terminal velocity <100m/s.
« Last Edit: 02/13/2019 02:03 am by RobLynn »
The glass is neither half full nor half empty, it's just twice as big as it needs to be.

Offline RobLynn

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Re: Problems of SSTO and technologies to solve these
« Reply #232 on: 02/13/2019 01:53 am »
Yeah, and other huge impact of rocket mass is the vehicle centre of mass - and requirement for heavy wings or other planform area to balance all the tail mounted engines.  Maybe it is worth chasing engine T/W uber alles?  A TAN version of Raptor might get up to 3-400:1 T/W with Vac Isp of ~380s.
With the proviso this is always in the context of a VTOL SSTO system.

HTOL SSTO systems can have options.

HTOL really only makes sense if airbreathing is involved - which comes with huge aero and gravity losses, high dynamic pressures , high developmental complexity and long development cycles all of which mean huge cost with marginal likelihood of success (technical or commercial).  HL might be viable, though with terrible abort options and typical estimates for wings and wheels at around 10% of landed mass vs as little as 5% for a 'fluffy' rocket powered VL and ability to land anywhere,  it is now tough to make a convincing case for HL.

The arguments for HL used to hold more water in pre-spaceX day due to lack of familiarity, but propulsive VL has been very well proven now and is almost certainly going to be the standard going forward.
The glass is neither half full nor half empty, it's just twice as big as it needs to be.

Offline john smith 19

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Re: Problems of SSTO and technologies to solve these
« Reply #233 on: 02/14/2019 06:56 am »

HTOL really only makes sense if airbreathing is involved - which comes with huge aero and gravity losses,
Most of which are swallowed during the winged portion of the flight plan.
Quote from: RobLynn
high dynamic pressures , high developmental complexity and long development cycles all of which mean
OTOH thousands of HTOL "reusable vehicles" have been developed, versus a handful of capsules and an even smaller number of VTHL winged vehicles for orbital return. Note only a winged vehicle (Shuttle) have down mass in the 10s of tonnes range.

Quote from: RobLynn
huge cost with marginal likelihood of success (technical or commercial). 
An assertion with no actual evidence to back it.
Quote from: RobLynn
HL might be viable, though with terrible abort options and
Another assertion. You might like to look at the survival rates of airplane crashes versus helicopter crashes, factoring the volume traffic in each mode of transport.
Quote from: RobLynn
typical estimates for wings and wheels at around 10% of landed mass vs as little as 5% for a 'fluffy' rocket powered VL and ability to land anywhere,  it is now tough to make a convincing case for HL.
The theoretical ability to "land anywhere" of a VTOL rocket has only ever been demonstrated on the Moon. It was an argument Bono & Gatland used in the 1960's but AFAIK it's never swayed any VC investor.
Needing a runway is not a handicap on Earth as long as the consumables to get the vehicle ready to get it back to its home base are readily available. Shuttle in this respect (as in so many others) was a poor role model.  :(
Quote from: RobLynn
The arguments for HL used to hold more water in pre-spaceX day due to lack of familiarity, but propulsive VL has been very well proven now and is almost certainly going to be the standard going forward.
"Well proven" by rocket standards is in the 10s of landings. Try 100s of 1000s for 777 or A380 sized HTOL vehicles. That's "well proven" by the standards of every other commercial transportation system.

I think it's very telling that despite arguably the best design team in US rocket building industry (in terms of current applied skills experience, as opposed to something, somewhere, inside the corporate memory that was done long ago) and a very strong desire by the Chief Designer to avoid wings at all costs SX still ended up with a winged US design.

Wheather they could have if they didn't have to have a US with the AR of a conventional rocket is debatable, but how exactly would that have worked?

If "Be usable on any large body in the Solar System" is a design constraint then VTOL is the obvious way to go.

If not then leveraging the huge design base of HTOL (solutions as well as issues) remains a viable option. Between 30 and 50% less thrust for the same GTOW, with very limited black zones and intact recovery.

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Online Tulse

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Re: Problems of SSTO and technologies to solve these
« Reply #234 on: 02/14/2019 02:03 pm »
I think it's very telling that despite arguably the best design team in US rocket building industry (in terms of current applied skills experience, as opposed to something, somewhere, inside the corporate memory that was done long ago) and a very strong desire by the Chief Designer to avoid wings at all costs SX still ended up with a winged US design.
As I understand it (and I may be wrong), the "wings" on Starship won't provide any lift, but are simply control surfaces, much like the gridfins on the Falcon 9.  I don't think they're a very good argument for the utility of lifting surfaces on an orbital craft.

