Was: Exploding SSTOs, Roton in New Scientist
This is a belated reply to two messages by Mark@unicorn.com. I think this is droning on a bit much, so this is the last hunk of bandwidth I'll use on this subject. Mark, feel free to reply, but I won't be answering on the mailing list. If you want to continue by private email, feel free.
>Philip Witham [firstname.lastname@example.org] wrote:
>Ok, let me repeat this one more time and see how simple I can put it to
>see if you finally get it.
I was being polite about "not understanding", my mistake. I won't do that again. Looks like I did get your points the first time, you are repeating yourself.
>If one of Truax's multi-stage monsters suffers an engine failure, whether
>or not it's catastrophic, it has two choices.
> It aborts to orbit or it crashes and burns.
Clearly true (for Truax's designs) early in the flight (aerodynamic instability would destroy the vehicle if the engine shut down in the atmosphere.) Not so, later in the flight where the second stage can abort.
But again: I am not just talking about Truax's designs. I am talking about the advantages of simplicity, and overall costs, including a healthy respect for the cost of development of new ideas.
SSTO has several apparent advantages: No staging or second stage ignition problems. No need to mate stages on the ground. Everything (usually) returns to the launch site. Since no SSTO has ever carried a payload and proven these advantages, any overall advantage is theoretical only.
I am all for good new ideas, as long as private funding can carry them through to actual use and repayment of the investment. An abort capability may not pay for itself. As SSTO's go, the ROTON is the simplest I've seen. But even the ROTON does not look as simple to develop as a two stage pressure fed vehicle.
>That's all it can do, because there's no way you can
>land a multi-stage monster like that.
As I said, in flight abort is an independent design question. For a vehicle to abort in the atmosphere, you need it to be aerodynamically stable, I.E., it needs fins. To abort a pressure fed vehicle of any size (in vacuum or if the vehicle has fins): 1. Cut off propellant valve. 2. Dump fuel. 3. Wait till the right moment, then deploy drag device (chute, parafoil, or ballute). 4. Tow back from landing site. This assumes the landing is at sea. For SSTO's, the assumption is that you are in glide (or working engine) range of a landing site. If you don't have a working engine, remember that the earth's surface is 3/4 ocean. For two stage vehicles, if they are still connected, you have to separate the stages (after slowing down, if in atmosphere)
> It's not a huge, tall, narrow monster rocket that's only designed to fall into
>an ocean, it's actually supposed to land intact at the end of its flight.
Big dumb boosters are designed to land intact at the end of flight. Truax's designs are rather wide, squatty vehicles, actually, and again, since the tanks are built for high pressures, they are far from fragile. Clearly, any SSTO has less structural margin, it requires the weight to be minimized.
(reply to separate message:)
Re: towing back from the landing site,
>And how much in interest on loans, salaries for people waiting at the
>launch site and twiddling their thumbs, etc, etc, while you wait for it
>to come back for re-launch? Probably far more than hundreds of thousands
Truax's scheme is to have a small fleet of vehicles, and they are in circulation. Not much idle time. But you are making one of Truax's own arguments, for simpler rockets. The size of the standing army of technicians, engineers, and managers, and their facilities, is proportional to the complexity of the vehicle. Among other things, obviously. But you do make an argument for any vehicle with quicker re-cycle times. At least, it would reduce the number of vehicles you need in the fleet. That's good, because the more complex SSTO's would cost more to build in the first place.
>There's no point whatsoever in throwing
>around forty-year-old $50-per-pound figures when we're comparing it to
Yes. Anybody have the data or a web site on this?
>>The smaller Excalibur is a $20,000,000 launch in recent dollars,
>>putting up 100,000Lbs.
>And where does that cost figure come from and what does it include?
That figure included amortized development and such. Profit making launch price.
>That's why they concentrate so much on simplicity and fast turnaround.
Now we are in complete agreement.
>>And you are right, I ignored vertical landing SSTO's. I would add then,
>>that vertical landing is an advantage of having vertical landing
>>capability (!), and not of having one stage to orbit.
>Name one multi-stage launcher which can land intact. Just one will do. Of
>course there aren't any, because they're not designed to, and being big,
>tall, unstable designs they're never likely to. They could be designed
>to do so, but that would significantly reduce their payload.
Name one SSTO (capable of making orbit with a payload) that has ever landed intact. Just one will do. Of course there aren't any, because no SSTO has ever made it through development. One could design an SSTO, but that would significantly reduce the payload.
>Yet you're arguing against a simple single-stage design which lands at
>the launch site ready for maintenance, refuelling and relaunch in favor
>of a complex multi-stage rocket which has to be towed back from a
>landing in the sea, checked for damage, and reassembled.
Yes. It is obvious that this is a point of simplicity for SSTOs. What is not obvious is that an SSTO must achieve high performance from its engine(s) and structure, and that this makes the actual complexity and development cost of an SSTO greater than a dumb (pressure fed) two stage booster.
>But it's never meant to land in its launch configuration, so any changes
>you make will cost you payload. The reusable SSTO has to land in its
>launch configuration (possibly after dumping fuel), so there's no extra
Yes. A reusable SSTO carries the penalty in all cases. A flimsy pump fed tank structure (SSTOs, Atlas's, Saturn-V, etc) cannot survive landing with the lightest weight recovery option available, I.E., in the ocean with a 'chute or parafoil.
No simple development of a Saturn-V is ever going to land in its
True. It's a pump fed booster.
>But they're not working to the same design goals. That's the whole point.
That makes comparison useless, doesn't it? I *am* talking "the same design goals", in particular, lowest cost space transportation after all costs are included.
>Uh, Roton is a LOX/Kerosene rocket; they don't even push it to LOX/LH2.
>> more reliable than two pressure fed stages with fewer total engines
>>and enormously fewer parts, and large strength margins?
>Is the SSME more reliable than almost any other rocket engine regardless
>of design? Uh, yes. Yet it's also probably the most complex engine ever
>built. Similarly, a modern jet engine is far more reliable than any old,
>simple, biplane piston engine. A complex engine can be more reliable than
>a simpler design.
Has any engine development ever cost as much as the SSME? Can the cost of a turbofan be compared to an old 9 cylinder radial? Spend the same time and money on a pressure fed rocket motor and clearly the reliability would be higher still than the SSME.
>If an SSTO engine fails to ignite you just
>shut down the rest, climb out of the pilot's seat and grab a coffee
>while the engineers fix it.
Yeah. They'll just stick some chewing gum on the pump casing.
>>I'd like to see the statistics on that, do you have a reference handy?
>Uh, reading numerous reports on the reasons why launchers failed.
Maybe I did not make it clear: I truly would like to learn something from you here, you have me intrigued on this point. It goes against what I have learned. Can you point me to a specific reference for this data?
>>With two stages, the payload (or next stage) is about 10-20% of
>>the stage liftoff mass.
>So? Your first stage increases in mass by 3% and you've lost your payload.
>Your Roton mass increases by 2% and you've lost your payload. What's the difference?
No. Increase the first stage mass by 3 % and the *whole second stage and payload* shrinks by perhaps 20% (15% of total liftoff mass becomes 12% of liftoff). To eliminate the payload on a two stage vehicle, you would need to be about 15% off on the Isp or weight. This is a consequence of the basic math, SSTO is very difficult to do and sensitive to Isp and structural weight.