Author Topic: IAC 2017 -- ITS v0.2  (Read 40350 times)

Offline RobW

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Re: IAC 2017 -- ITS v0.2
« Reply #40 on: 05/30/2017 02:58 AM »
I think RobW was talking about the tanker.
He was, thanks.

My thought was that, in normal use, the last tanker will have some fractional amount of prop left over after topping off the spaceship. If the spaceship had some amount of boil-off, you would just load more of the tanker's excess prop to compensate.

Having said that, if you have more than one spaceship in the same orbit, you don't really have any fractional/spare prop, because whatever is left over from tanking one spaceship can then be used in the next.

But either way, the question is: do you want to develop zero-boil-off main tanks, or can you live with (marginally) more tanker flights instead. That depends where the $ fall out in the trade.
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Offline Robotbeat

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Re: IAC 2017 -- ITS v0.2
« Reply #41 on: 05/30/2017 03:10 AM »
If you know you're going to have an extra half a tank, you can set your rendezvous orbit to a slightly higher apogee so as not to use exactly so many tanker flights to their fullest extent.
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Offline RobW

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Re: IAC 2017 -- ITS v0.2
« Reply #42 on: 05/30/2017 03:56 AM »
That's a really elegant way to get the most out of your tanker flights. Deliver exactly the right amount of [edited to correct a bad habit of saying fuel when I really mean] propellant, using any excess performance of the tanker's to go to a more energetic rendezvous orbit.

I understand now why there'd be no 'free' propellant to compensate for boil-off in the spaceship. Even though the spaceship always performs its departure burn with full tanks, any boil-off prior to leaving, while it waits in orbit with a partial prop load for more tanker flights, still translates to a longer transit time to Mars vs the same spaceship with zero-boil-off tanks.

The TANSTAAFL principle holds :). No free lunches and no free prop to compensate for boil-off either.
« Last Edit: 05/30/2017 04:00 AM by RobW »
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Offline su27k

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Re: IAC 2017 -- ITS v0.2
« Reply #43 on: 05/30/2017 04:23 AM »
Here's my idea of ITS v0.2, basically replacing the booster with another Ship:
1. The ship structure will need to be strengthened, but can reuse the tank tooling.
2. Each of the 6 vacuum engines is replaced by a cluster of 3 sea-level engines, result in 21 engines
3. The sea level thrust is 63MN, T/W at lift off is about 1.2. The thrust level is close to Nova C8 and I assume is acceptable for LC-39 flame trench.
(edit) 4. Can also reuse the Ship landing legs and get rid of the launch/landing mount concept.

Guesstimate for the Ship/Booster:
Length: ~30m
Diameter: 12m
Dry Mass: ~120t
Propellant Mass: 2500t
Raptor Engines: 21
Sea Level Thrust: 63MN

Plugin the numbers to  http://silverbirdastronautics.com/LVperform.html:
1st stage
Dry Mass: 120000
Propellent: 2500000
Thrust: 63000
Isp: 334

2nd stage
Dry Mass: 150000
Propellent: 1950000
Thrust: 31000
Isp: 382

Gives expendable performance:
Launch Site:     Cape Canaveral / KSC
Destination Orbit:     200 x 200 km, 28 deg
Estimated Payload:     193412 kg
95% Confidence Interval:     140753 - 256049 kg
« Last Edit: 05/30/2017 05:29 AM by su27k »

Offline Nathan2go

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Re: IAC 2017 -- ITS v0.2
« Reply #44 on: 05/30/2017 05:08 AM »
The ITS as described at IAC 2016 is awesome. The only two things I think you could reasonably object to are the raw size (impracticality large thrust for LC39a and Brownsville), the ambitious landing tech, and maybe the lack of abort.
...
But the basic idea behind ITS just kicks most architectures' butts so hard that I don't think people have yet realized how much better this approach is. Just two stages, with a couple variants of the upper stage, is all the vehicle you need.
Yes, I agree that ITS is awesome, and of all the options we've seen thus far, is by far the most likely to dominate in a future with daily rocket launches and off-world colonies.