Offline Lars-J

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Re: Problems of SSTO and technologies to solve these
« Reply #235 on: 02/14/2019 05:40 pm »
I think it's very telling that despite arguably the best design team in US rocket building industry (in terms of current applied skills experience, as opposed to something, somewhere, inside the corporate memory that was done long ago) and a very strong desire by the Chief Designer to avoid wings at all costs SX still ended up with a winged US design.
As I understand it (and I may be wrong), the "wings" on Starship won't provide any lift, but are simply control surfaces, much like the gridfins on the Falcon 9.  I don't think they're a very good argument for the utility of lifting surfaces on an orbital craft.
A valiant effort, but you are not going to change his mind. He has decided that they are wings and that SpaceX has validated wings forever.  :o

Offline john smith 19

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Re: Problems of SSTO and technologies to solve these
« Reply #236 on: 02/14/2019 09:13 pm »
As I understand it (and I may be wrong), the "wings" on Starship won't provide any lift, but are simply control surfaces, much like the gridfins on the Falcon 9.  I don't think they're a very good argument for the utility of lifting surfaces on an orbital craft.
I do so admire anyone who can confidently describe a set of control surfaces that have to operate over a range of 23 Mach numbers, with operating temperatures into the  1200-1400c range, creating control forces in the 10s, if not 100s of tonnes range, as "simple"

Naturally I look forward to SX proving you right, and demonstrating all the things they learned as they discovered you couldn't make a 3m dia US economically recoverable.

As always time will tell how easy (or not) this proves to be.

However perhaps we could try getting back to the topic of this thread, which (I think) implies that the OP would like to do SSTO on Earth, not Mars.
« Last Edit: 02/15/2019 08:18 pm by john smith 19 »
BFS. The worlds first Methane fueled FFORSC engined CFRP structured A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP stainless steel structured booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C apply. Believe no one. Run your own numbers. So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

Online Tulse

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Re: Problems of SSTO and technologies to solve these
« Reply #237 on: 02/15/2019 01:46 pm »
As I understand it (and I may be wrong), the "wings" on Starship won't provide any lift, but are simply control surfaces, much like the gridfins on the Falcon 9.  I don't think they're a very good argument for the utility of lifting surfaces on an orbital craft.
I do so admire anyone who can confidently describe a set of control surfaces that have to operate over a rage of 23 Mach numbers, with operating temperatures into the  1200-1400c range, creating control forces in the 10s, if not 100s of tonnes range, as "simple"
They are not "simple", but they are "simply" control surfaces, and not wings, a point you don't seem to dispute but which you seemed to claim earlier.  Let's try to discuss in good faith, rather than score rhetorical points.

Offline john smith 19

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Re: Problems of SSTO and technologies to solve these
« Reply #238 on: 02/15/2019 09:19 pm »
As I understand it (and I may be wrong), the "wings" on Starship won't provide any lift, but are simply control surfaces, much like the gridfins on the Falcon 9.  I don't think they're a very good argument for the utility of lifting surfaces on an orbital craft.
I do so admire anyone who can confidently describe a set of control surfaces that have to operate over a rage of 23 Mach numbers, with operating temperatures into the  1200-1400c range, creating control forces in the 10s, if not 100s of tonnes range, as "simple"
They are not "simple", but they are "simply" control surfaces, and not wings, a point you don't seem to dispute but which you seemed to claim earlier.  Let's try to discuss in good faith, rather than score rhetorical points.
Well I think Musk described them as "wings" around 3:11

, along with the "MNs of force" needed to actuate them. If the vehicle they are attached to weighs hundreds of tonnes, and the forces needed to move those surfaces are also in the hundreds of tonne range, that seems quite like a set of wings to me, albeit it all moving wings, which are pretty well unique to this design.

As usual in this context "unique" is a synonym for "likely to have unknown problems which can only be found by flight testing."

"Control surfaces" needing MN of force to move them were the issue that killed HOTOL, to the extent that by the end of the project HOTOL was viewed as a vehicle to "Get the hydraulic system into LEO."  :(

While you might expect these surfaces to be in motion only during the entry and landing phase I'm not sure that's certain. IIRC the Shuttle stack needed all orbiter control surfaces in motion to keep it under control, along with the SRB TVC's Hopefully BFS will do better.


« Last Edit: 02/15/2019 10:12 pm by john smith 19 »
BFS. The worlds first Methane fueled FFORSC engined CFRP structured A380 sized aerospaceplane tail sitter capable of flying in Earth and Mars atmospheres. BFR. The worlds biggest Methane fueled FFORSC engined CFRP stainless steel structured booster for BFS. First flight to Mars by end of 2022. Forward looking statements. T&C apply. Believe no one. Run your own numbers. So, you are going to Mars to start a better life? Picture it in your mind. Now say what it is out loud.

Offline Nighthawk117

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Re: Problems of SSTO and technologies to solve these
« Reply #239 on: 02/16/2019 02:07 am »
Luckily, XS-1 is the spiritual successor to X-33.

No, it is not.

1. Both vehicles are SSTSO - Single Stage to Sub-orbit - and share a similar flight profile - VTHL
2. Both vehicles rely upon composite LH2 tanks to save weight
3. Both are powered by main engines from Rocketdyne
4. Both are products of USAF Plant 42
« Last Edit: 02/16/2019 02:16 am by Nighthawk117 »

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