DG-like technology rocket...

N_Molson

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I recently got into Velcro Rockets fun again. My idea was : let's try to do a rocket with Deltaglider-like technology.

Of course I wanted a reusable SSTO, at this technology level.

So the thing is equipped with 5 DG-engines (200 kN x5, Isp 40,000, IspSL 30,000). I found it was OK for a liftoff mass of 40-50 tons. I set an empty mass of 25 tons and a propellant mass of 20 tons.

I chosen a 6-meters diameters because I wanted it to be able to carry large payloads. In that case, an adapter and a fairing would be secured on top of the launcher, then detached before re-entry. Still, even in that configuration, the rocket would keep its aerodynamic shape, even at the cost of some volume lost (could be used for expendable upper stage engines or tanks anyway). There is a landing gear that is of course supposed to be retractable. The black paint is supposed to be made of thermal coating, and there would be thermal tiles on the nose, on the "engines cases" and on the leading edge of the fins, the most exposed parts to heat.

The way I see things, the lower part would be occupied by the engines and the nuclear fusion devices. Above would be the Unobtainium propellant tank. In fact there is plenty of room for a propellant much less dense (and way cheaper, like LH2 ?) than in any DG, where space is precious.

The biggest challenge I face is unsurprisingly re-entry. I've entered correct cross sections, and, as espected given its shape, the thing behaves like a missle and performs a ballistic re-entry, enduring 8-9 G in low atmosphere. That's not a problem for an unmanned vehicle. What worries me more are the 800 kPa of dynamic pressure recorded at the peak of the deceleration. It seems enormous to me and I fear it would crush anything, today's rockets don't go much above 40 kPa at maxQ. Also, there is very little time to orient properly the rocket and perform a powered landing.

So I wonder what's the best solution. A fully powered deorbit and landing seems crazy regarding fuel consumption, even with that kind of tech : the engines would be very exposed. I'm thinking about some massive airbrakes that would deploy between the "engines cases", increasing the frontal cross section. But I guess it would not be possible to deploy them above the worst part of the re-entry. Maybe it would help to deploy them from orbit, retract them when the DNP becomes intense, then deploy them again when things calm down.

I'm wondering what are your 2 cents about this very hypothetical spacecraft engineering problem. Note that the screenshots depict a "test article", with non-retractable landing gear and no upper stage, payload or fairing. :hmm:

Experiments showed me that the propellant mass is correct, I can achieve orbit and have enough to spare for a powered landing after the ballistic re-entry (knowing this one is carrying no payload).











Re-entry :







 
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N_Molson

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deployable wings? Inflatable heat shield?

I don't want wings. Else it is a DG. Inflatable heat shield ? No. Inflatable heat shield ? Not for a 30 tons craft.
 

Loru

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maybe lifting body then???
 

T.Neo

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The simple answer to the re-entry problem is to study the design aspects of the large number of VTOVL SSTO concept that have come before. Things like the DC-X or the Bono concepts would be a good start; if you're looking at base-first re-entry, you may want to consider a plug-nozzle engine.

If we assume a dV to orbit of 9200 m/s and DG engines (effective exhaust velocity ~40 000 m/s) a mass ratio of less than 1.3 is required. Doubling the dV gives a mass ratio of around 1.6. This is worlds away from the mass ratios required for a chemical SSTO. Even the LM descent stage had a higher mass ratio.

Fully propulsive landing is completely feasible from a mass ratio perspective, it if were an undesirable design choice it would either have to be because increased wear on the engines will cost more than maintaining a suitable TPS, or because the propellant is really expensive and conserving it will considerably reduce mission costs.
 

Urwumpe

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Fully propulsive landing is completely feasible from a mass ratio perspective, it if were an undesirable design choice it would either have to be because increased wear on the engines will cost more than maintaining a suitable TPS, or because the propellant is really expensive and conserving it will considerably reduce mission costs.

It is also no simple "either that or that" question - it would in reality always result in a combination of both, since you generally need a heatshield at the engine end.
 

N_Molson

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No lifting body. This is a rocket. :)

Mhh OK so I should probably cut that pointy nose and install some strong deployable stabilizers/airbrakes there. The aft could be more aerodynamic and heat-protected, with only 5 holes for the engines exhausts, and landing gear "hatches". So a half-powered descent maybe, I'll try that. :hmm:
 

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No lifting body. This is a rocket. :)

That's actually the part I find questionable. Once you have the ISP to do SSTO, I have trouble thinking of reasons that one would want a rocket rather than a lifting body.
 

N_Molson

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That's actually the part I find questionable. Once you have the ISP to do SSTO, I have trouble thinking of reasons that one would want a rocket rather than a lifting body.

Answering this was actually a part of the experiment. I guess you're right, the concept of rocket gets obsolete with such technologies.

