Author Topic: Operating the ITS spaceship or tanker as an SSTO launch vehicle.  (Read 41151 times)

Offline Kaputnik

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If they're using the BFS in SSTO mode for testing then they could replace some or all of the Vac engines with SL ones.

No, they won't be doing that:
Quote from: Elon Musk
Will be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance. Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep space Raptor engines.
(my bold)

https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodec8l/

The deep space Raptors, which your quote shows they don't need, *are* the Vac engines.
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline R.Simko

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If you are talking about using 12 engines for SSTO, it would seem to me, that this would be the perfect time to do a stage stretch.  You would have the extra lifting power for the stretch and it would allow you to carry a lot more fuel.  Could someone run the numbers on it. 

Thanks:)
« Last Edit: 10/15/2017 12:38 pm by R.Simko »

Offline LucR

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If they're using the BFS in SSTO mode for testing then they could replace some or all of the Vac engines with SL ones.

No, they won't be doing that:
Quote from: Elon Musk
Will be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance. Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep space Raptor engines.
(my bold)

https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodec8l/

The deep space Raptors, which your quote shows they don't need, *are* the Vac engines.
Yes, that was my point, he didn't say anything about *replacing* them, just not using them...

Offline Hominans Kosmos

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If they're using the BFS in SSTO mode for testing then they could replace some or all of the Vac engines with SL ones.

No, they won't be doing that:
Quote from: Elon Musk
Will be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance. Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep space Raptor engines.
(my bold)

https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodec8l/

The deep space Raptors, which your quote shows they don't need, *are* the Vac engines.
Yes, that was my point, he didn't say anything about *replacing* them, just not using them...

BFS with 3 engines does not have enough thrust to carry the fuel to fly hundreds kilometers. Elon's comment is clearly meant to communicate the use of sea level capable engines.


Offline guckyfan

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Elon did say they replace the vac engines with SL engines for the suborbital hops.

They will use the vac engines for SSTO test flights. As he also said

Quote
The "vacuum" or high area ratio Raptors can operate at full thrust at sea level. Not recommended.

SSTO can not be operational practice. But it can be used for a few test flights and in emergencies. After that the engines or at least the engine bells need inspection and/or replacement.

Offline philw1776

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If they're using the BFS in SSTO mode for testing then they could replace some or all of the Vac engines with SL ones.

No, they won't be doing that:
Quote from: Elon Musk
Will be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance. Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep space Raptor engines.
(my bold)

https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodec8l/

The deep space Raptors, which your quote shows they don't need, *are* the Vac engines.
Yes, that was my point, he didn't say anything about *replacing* them, just not using them...

BFS with 3 engines does not have enough thrust to carry the fuel to fly hundreds kilometers. Elon's comment is clearly meant to communicate the use of sea level capable engines.

I'm not sure that's correct.  Load 0 tonne payload BFS up with 330 tonnes of propellant and the 3 SL engines have a T/W of 1.26 and the rocket equation gives 5.7 Km/sec. If it's an ASDS landing, it could be hundreds of Km.

Even better, Musk said that with just SL engines they'd be doing test hops of a few hundred Km.
 
« Last Edit: 10/15/2017 02:34 pm by philw1776 »
FULL SEND!!!!

Offline Robotbeat

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Particularly with a lighter BFS without a heatshield.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Robotbeat

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If they're using the BFS in SSTO mode for testing then they could replace some or all of the Vac engines with SL ones.

No, they won't be doing that:
Quote from: Elon Musk
Will be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance. Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep space Raptor engines.
(my bold)

https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodec8l/

The deep space Raptors, which your quote shows they don't need, *are* the Vac engines.
Yes, that was my point, he didn't say anything about *replacing* them, just not using them...

BFS with 3 engines does not have enough thrust to carry the fuel to fly hundreds kilometers. Elon's comment is clearly meant to communicate the use of sea level capable engines.
Yes it does.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline DreamyPickle

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Even just a few tons to LEO would be useful as an extremely cheap way to deploy earth observation satellites or parts of the StarLink constellation. It's strange they don't seem to be publicly pursuing this as a real system.

A lot of the numbers involved are just optimistic estimates, maybe they're waiting for a full design of the first BFS.

Offline intrepidpursuit

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He specifically said that earth is not the place for SSTO. I don't understand the fascination with this idea. The booster is designed to fly extremely cheaply.

They are doing the suborbital hops to test the BFS. They wouldn't change out engines or stretch the tanks just for a test. They'll leave off what they don't need to save weight, including the VAC engines and maybe the heat shield. I suspect they want to simulate the last part of EDL  though and if they are flying a "few hundred km" up and out then they will likely need the shield.

