An SSTO as "God and Robert Heinlein intended".

[T.Neo]

Again, you just read a title without reading what the article was about.

Except... I did read the article. :shifty:

I could say the new year starts on my birthday. That doesn't mean it's valid.

You could always invent a new calender with your birthday as the first day of the year.

 

[Urwumpe]

Again, you just read a title without reading what the article was about.

Actually, your posts have been soo repetitive, that it doesn't really require reading all of it. The first and the last paragraph are good enough.

But seriously: Maybe you didn't realize it yet, but you are already talking to yourself there, because most wiser people already went while shaking their heads. The few answers here are done by people who have yet to reach wisdom (I count myself to them), who still hope that you will one day learn the scientific method and be open for arguments instead of using your own "big dumb booster" solution of using quantity of your posts over quality.

Get a blog for this, A forum is a place for discussion and the dis- in discussion requires more than one person taking part, so the positions can be apart.

That you quote yourself, is also just a sign of helplessness, not of a discussion - you discuss only with yourself while agreeing with you. Which isn't a discussion, but actually talking to you.

A wiki would maybe be even much better, since you can then edit your data-free claims. And even better: Others could fill the gaps in your argumentation with something better than "something magical happens here". But please - stay away from OrbiterWiki then. Use Wikipedia, there the moderators are much more patient with you than I will ever be.

 

[RGClark]

Except... I did read the article. :shifty:

And you interpreted the article to say individual human lifespans could be made indefinitely long by going to space?


Bob Clark

---------- Post added at 01:38 AM ---------- Previous post was at 01:31 AM ----------

But seriously: Maybe you didn't realize it yet, but you are already talking to yourself there, because most wiser people already went while shaking their heads. The few answers here are done by people who have yet to reach wisdom (I count myself to them), who still hope that you will one day learn the scientific method and be open for arguments instead of using your own "big dumb booster" solution of using quantity of your posts over quality.

Perhaps. But I have yet to see anyone on this forum to actually do the calculation using the most weight optimized stages and the most sea level to vacuum efficient engines we have now.
Anyone on this forum could do that calculation but no one on this forum has done it.

Bob Clark

 

[T.Neo]

And you interpreted the article to say individual human lifespans could be made indefinitely long by going to space?

No.

Perhaps. But I have yet to see anyone on this forum to actually do the calculation using the most weight optimized stages and the most sea level to vacuum efficient engines we have now.
Anyone on this forum could do that calculation but no one on this forum has done it.

Perhaps because nobody cares?

Your claim is that SSTO is important because it can magically slash launch costs to roughly $100/kg. Claims like these made by anyone are dubious, and you haven't really put forth much of a justification for this claim either.

 

[Urwumpe]

Perhaps. But I have yet to see anyone on this forum to actually do the calculation using the most weight optimized stages and the most sea level to vacuum efficient engines we have now.

Anyone on this forum could do that calculation but no one on this forum has done it.

There you see a strong misconception. This world works like that: You make the claims, you do the math to prove them. you can ask people for help, that is ok and if you are polite and not start commanding people around (which you have a tendency for), people will help you with your problems. We are not your employees here, and we also have no moral obligation to prove your wild claims. Most people here don't believe in SSTOs, because they can experimentally see how much better even a reusable TSTO already is.

As you can maybe notice here: People can design rockets. Most are multi-staged, and they use pretty good optimized stage ratios often. Maybe you should try the same. I have pushed you towards making your own Orbiter add-ons to experimentally verify your claims, but this also never happened.

I somewhere had defined the Urwumpe limit for reusability, a pretty good mathematical function about the cost efficiency of reusable spacecraft (that badly cries for getting put on a paper and peer reviewed). If I would just remember it in its full form... I just have the early calculations left here.

 

[Loru]

Perhaps. But I have yet to see anyone on this forum to actually do the calculation using the most weight optimized stages and the most sea level to vacuum efficient engines we have now.
Anyone on this forum could do that calculation but no one on this forum has done it.

Bob Clark

Exactly - anyone means that You can do the math also. You don't have to be add-on developer to experimentally verify your claims. As you claim SSTO is possible with current technology: do it.

Velcro rockets gives you simple and ellegant way to make your rockets (It comes with a lot of meshes so you don't have to mesh something) and has nice ISP-sl parameter that differs ISP in vacuum from ISP within Earth's atmosphere. All you need to do is do the math and put it into velcro cfg file.