Now I think that a bulky, unmanned lifting body freighter with a vast cargo bay could have its role. Something that would be less "overkill" than the XR-5 and wouldn't require to expose the life of a crew for routine cargo tasks (like Progress or ATV or Dragon do at another scale today).
 

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That's actually the part I find questionable. Once you have the ISP to do SSTO, I have trouble thinking of reasons that one would want a rocket rather than a lifting body.

A simple cylindrical rocket shape is more mass efficient than weirdly shaped lifting body so unless absolutely required you would want to keep the most mass efficient form. Also with engines like those on Deltaglider it would be perfectly feasible to do powered descent and land on tail like SpaceX plan to land Falcon9 first stage.
 

N_Molson

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it would be perfectly feasible to do powered descent and land on tail like SpaceX plan to land Falcon9 first stage.

Not at all and lets be clear about that once for all, SpaceX has no intention and has not the possiblity to make a Reusable Single Stage To Orbit.

And even with a DG-like technology, a fully propulsive descent is a waste of propellant. It would be a real pity not to use the atmosphere. Lifting bodies are not the same thing than spaceplanes, they can offer more internal volume.

I would imagine something with that kind of shape, but as big as the Space Shuttle, and fully automatic. Given how compact DG-engines are, and how dense DG-fuel is, it should leave a lot of volume, especially if you don't need a crew section. The navigation and sensors systems could be pretty compact too and fit in a few cubic meters, I'm sure.

 
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Linguofreak

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A simple cylindrical rocket shape is more mass efficient than weirdly shaped lifting body so unless absolutely required you would want to keep the most mass efficient form. Also with engines like those on Deltaglider it would be perfectly feasible to do powered descent and land on tail like SpaceX plan to land Falcon9 first stage.

The descent itself would be feasible. Actually landing would be made tricky by the temperature and velocity of the exhaust (the DG has the advantage for runway operations (not hover!) that it's not blasting 30 or 40 km/s exhaust straight into the runway surface, or past its landing gear). And, whatever the lifetime of the engines, flights with powered descents would use up twice as many hours of engine life as unpowered descents.

Plus, assuming 10 km/s delta V to launch or land, the propellant needed to land adds 7 or 8 tons (assuming a 45 ton launch weight and vacuum ISP all the way up, which won't be the case), and the added launch propellant needed to get the landing propellant into orbit (if you aren't refueling using fuel from the moon or somewhere with less of a DV budget than Earth surface -> LEO) adds 2 or 3 kp/s more. So between 15 and 22 percent of your launch mass gets used for propellant instead of cargo, which makes me think that the mass efficiency isn't as great as you think.

EDIT:

The biggest reason to use a tower rocket would be for manned flights if the engines were given to failing catastrophically, in which case it would probably be easier to build a launch escape system for the tower than for a lifting body. This is the primary reason that lifting bodies are unworkable for multistage launchers: they are subject to catastrophic failure modes that would be survivable or inapplicable for an SSTO lifting body and are harder to build workable LES's for than a tower.

---------- Post added at 20:46 ---------- Previous post was at 16:50 ----------

And even with a DG-like technology, a fully propulsive descent is a waste of propellant.

At this point I do have to admit why I asked for a Velcro Redstone in Addon Requests. It was an utterly silly project that's apropos to this thread: I wanted to see how the ballistics would work out for a powered descent with an S-II/Redstone stack.

And I *just* managed to land with the last drop of fuel on the Redstone.

With a better descent profile I would have had more fuel. In fact, I was surprised by how much DV the S-II gave the whole stack: enough to pretty much completely null out my orbital velocity: the Redstone's entire DV was used for descent from something like 80 km. It might even have been possible to land on the S-II alone.
 
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N_Molson

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With a better descent profile I would have had more fuel. In fact, I was surprised by how much DV the S-II gave the whole stack: enough to pretty much completely null out my orbital velocity: the Redstone's entire DV was used for descent from something like 80 km. It might even have been possible to land on the S-II alone.

The question is : what were your maximum dynamic pressure and speed peaks ?
 

Linguofreak

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The question is : what were your maximum dynamic pressure and speed peaks ?

I wasn't really watching dynamic pressure, but I targeted a descent rate of something like 200 m/s through most of the descent, and slowed things down closer to the ground. I probably could have used a faster descent higher up to conserve fuel (although part of my problem was that I misjudged my braking distance and ended up short of my target point).
 

N_Molson

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DnP is a very important parameter.
 

Linguofreak

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DnP is a very important parameter.

Certainly, it's just not one I had the luxury of time to monitor. That said, I don't think it got much higher than it would have flying a Redstone in the proper direction, and may not even have gotten that high, given that I was being super-paranoid about my descent rate.
 
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