SpaceX Falcon Design Updates

[edkyle98]

SpaceX recently updated its web site http://www.spacex.com/ with some updated design information for its Falcon launch vehicles. <br /><br />The information showed that SpaceX is planning to start using a substantially more powerful version of its Merlin engine starting in 2009. The higher thrust engine, which might be the regenerative "Merlin 1C" mentioned by Mr. Musk in an interview last year, has resulted in heavier launch vehicle designs that can handle heavier payloads. <br /><br />Of interest to me was the fact that the updated Falcon 1 launch mass was given as 38.56 tonnes, a substantial increase from the previous 27.2 tonnes. This data wouldn't apply to existing Falcon 1 vehicles, but only to those launched in 2009 or later. An increase of more than 11 tonnes implies that the Falcon 1 would have to be stretched substantially to accommodate more propellant, but the SpaceX information shows no such growth in height. Both old and new Falcon 1 vehicle heights are/were given as 21.3 meters. The Falcon 1 first stage currently carries only 21 or so tonnes of propellant, so one has to wonder where the extra 11 tonnes will go!<br /><br />An 11+ tonne increase in gross liftoff weight also implies that payload capacity to LEO will increase beyond the current 570 kg (from a 28 deg inclination launch site) to something approaching, and perhaps exceeding, 650 kg. That would match or beat Minotaur 1 and Russia's Start-1, but the SpaceX web site still shows only 570 kg to LEO for Falcon 1.<br /><br />For background information, here is a link to my Falcon 1 web page, which provides data for the existing launch vehicle. <br /><br />http://www.geocities.com/launchreport/falcon.html<br /><br />A review of this page will show why I am presuming that most of the mass growth would be in the first stage. The second stage only weighs 3.745 tonnes, and already

 

[josh_simonson]

It could be that the Falcon 1 tanks were designed a little oversized from the begining in anticipation of an upgraded Merlin (which they've been looking at for several years). We know they use a common bulkhead, so a scale photo of the booster stage should allow a reasonably accurate calculation of fuel volume and hence full load weight. <br /><br />I doubt it's a typo, but that's possible too considering the payload capability vs growth are inconsistent.

 

[gunsandrockets]

"Of interest to me was the fact that the updated Falcon 1 launch mass was given as 38.56 tonnes, a substantial increase from the previous 27.2 tonnes. This data wouldn't apply to existing Falcon 1 vehicles, but only to those launched in 2009 or later. An increase of more than 11 tonnes implies that the Falcon 1 would have to be stretched substantially to accommodate more propellant, but the SpaceX information shows no such growth in height. Both old and new Falcon 1 vehicle heights are/were given as 21.3 meters. The Falcon 1 first stage currently carries only 21 or so tonnes of propellant, so one has to wonder where the extra 11 tonnes will go!" <br /><br /><br />Obviously something doesn't match up. If the mass has gone up then the size has gone up too, whether in the first or the second stage. I stand by my earlier comments for the most logical place to put that extra mass -- the second stage powered with a Merlin engine instead of a Kestrel engine.<br /><br />Since the 1st stage is hoped for recovery via parachute and water landing, enlarging the 1st stage at least as far as increasing it's delta V wouldn't be a very good idea. The faster the 1st stage goes up the faster it will come down! <br /><br />The Falcon I second stage, on the other hand, is expendable. Though with such a larger second stage, could the Kestrel upper stage engine handle that mass increase? What if the enlarged upper stage replaced the Kestrel with a Merlin engine? That way SpaceX could shut down one of their engine production lines, and save money. <br /><br />

 

[josh_simonson]

From a photo I estimated the tank length at 9.75m, which resulted in enough volume to hold 21400kg of propellants, so it doesn't seem like there's much extra space in there.

 

[josh_simonson]

Even if it were 100% lox, which is denser, it'd still weigh only 22200kg. They're already using a common bulkhead, so there's no space to squeeze out there either.