Then you can release it and anyone can check your SSTO concept.

 

[RGClark]

Perhaps because nobody cares?

Your claim is that SSTO is important because it can magically slash launch costs to roughly $100/kg. Claims like these made by anyone are dubious, and you haven't really put forth much of a justification for this claim either.

I don't believe that.
The calculation is on the high school level. And it is an interesting topic. The readers on this forum are very scientifically interested people.


Bob Clark

 

[Urwumpe]

I don't believe that.
The calculation is on the high school level. And it is an interesting topic. The readers on this forum are very scientifically interested people.

If they are on the high school level... at which level of education did you stop, so that you are incapable of doing them yourself?

Scientifically minded people will also not do the math for you. You calculate first, and they will check your calculations later, and point out the flaws and errors. It is always like that.

 

[RGClark]

On the Space Flight news subforum it was discussed that Sierra Nevada is developing the Dream Chaser spacecraft based on the HL-20:

http://www.nasa.gov/topics/technology/features/hl20-recognition.html

http://www.nasa.gov/centers/langley/news/factsheets/HL-20.html

[ame="http://en.wikipedia.org/wiki/Dream_Chaser"]Dream Chaser - Wikipedia, the free encyclopedia[/ame]

The HL-20 was stated as having a mass of 9,979 kg while the Dream Chaser is listed as having a mass of 9,000 kg. The Dream Chaser is to be of largely composite design. Composite design allows a rocket to have SSTO performance if it is of sufficient size.
Anyone know if that mass of 9,000 kg is the dry mass or gross mass of the Dream Chaser, meaning containing propellant? If it is the dry mass as I'm assuming it is, then filling the entire internal volume with dense, hydrocarbon, propellant would allow you to get a high Mach suborbital vehicle. And scaling it up twice would allow you to get a fully orbital vehicle as is the case with the X-37B.


Bob Clark

 

[Urwumpe]

Composite design allows a rocket to have SSTO performance if it is of sufficient size.

Can you explain how you arrive at this conclusion?

(PS: I know what you are likely to say, so I warn you, I am aware of the big text bubble with "something magical happens here")

Also, the full weight of the HL-20 was 25,000 lb, a tiny bit more than what you state for the HL-20, the exact mass of the Dream Chaser is unknown, there are only guesstimates for the suborbital version without heat shield and simpler subsystems, based on the fact that it was meant to be launched by a white knight.

 

[Ghostrider]

SSTOs do not make launches cheap. They make the rockets simpler because they do away with the staging stuff. However, with chemical rockets it's not feasible and even if you could fly the thing into orbit, what about the payload? SSTOs also make sense if you're also recovering it intact, otherwise you're leaving a big bad rocket stranded Up There waiting to come down and burn your investment or something else on the ground.

If we want practical SSTOs we need nuclear engines. However, we'll have to call them "fuzzy fluffy bunny rockets" or the public opinion will commit mass suicide in order not to risk being killed by radiation.

 

[C3PO]

Can you explain how you arrive at this conclusion?

(PS: I know what you are likely to say, so I warn you, I am aware of the big text bubble with "something magical happens here")

 

[Ghostrider]

Can't blame the gnomes for being creative in their business plans.

However, any authorized entity who ever comes near my preciousss underwear will be terminated with extreme prejudice.

 

[T.Neo]

I'm failing to see the "chemical SSTO is impossible" argument here. There is nothing that makes a chemical SSTO impossible, there are just a whole lot of things that make it impractical outright or unfeasible from an operational perspective. Or just plain less attractive than a staged system.

Nuclear propulsion solves some problems, but it just creates others. Apart from the political aspect, you have to deal with radiation from the engine, thrust/weight ratios, and operating what is basically a flying nuclear reactor. Combined with a rocket engine.

And... during operation, radiation will transmute elements in the engine structure into radioactive isotopes, and there will be radioactive fission products in the spent fuel as well. Refurbishing a used nuclear rocket engine would either poison your engineers, or require expensive and time-consuming robotic maintainance.

NERVA got low fallout rates, but NERVA also had an extremely poor thrust/mass figure. DUMBO was better, but apparently kept the fuel contained in something like a thin foil, drastically increasing potential for fallout.

If high thrust/mass and negligible fallout cannot be achieved with a single system, then NTR is entirely impossible for ground launch in a sane world (and various types of insane ones too).