The issues of large size and landing tech don't worry me.  If needed, build an off-shore launch platform.  If the landing cradle doesn't work out, use proven F9 style legs instead (although the F9 landing accuracy is already good enough to straddle a flame trench); 2-stage-to-orbit rockets are not overly sensitive to a weight increase of any one piece.

I suspect the launch abort system though, will be necessary for the first couple hundred crewed flights.  Rocket explosions are just too visually compelling to be ignored by the general public, and I don't think 97% demonstrated success rate (i.e what you prove with dozens of successes) is high enough to forgo a launch abort system.

A launch abort system does not have to fundamentally change the architecture, it's just a weight/cost/complexity/testing addition.  I don't think it degrades the design too badly if the passenger section of the ITS ships is designed to separate during a launch abort, it's already a separate pressure vessel.  The unused propellant for the liquid fuel abort engines can be transferred in flight to the main propellant tanks just before MECO.

I suppose it still remains to be seen whether the ITS ship landing system will have comparable safety during entry/decent/landing as would a capsule.

Online WBY1984

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Re: IAC 2017 -- ITS v0.2
« Reply #45 on: 05/30/2017 05:59 AM »
Why can't the entire ITS serve as an escape system? It's certainly got a lot of engines at the back to escape a failing booster?

Offline Ictogan

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Re: IAC 2017 -- ITS v0.2
« Reply #46 on: 05/30/2017 06:09 AM »
Why can't the entire ITS serve as an escape system? It's certainly got a lot of engines at the back to escape a failing booster?
Not enough TWR to escape an explosion booster and engines don't ignite fast enough.

Offline guckyfan

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Re: IAC 2017 -- ITS v0.2
« Reply #47 on: 05/30/2017 06:12 AM »
Carrying a LES all the way to Mars and back will cost a lot of performance. If it is deemed necessary there could be a version of tanker and a kind of capsule for up to 100 passengers crammed in for a few hours at the top. It could deliver the passengers last and top off any boiloff from the waiting time in LEO. It is one more launch per Mars flight but it would not degrade Mars performance and another LEO launch is not a very large share of the total flight cost. It would be mostly development cost for this variant.

I don't think it will be really necessary. Before large crews fly there would be hundreds of launches, not dozens and the reliability proven, hopefully.

Online AncientU

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Re: IAC 2017 -- ITS v0.2
« Reply #48 on: 05/30/2017 10:20 AM »
I don't expect development of long term on orbit main tank cryo storage because it's not critical to ITS.

It is though, for varying definitions of long term. ITS will need to be refueled over a handful of tanker flights which could take a bit of time

Not if you can live with the boil-off losses during the fueling campaign. The trade-off is between developing long-term, main tank cryo management, and launching a bit more fuel during the refueling campaign. If the fuel needed for the Mars ship is not an exact multiple of the tanker capacity, you already have some 'free' excess fuel that can go toward making up for the boil-off.

They'll need something approaching ZBO tankage.  Recall that they are discussing assembling a 'fleet' of vessels which will all depart together.  Cannot have the first fueled suffer significant boil-off while waiting for the rest. 

Assembling in a high orbit (EML-1/2) would help greatly as the Earth's heat load would be reduced to a couple percent of what it was in LEO.

Also, when fueling on Mars, there must be some high insulating value in the spaceship tankage to avoid becoming a carbon dioxide ice block.  (Lox and liquid methane are at temps below the carbon dioxide freezing point.)
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Offline Kaputnik

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Re: IAC 2017 -- ITS v0.2
« Reply #49 on: 05/30/2017 11:09 AM »
How feasible would active cooling be, using the 200kw of solar power available on the ITS?
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Offline pippin

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Re: IAC 2017 -- ITS v0.2
« Reply #50 on: 05/30/2017 12:14 PM »
How feasible would active cooling be, using the 200kw of solar power available on the ITS?
What is this "active cooling" using power? Any power you use needs to be radiated off, too, otherwise it's rather heating than cooling.