Offline Elmar Moelzer

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My take is that an SSTO variant may become more relevant again in a few years, once the Raptor engine has (inevitably) improved over the current version and once SpaceX has learned more about their composite tanks. Looking at the huge strides that the Merlin 1D has made since the very first version and assuming only a fraction of that improvement for the Raptor, an ITS/Tanker derived SSTO with a large enough payload to LEO seems feasible. I think that for LEO payloads an SSTO with a F9 sized payload would be cheaper to operate than the BFR/BFS- two stage system. Since it would be smaller, it would probably also offer some operational advantages. Missions beyond LEO might require on orbit refueling and then it is a question whether two stages are better than two SSTOs...

Offline hkultala

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My take is that an SSTO variant may become more relevant again in a few years, once the Raptor engine has (inevitably) improved over the current version and once SpaceX has learned more about their composite tanks. Looking at the huge strides that the Merlin 1D has made since the very first version and assuming only a fraction of that improvement for the Raptor, an ITS/Tanker derived SSTO with a large enough payload to LEO seems feasible. I think that for LEO payloads an SSTO with a F9 sized payload would be cheaper to operate than the BFR/BFS- two stage system. Since it would be smaller, it would probably also offer some operational advantages.

What LEO payloads are you talking about?

And what is your definition of LEO?

Theoretical 185km orbit on optimal inclination from your launch site is much easier than the orbits where the "LEO payloads" are really wanted. Going to lower inclination requires expensive plane change manouver. Going to higher inclination means losing lots of the earth rotation velocity boost.

So, even if an expensive and heavy (85 tonne craft + 10 tonne landing fuel) SSTO craft could lift 15 tonnes to "easy LEO" and go around the world with it, it's payload to orbit where the customers want might be ZERO.

For example, most launches from Vandenberg are going to very high inclination polar orbits. Requires hundred of meters/sec more delta-v than the "easy LEO".

And ISS is at, 400 kilometers? And 51 degrees inclination? Thats about 24 degrees more than optimal from the cape. So again,  more delta-v needed than just "easy LEO".

Quote
Missions beyond LEO might require on orbit refueling and then it is a question whether two stages are better than two SSTOs...

No, there is nothing questionable. If it would be refuelling on orbit, then SSTO makes no sense.

Assume the payload to LEO on SSTO mode is 15 tonnes, craft weights 85 tonnes and landing fuel is 10 tonnes.

Lets think about 6 tonne satellite going to GTO:

LEO to GEO is 2.5 km/s.

Craft + 6 tonne payload is 101 tonnes.

With isp of 370 s this means this means that the refuelled mass on LEO is  201 tonnes, and mass on LEO before refueling is
85+ 10 + 15 = 110 tonnes (some fuel left in tanks)

So, to refuel to the craft enough times in  LEO to reach GTO, needs 91 tonnes of fuel. This mean 6 tanker flights with SSTO tanker. Or maybe 4 if our tanker is lighter and can carry 25.5 tonnes instead of 15 tonnes to LEO

So, total 5-7 launches and refueling operations needed.

Just one launch with fully reusable booster is many, many times cheaper.



When the system matures, the cost of one TSTO launch will probably be cheaper than 2 SSTO launches, because the boosters can be reused very rapidly, where the ship has to stay in orbit for much longer time to return to same location, and it returns with much higher thermal load, and it's engines burn for longer duration etc.

So always, if an SSTO cannot reach the destination without refueling, then it makes much more sense to first add the booster, and only refuel (with fuel lifted with the help of a booster) if it cannot reach the destination even on TSTO mode.
« Last Edit: 10/23/2017 03:06 pm by hkultala »

Offline MP99

If BFS can carry a reasonable payload to LEO, it could carry a larger payload to suborbital as required to do point to point passenger service. Every m/s saved makes a surprising difference.

It would make much more sense to do this, as it avoids relying on 31 engines on the core (and the cost of operating that core), a safety critical staging event, and mating ops before BFS can fly again.

I suspect that BFS may also be able to optimise its trajectory (vs injecting to orbit) somewhat to maximise payload.

This makes me doubt that BFS can get any meaningful payload to LEO (edit: because it's not shown on the P2P video), even with the 300 bar Raptor (I expect this to be in service before any passengers are carried).

If it can, however, this might make for a safer way to launch Mars crews, which would rendezvous with a BFS that had previously launched with their cargo.

Cheers, Martin

Sent from my Nexus 6 using Tapatalk
« Last Edit: 11/05/2017 08:16 pm by MP99 »

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