 

[Loru]

And one more thing about SSTO with current technology.

Most current engines are throtable in range 70-100%. Asuuming you're using 1 stage with one set of engines (not to carry on dead mass into orbit) you encounter more problem:

During lift-off you need a big thrust to weight ratio to overcome gravity and clear the atmosphere.

This high thrust gives you a problem in latest stage of boost phase. Engine - that has to work with at least 70% of it's nominal thrust, will create so much acceleration that many components (human crew included) may not survive boost phase.

Let's do the numbers.
As the example I'm using first stage of my ETS launch vehicle which has powerfull 1st stage(current configuration):

Themis LV Stage I

structural mass + engine               30 000 kg 
RP1/LOX Isp 309              400 000 kg 
                             430 000 kg total
Engine:
1 x RD-180: 3.83 MN at sea level

Let's add typical Soyuz spacecraft as payload: 7000kg

At full thrust rocket is incapable of launching:
T/W Ratio (lift off) = 8.76 means it can't even lift itself from a pad

At full thrust at burnout
T/W Ratio (burnout) = 103 - that's 10Gs of acceleration

Giving unique fact that rd-180 is throtable down to 40% we've got ~4 Gs at burnout (maybe that's the reason I've chosen this engine) but throttling down to 4Gs increases gravity loses a lot.

With 100% thrust and 7ton payload rocket has 8179 km/s of dV (freefall) but due to fact it has to lift itself from the surface I estimate real dV at around 5km/s with boosters that helps it lift off.

Note: T/W ratios given in [N/kg]

 

[T.Neo]

Giving unique fact that rd-180 is throtable down to 40% we've got ~4 Gs at burnout (maybe that's the reason I've chosen this engine) but throttling down to 4Gs increases gravity loses a lot.

Why would throttling down to ~4Gs late in the ascent be a problem? STS throttles down to 3Gs for astronaut comfort (the Orbiter Vehicle, at least, is rated for something like 5Gs), and it doesn't seem to have too many problems with gravity losses.

The RS-68 can throttle down to 58% of rated thrust, I'm not sure what range the SSME can throttle down to, but it must be relatively similar.

Perhaps a comparison of several different unmanned launch systems would be a good example of a baseline acceleration limitation for an SSTO.

Limiting acceleration is important because by the very nature of a vehicle it would have as lightweight a structure as possible- but as you accelerate you are also depleting your propellant and perhaps reducing the strain on the structure in some cases.

Perhaps engines can be made to be throttled down even more; the DC-X used modified, throttleable RL-10 engines. If you want to be really nasty, you could drop engines off, which kills your SSTO status, if anything, and makes reusability far more difficult.

But another issue is, if you throttle down enough, or even shut down an engine(s), you are suddenly finding yourself flying a vehicle with a good deal of parasitic mass onboard...

 

[Loru]

Why would throttling down to ~4Gs late in the ascent be a problem? STS throttles down to 3Gs for astronaut comfort (the Orbiter Vehicle, at least, is rated for something like 5Gs), and it doesn't seem to have too many problems with gravity losses.

Gravity losses are problem in the begining of flight when you have to quickly clear dense parts of atmosphere and gain tangential speed.

It isn't a problem for the shuttle (SRBs at lift-off) but for SSTO it will be a problem of efficiency. Remember that ~70% of lift-off thrust of STS is from boosters.

My point is that designing an engine for SSTO that will be efficent (high ISP) on 100% throttle and 20% and will provide enough thrust to minimize gravity losses (high thrust) is very difficult.

Probably I haven't written that very well.

Also I've used ETS as reference because I'm quite familiar with this rocket and it's capabilities (over 100 test launches during dev process)

But another issue is, if you throttle down enough, or even shut down an engine(s), you are suddenly finding yourself flying a vehicle with a good deal of parasitic mass onboard...

Exactly - throttled down engines work usually below theirs maximum efficiency.

PS - I'll try to do the same for umanned boosters/stages and post results here.

---------- Post added at 11:02 PM ---------- Previous post was at 10:56 PM ----------

BTW my most with ETS calculations was intended to show that doing numbers for orbiter isn't that hard. There are plenty of tools to do it both online and on OHM to do it.

 

[T.Neo]

Gravity losses are problem in the begining of flight when you have to quickly clear dense parts of atmosphere and gain tangential speed.