 

[edkyle98]

>"I stand by my earlier comments for the most logical place to put that extra mass -- the second stage powered with a Merlin engine instead of a Kestrel engine."<<br /><br />One problem with this idea is that Merlin weighs about as much as the current second stage does empty. Another problem is that Merlin produces too much thrust for the upper stage application. It would produce several 10s of Gs near the end of its burn no matter how large the second stage was. A third problem is that such a design would be far from optimum. <br /><br />For any two-stage vehicle, there is an optimum division of mass between the two stages to achieve maximum payload. The division depends on many factors, but rests heavily on the specific impulse of each stage. For a Merlin/Kestrel design, the optimum division is for the first stage to weigh about eight times more than the second stage. Even if a throttled-back Merlin could be used as a second stage, the optimum mass division would still only be 7:1, meaning that the second stage of a 38-ish tonne launch vehicle would only weigh roughly 4.6 tonnes, which isn't much more than the existing Kestrel-powered second stage.<br /><br /> - Ed Kyle

 

[comga]

Let's not get ahead of ourselves. My guess is the simplest one I can imagine. No switching propellants. No excess volume in the original rocket. No tricks with the fuel (like the one I think they are pulling with the Helium.)<br /><br />They have found that the turbopump has more capability than originally planned. They also called the ablative chamber their biggest mistake to date. (A bit of hyperbole) So now that they are chasing COTS, they are going to a regeneratively cooled engine with over 1E5 lbs of thrust. <br /><br />Long term they won't want to have two production lines, one churning out dozens of Merlin 1-Cs and one doing a couple of 1-As. So they plan to phase out the 1-A and build a larger larger single engine rocket. I will call it the Falcon 1.5, in a nod to Musk's reference to the current vehicle as the Falcon 1.1. They have done enough engineering to calculate a launch mass to four decimal places, but not much else. Not even a sketch.<br /><br />I hope they don't waste time on the "1.5" with so much else on their plate. It is now (again) one month or so to the launch of the Falcon-1 and only ~16 months to the launch of the first Falcon-9. I don't see how they can do it all, but I sure hope they can and wish them luck.

 

[solarspot]

"They also called the ablative chamber their biggest mistake to date."<br /><br />The Boeing Delta IV booster uses the RS 68 engine. It's ablatively cooled and thus costs a small fraction of the cost of the (less powerful) Space Shuttle Main Engine. Why would SpaceX replace the ablative if it's saving them cash?? That is what the Falcon is designed to do- be cheap. NASA is the one pushing overly-high-performance engines and million dollar lightweight fuel tanks...

 

[rocketman5000]

I think it goes back to the development time. From what I understand it was difficult to model the ablation.

 

[josh_simonson]

If that was the case, they'd be sticking with it rather than punting to the regeneratively cooled nozzle. The ablative chamber must also be underperforming on a performance/cost basis compared to the regenerative one.<br /><br />

 

[rogers_buck]

If it is a closed system like the N-1 with higher nozzle pressures, they just might be planning to hit the marks with a more efficient engine? I'm assuming that "regenerative" here means that a premix combustion is used to power the turbopump inline with the main engine?<br />

 

[rybanis]

Sorry buddy, SSME has more thrust than the RS-68.<br /><br />If I remember correctly, the regen chamber allows for the engine to have a higher chamber pressure, hence more thrust. <div class="Discussion_UserSignature"> </div>

 

[scottb50]

The RS-68 produces more power than the SSME. <div class="Discussion_UserSignature"> </div>

 

[comga]

"I'm assuming that "regenerative" here means that a premix combustion is used to power the turbopump inline with the main engine?"<br /><br />Actually, I believe that it is regenerative *cooling*. IIRC the regenerative cooling is flowing the oxidizer around the bell. They power the turbopump with a separate combustion line that flows out an exhaust actuated to provide roll control.

 

[rybanis]

Awwww, I see that you're right. <br /><br />I forgot, higher ISP, less thrust. <div class="Discussion_UserSignature"> </div>

 

[josh_simonson]

>they just might be planning to hit the marks with a more efficient engine?<br /><br />If you made a 1.7m tube the length of the falcon 1, including engine and interstage (but not the payload shroud) and filled it with LOX (which is denser than RP1), it would weight around 38t. There is no way to shoehorn that much fuel into the existing tankage of the falcon 1 without streching it or widening it. This means either the length, height or weight is wrong. Engine performance has nothing to do with this.