Online stcks

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Re: IAC 2017 -- ITS v0.2
« Reply #51 on: 05/30/2017 12:32 PM »
Not if you can live with the boil-off losses during the fueling campaign. The trade-off is between developing long-term, main tank cryo management, and launching a bit more fuel during the refueling campaign.

That's a good point, but it presumes a quick enough launch cadence for refueling to be able to even make the trade. Will be interesting to see how (if) it is done

Online envy887

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Re: IAC 2017 -- ITS v0.2
« Reply #52 on: 05/30/2017 12:50 PM »
How feasible would active cooling be, using the 200kw of solar power available on the ITS?
What is this "active cooling" using power? Any power you use needs to be radiated off, too, otherwise it's rather heating than cooling.

Refrigeration. Of course the process generates extra heat has to be radiated away.

Offline pippin

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IAC 2017 -- ITS v0.2
« Reply #53 on: 05/30/2017 12:59 PM »
How feasible would active cooling be, using the 200kw of solar power available on the ITS?
What is this "active cooling" using power? Any power you use needs to be radiated off, too, otherwise it's rather heating than cooling.

Refrigeration. Of course the process generates extra heat has to be radiated away.
Refrigeration requires a heat sink.
No such thing in space.
Yes, you can increase the temperature differential to radiators by using some high-temperature cycle to increase radiation but that increases the heat load on parts of your system quite a bit.
Even radiating away these 200kW of power is no small feat.

The one really feasible way to cool stuff in space is by shedding heated material (fuel) but that requires exactly the boiloff you want to prevent. Ok, you could make the process a bit more efficient by shedding hotter gas but I doubt that will have a lot of effect. You'd have to increase temperature quite a lot.
Let's not forget all these temperature differentials are in Kelvin so you need a lot of change to get a big result, heating up your shredded gas to hundreds or even >1000 degrees just to cool other parts, not sure that's very practical.
« Last Edit: 05/30/2017 01:00 PM by pippin »

Offline Robotbeat

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Re: IAC 2017 -- ITS v0.2
« Reply #54 on: 05/30/2017 01:09 PM »
Space is a heat sink, just an inefficient one since you have to radiate. So refrigeration is still most certainly possible.
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Offline pippin

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Re: IAC 2017 -- ITS v0.2
« Reply #55 on: 05/30/2017 01:10 PM »
Well, yes, possible, but veeeery inefficient

Offline Robotbeat

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Re: IAC 2017 -- ITS v0.2
« Reply #56 on: 05/30/2017 01:13 PM »
Well, yes, possible, but veeeery inefficient
Not sure what your point is. It's certainly possible to refrigerate a cryogen to liquification, especially if it's not hydrogen or helium.
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Offline pippin

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Re: IAC 2017 -- ITS v0.2
« Reply #57 on: 05/30/2017 01:15 PM »
Yea, if you've got a square kilometer of radiation surface or it's small volume or you're willing to shed a lot of gas in exchange. All as I said above.

Offline Robotbeat

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Re: IAC 2017 -- ITS v0.2
« Reply #58 on: 05/30/2017 01:16 PM »
Yea, if you've got a square kilometer of radiation surface or it's small volume or you're willing to shed a lot of gas in exchange. All as I said above.
No, that's hyperbole.
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Offline pippin

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IAC 2017 -- ITS v0.2
« Reply #59 on: 05/30/2017 01:19 PM »
No, it's not. ITS as shown in the early drafts was clearly already lacking radiator surface area even without trying to shed enough heat load for a phase change.
If you use tens of kW of power for refrigeration, radiating off that heat will require increasing the radiation surface area significantly- or to beef up the temperature differential with all the problems involved (use of different fluids and all)
« Last Edit: 05/30/2017 01:22 PM by pippin »

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