But we're not talking about the beginning of the flight, where you are passing through the thick lower-altitude parts of the atmosphere. At that point, your vehicle is still carrying a large amount of propellant, is accelerating relatively slowly, and it needs a large amount of thrust to accelerate.

We're talking about late in the flight with most of the propellant expended. With the engines running at full capacity, the acceleration would either be too much for the astronauts, at least, or even too much for the spacecraft structure to withstand.

Exactly - throttled down engines work usually below theirs maximum efficiency.

I was talking about lugging around engine mass that from a physics perspective you no longer need, but you make a good point. If throttling down the engine reduces exhaust velocity in some way, it is an extra problem.

BTW my most with ETS calculations was intended to show that doing numbers for orbiter isn't that hard. There are plenty of tools to do it both online and on OHM to do it.

Seconded. With Math (tm) And A Bit Of Knowledge, You Too Can Become An Amateur Rocket Scientist!*

*WARNING: Actual scientific validity not guarunteed.

 

[Loru]

I was talking about lugging around engine mass that from a physics perspective you no longer need, but you make a good point. If throttling down the engine reduces exhaust velocity in some way, it is an extra problem.
*WARNING: Actual scientific validity not garunteed.

Well... AFAIK reducing throttle decreases chamber pressure (less propellant feeded into chamber) -> with non variable nozzle throat you have smaller exhaust velocity. I'm 99% sure that that will decrease ISP of the engine.

 

[T.Neo]

Ok, let's play around with some numbers here. Let's take the STS ET and some SSMEs;

If we have a liftoff mass of 800 tons, we will need 6 SSMEs to accelerate the vehicle at roughly 4.17 m/s^2, 5 SSMEs to accelerate the vehicle off the pad at roughly 1.84 m/s^2. The Saturn V accelerated off the pad at ~2.2m/s^2.

Now onto mass: the ET has an empty mass of 26 500 kilograms. Let's say, the intertank has a mass of about 5400 kilograms, and that the engine support structure will have a similar mass.

An SSME is cited to mass 3177 kilograms. Five SSMEs mass 15 885 kilograms, six SSMEs mass 19 062 kilograms.

All together this is a mass of 50 962 kilograms, not including payload mounting structures and/or fairings.

We will assume that 2 000 kilograms of propellant remains in the tanks after reaching orbit.

Since ISP varies with altitude, we will assume (simplistically) that through 2000 m/s of dV the exhaust velocity is at 3600 m/s (sea-level value), and through 8000 m/s of dV, the exhaust velocity is at 4350 m/s (almost 100 m/s lower than the vacuum rating of the SSME).

Based roughly on these figures, it seems that the vehicle would use under 532 tons of its propellant to get to orbit without payload.

Working backwards using these figures and assuming a total (used) propellant figure of 730 tons, we can figure out that the vehicle can put maybe 20 tons tons of payload into LEO.

This is disregarding things like fairings and payload attachments.

Now, do you really want whole huge ET-based launcher just for launching a measly 20 tons? In practice, less than 20 tons?

If we assume that the modified ET costs $60 million and each SSME costs $50 million, we get a cost to LEO of $18 000 per kilogram, which is far from the oft-repeated $100/kg figure. This is considering hardware cost only, and neglecting launch-related and other costs.

And there is no ability to make this vehicle reusable, of course. Already adding such simple systems as a fairing would cut into the payload figure severely, adding a TPS, parachutes, and some sort of landing system would obliterate it entirely if not make the vehicle outright incapable of attaining orbit.

Six SSMEs running at 67% will accelerate the vehicle at nearly 13 Gs just before cutoff. Three SSMEs running at 67% will accelerate the vehicle at 'only' 6 Gs just before MECO, and only two SSMEs still operating will still accelerate the vehicle at over 4G. To get under 3G, you will have to shut down all but one engine, meaning that the other five engines will be nearly 16 tons of parasitic mass.

At this rate, it would likely be more efficient both physically and financially to design a breakaway ring of expendable RS-68-like engines, and turn the vehicle into a 1.5 stager.

On the other hand, one could argue that the ET is pretty unoptimised for the task, and a propellant tank that uses shared bulkheads for example, and a semi-pressure-supported structure (like the Falcon propellant tanks) would be better. This would likely improve the associated problems, but not get rid of them entirely.

I'm sure I'm missing something, somewhere. Please feel free to criticise, comment on, and test my ideas.