Question Fully reusable two-stage-to-orbit or reusable 1st/expendable 2nd stage?

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mikusingularity
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So we have SpaceX's goal of having a fully reusable rocket TSTO.

But which one gives you more bang for your buck? The main issue here seems to be loss of payload capacity due to the need to return rocket stages vs. savings from not discarding them.
 
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EnDSchultz

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I think SpaceX needs to prove they can economically refurbish first stages without sacrificing vehicle reliability before even contemplating recovering the second stage. Seems like most of the vehicle's value is in the first stage (9 engines versus only 1 for the second) anyway.

I suspect it will turn out that trying to recover the second stage will have seriously diminishing returns because of the weight of components (heatshield, RCS control system) cutting into payload even further, and the greater mechanical and thermal stresses of returning from orbit making refurbishment much costlier.
 

PhantomCruiser

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Agreed.

One thing they have going for them is the relative simplicity of their rocket motors. The SSME was so far on the edge of what is capable that is was really, really expensive. Add to that the extensive overhaul (rebuild essentially) of the SSME for it to be flight ready (and thus "reusable"). How much cost went into the maintenance/overhaul of an SSME vs a brand new SSME? (I don't know this myself, but I suspect others here do)

The tank, is well... a tank. Not anything overtly special, but it will required some pretty extensive NDI (non destructive inspection) before they can certify it ok for x many flights. Hoses, tubing, fuel lines, etc. will also have to be inspected before taking another trip into the black.

I think (and I'm a technician, not an engineer; engineers dream things up but it's the techs that make it work) that if they add all the weight they'd need to be "fully" recoverable (meaning both stages), that the payload capability would drop off to the point that it'd take a Falcon heavy to get a big commsat to GTO, and the "regular" F9 would be relegated to throwing things into LEO.

Of course, they have these grand plans for a bigger engine; I've seen a few graphics with the Falcon with only one engine (a Merlin 2 IIRC). http://www.spacelaunchreport.com/sls3.html

OK, so that report is out of date, but it shows the direction Elon and company would like to go. I'm sceptical, I'm a maintenance guy, so I'm always sceptical of engineers (apology to any and all engineers here, but your maintenance guys are sceptics too; ask them, if their answers won't hurt your feelings).

For now, and at least the next 5 years or so, I really don't forsee the level or reusability that was indicated by that (admittedly awesome) video that SpaceX put together. I can see them getting a "soft" water landing, or maybe landing on an island out in the middle of nowhere. What kind of regulations will the FAA dream up for an unmanned pressurised tank of hydrolox (or kerolox, methane, whatever) into a populated area?

I'm also always happy to be proven wrong. I hope Elon and co. achieve everything they strive to do.
 

Urwumpe

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By the plain numbers, building a reuseable TSTO is no problem. The business end is the problem. Kistlers K-1 got very far, but then died on the business end. The same problem will also come to SpaceX there. If the TSTO can't deliver the same payloads into orbit as the expendable version, it could launch for free (and an eel extra) and would still not get accepted as launcher. It could do small university payloads, smaller NASA satellites, but the big business would not come, because the launch costs are no longer the biggest issue in the satellite design.

If you split a big satellite into two smaller ones, this would then reduce the launch costs, but would make the operational costs much higher for the next 15 years and also increase the costs for both satellite busses, since you can't just split the satellite into two halves. Many parts have to exist then in both satellites as often as in the initial bigger one. Reducing the lifetime of the satellite and building cheaper satellites will also not work out: You create a huge space debris problem then.

So, if your TSTO does not reduce the whole lifecycle costs of a satellite, it will be no option for the operator. And if using the TSTO means a 20% decrease in revenue because you have to put less commercial payload into it, you also have to consider this in the launcher evaluation.

A TSTO has to do everything good, that any other launcher can do good. And then do many things better - the B.A terms are "expected product" (what the customer expects to get) and "extended product" (what your product offers beyond the expectations). Not be reused for the sake of being reused.
 

kamaz

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For trips to LEO, I'd go for a three-stage vehicle with the middle stage being expendable. Here's why: the complexity involved in recovery.

The first stage is relatively easy to recover, because it is slow, and can land on parachutes (see SRBs) near the launch site.

The third stage would be something like X-37B or DreamChaser, it only needs fuel for circularization burn and deorbit, so a small reusable spaceplane with a good mass ratio is possible. Lands on runway, no problem with recovery.

But the second stage is a problem, because it will reenter at near-orbital speed (so it needs thermal shielding, this will spoil the mass ratio) and then it has to be shipped half across the world. Unless you give it wings, which will spoil the mass ratio even more.

For sending comsats to GTO, I think that recovering the second stage is not worth it for the reasons given above.
 

T.Neo

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That's why a TSTO is better; the second stage, although it has to undergo 'full' orbital re-entry, can do a 'once-around' recovery, whereas the second stage of a three-stage vehicle will have both a higher re-entry velocity than the first stage, and be unable to be recovered near the launch site.

A spaceplane third stage contributing a relatively minor amount of delta V will also limit payload options (good for crew or cargo to a space station, probably not good for satellites) and potential orbits that one could launch to. A three stage vehicle will also be more complex to manufacture, refurbish and prepare for launch.

Of course, ideally one would not even bother with sending the second stage of a TSTO RLV beyond LEO- instead, payloads intended for higher orbits would be passed on to an in-space tug, refuelled separately, which would place the payload in the intended orbit and then return to LEO for the next mission.

This of course would require that payloads are able to survive for some time in LEO while not in independent flight, which IIRC modern payloads are not typically designed for. If such a system could lead to decent cost reductions though, this difficulty would be offset.
 

kamaz

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That's why a TSTO is better; the second stage, although it has to undergo 'full' orbital re-entry, can do a 'once-around' recovery, whereas the second stage of a three-stage vehicle will have both a higher re-entry velocity than the first stage, and be unable to be recovered near the launch site.

As I have learned from playing Kerbal Space Program, the second stage is the critical component of the stack.

The first stage basically needs to have large TWR, but it can have low delta-v.

The third stage operates in orbit, so it should have large delta-v, but it can have low TWR.

But the second stage must have both high delta-v and TWR above 1.0 (with payload attached). In fact, most of the delta-v during ascent is contributed by the upper stage. This, in turn, requires that the upper stage must have high mass ratio itself -- i.e. Centaur has about 10:1.

So my point is that trying to make the second stage recoverable will compromise the mass ratio, which means that your payload will go down.

At this point it's down to economic calculation. For example, if the payload is too small to inject a comsat into GTO, then the vehicle has very limited practical use, even if it is cheap.

An exception here is the situation where you are launching something which must be brought back down (i.e. humans). In this case, it makes a lot of sense to put a reusable spaceplane atop of a stack comprised of an reusable first stage and expendable second stage. I.e. DreamChaser.

It would be more sexy for the space plane to act as a second stage, not a third stage. However, the delta-v required means that the vehicle becomes too large to fit on any conceivable first stage. In other words, if you want to have a TSTO, then you need either nuclear propulsion or air-breathing scramjet.
 
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T.Neo

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In other words, if you want to have a TSTO, then you need either nuclear propulsion or air-breathing scramjet.

No. That holds true for an SSTO, but the general consensus is that a reusable TSTO can be built largely with modern materials and chemical propulsion (the question is simply whether or not an RLV could be economically viable) unless one is assuming very conservative values.

I would not recommend KSP as an analytical tool for the design of reusable spacecraft.

Adding recovery systems will decrease payload in any case- even an increase in mass on the first stage will decrease payload, obviously, though at a far lower ratio than the 1:1 losses on the final stage. Any design where the final stage is reused will suffer 1:1 losses of payload capacity due to the need for recovery system mass on the final stage.

A three stage design will only increase complexity, impose limitations and heighten costs, while providing little or no advantages that could not be gained via other methods. It will also greatly increase complexity in terms of development, manufacturing, recovery, refurbishment and launch preparations.

From this perspective an SSTO is simplest, but this is negated by the physical impracticality of an SSTO with foreseeable technology. Thus a TSTO represents an optimum between physical and operational practicality.

The best course of action is simply to attempt to overcome the issues of making the second stage reusable.
 

C3PO

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But the second stage must have both high delta-v and TWR above 1.0 (with payload attached).

You don't need TWR above 1 to get into orbit, but it does reduce the gravitational losses. Extreme cases like STS or ATV launches have a TWR that are no way near 1, but it requires the 1'st stage to go high to give the 2'nd stage time to do it's magic. This reduces the tangential velocity at staging.

But I agree with your main point. :thumbup:
 

MattBaker

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As I have learned from playing Kerbal Space Program

Problem in KSP is that you only need 4,500 m/s of delta V to get into orbit. With these conditions everyone can create a reusable launch vehicle.
 

C3PO

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Problem in KSP is that you only need 4,500 m/s of delta V to get into orbit. With these conditions everyone can create a reusable launch vehicle.

While that may be true, the relative differences in design are still valid.
 

kamaz

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No. That holds true for an SSTO, but the general consensus is that a reusable TSTO can be built largely with modern materials and chemical propulsion (the question is simply whether or not an RLV could be economically viable) unless one is assuming very conservative values.

Well, I should have said a practically usable TSTO. Yes, you can engineer a TSTO which can put itself into orbit, but usability of such vehicle is quite limited.

The issue is that the second stage must provide a lot of delta-v, and must provide it fast, before the vehicle falls down from the parabolic trajectory the first stage put it on. (A naive way of launching is to launch straight up and burn until the first stage is spent, then jettison it, and burn horizontally using the second stage.) This means that the second stage must have powerful engines and large fuel tanks.

If you want to recover the stage, you must engineer the tanks so they can withstand reentry forces while empty and put thermal protection on them. In other words, you will end up building something like this:



X-33/VentureStar was a SSTO and not a TSTO, but it failed because of this very problem: how to build a tank which is both light enough so the vehicle can fly and sturdy enough so it can withstand re-entry and landing.

And you can avoid this problem entirely if you treat the second stage as expendable, because you can use baloon tanks which are very light.

This is also the same reason why Shuttle got its expendable External Tank.
 

Sky Captain

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I bet if/when Space X manage to reuse first stage they will find out reusing second stage is not economical because it will eat too much payload. Probably to a point where fully reusable Falcon 9 could not put Dragon into LEO. Second stage also have only one engine so it is much cheaper than first stage. Any potential savings from recovering second stage are much less than savings from recovering first stage while second stage recovery far more difficult and expensive.
 

Urwumpe

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Well, if you could make a first stage, that could haul a second stage that goes to GTO, you could also sure make a second stage, that only goes to LEO, but can be reused.

The margins are slim, but not physically impossible. It would just no longer be the same rocket, but rather a rocket family based on the same first stage as platform.
 

Sky Captain

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Well, the question then would will it be economically desirable to fire up Falcon Heavy to put 4 ton sattellite in GTO and recover all parts of launcher vs using Falcon9 with reusable first stage but expandable second stage to do the same.
 

Urwumpe

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Well, the question then would will it be economically desirable to fire up Falcon Heavy to put 4 ton sattellite in GTO and recover all parts of launcher vs using Falcon9 with reusable first stage but expandable second stage to do the same.

if the reuse quota is good enough: Yes.

If not: No.

Sometimes the world can be simple.
 
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